What type of electrical energy and what voltage should I choose?

In order to choose the right elements for your solar energy system (your solar electric array, stuff batteries, inverter, water pump, etc), you must begin with what your personal energy demands are going to be.

Contributing factors:

  • Whether you are using a grid tie-in system (you will still have access to electricity from the power company) or a stand-alone solar energy system (you will supply all your own power)
  • The availability of sunshine in your particular location
  • Your ability to use energy wisely–to conserve and exercise efficiency (especially in periods of extended cloudy weather). Using the most efficient appliances and using them when you really need them will keep the cost of your system down.

Make a list of all your electrical appliances, DC and AC if an inverter[link to explanation] is in your plans. Most AC appliances have their ratings on a tag in the back of the appliance. They are usually rated for the maximum potential of energy use, so the actual operating wattage may be only half as much.

Use the following chart of common household appliances to help estimate your loads. Note the differences between some appliances and their DC counterparts.
Energy amounts of household appliances

If you plan to use a stand-alone system

For success in using a dependable yet small amount of energy delivered free every day, you have to live within your means. The following is a review of common household appliances (courtesy of Alternative Energy Engineering). Read this before estimating your energy demand.

Cooking, heating and cooling:

Conventional electric cooking appliances, space heating and water heating equipment use a prohibitive amount of electricity. Electric ranges use 1500 watts or more per burner, so bottIed propane or natural gas is a popular alternative for cooking. A large microwave oven has about the same power draw, but since food cooks much more quickly, the number of kilowatt hours used may not be too large. Propane and wood are popular alternatives for space heating. Good passive solar design and proper insulation can reduce the need for heat. For home cooling, central air conditioning uses a prohibitive amount of energy, though in a large system a small AC window unit may suffice on a very limited basis. Evaporative cooling is a more reasonable load in locations with low-to-moderate humidity. Appropriately placed fans will keep air moving in the house. A solar cooling plus: the largest amount of solar energy is usually available when the temperature is the highest.

Lighting:

Lighting requires the most study, since so many options exist in type, size, voltage and placement. The type of lighting that is best for one system may not be right for another. The first decision is whether your lights will be run on low voltage DC or 120 AC. In a small home, an RV, or a boat, low voltage DC lighting is usually the best choice. In addition to conventional size medium base low voltage bulbs, the user can choose from a large selection of DC fluorescent lights, which have three to four times the light output per watt of power used compared with standard incandescent types. Halogen bulbs are approximately 30% more efficient and actually seem almost twice as bright as similar wattage standard incandescents, because of the spectrum of light that they produce. Twelve and 24 volt replacement ballasts are available to convert AC Fluorescent lights to DC.

In a very large installation or one with many lights, the use of an inverter to supply AC power for conventional lighting is cost effective. (The increasing local availability of efficient AC compact fluorescent lights at a fraction of the cost of their DC counterparts makes this option more desirable.) In a stand alone system with AC lighting, the user should have a back up inverter or a few low voltage DC lights in case the primary inverter fails. AC light dimmers will not function on AC power from inverters unless they have pure sine wave output.

Refrigeration:

Gas powered absorption refrigerators are a good choice in small systems if bottled gas is available. Modern absorption refrigerators consume 5 to 10 gallons of LP gas per month. If an electric refrigerator will be used, it should be a high efficiency type. Sun Frost refrigerators use 300 to 400 watt hours of electricity per day, while conventional AC refrigerators use 3000 to 4000 watt hours of electricity per day at an average room temperature of 70 degrees F. The higher cost of good quality DC refrigerators is made up many times over by the savings in the smaller number of solar modules and batteries required.

Major appliances:

Standard AC electric motors in washing machines, larger shop machinery and tools, “swamp coolers,” pumps, etc. (usually 1/4 to 3/4 horsepower) require a large inverter. Often, a 2000 watt or larger inverter will be required. The inverter will get warm or hot when running these loads, which may shorten its life. These electric motors are sometimes hard to start on inverter power, they consume relatively large amounts of electricity, and they are very wasteful compared to high-efficiency motors (rectified permanent magnet motors) that use 50% to 75% less electricity. A standard washing machine uses between 300 and 500 watt hours per load. (It would require a 2000 watt inverter or larger to run it.) If the appliance is used more than a few hours per week, it is often cheaper to pay more for a high effiiciency appliance rather that make your electrical system larger to support a low-efficiency load. For many belt-driven loads (washers, drill press, etc,), their standard electric motor can often be easily replaced with a high efficiency type. These motors are available in either AC or DC, come as separate units or as motor-replacement kits. (Inquire for sources)

Vacuum cleaners typically consume 600 to 1000 watts, depending on how powerful they are, but most vacuum cleaners operate on inverters larger than 1000 watts because they have low-surge motors.

Small appliances:

Many small appliances such as irons, toasters and hair dryers consume large amounts of power when they are used, but by their nature require very short or infrequent use periods. If the system inverter and batteries are large enough, they may be useable.

Electronic equipment such as stereos, televisions, VCRs, and computers, has a fairly small power draw. Many of these are available in low voltage DC as well as conventional AC versions, and in general DC models use less power than their AC counterparts.

The following calculation will determine the total amp hours per day used by all the AC and DC loads in your system:

  1. If you are going to use an inverter then list all the AC loads, wattage and hours of use per week. Add up all the watt hours per week to determine DC watt hours per week:
    Appliance Description — Watts x Hours/Week = Watt Hours/Week
    (If an appliance is rated in amps, multiply amps by voltage (120) to find watts.)
  2. Multiply your total Watt Hours per Week by 1.10 to correct for inverter loss. This is your actual DC Watt Hours Per Week.
  3. Divide your Total DC Watt Hours Per Week by your System Voltage (12 or 24). This sum is your total Amp Hours per Week used by your inverter for the AC loads.
  4. Now list all the DC loads and calculate all the Watt Hours per Week.
    Appliance Description — Watts x Hours/Week = Watt Hours/Week
  5. Divide your Total DC loads’ Watt Hours per Week by System Voltage (12 or 24). This is the total Amp hours per Week used by DC loads.
  6. 6. Now, add the total DC and AC Amp Hours per Week. This is the total Amp Hours per Week used in your system.
  7. 7. Finally, divide the Total from step 6 by 7 and this will be the Total Average Amp Hours per Day.

*This final number will be used to estimate the amount of solar modules needed for your system.
In order to choose the right elements for your solar energy system (your solar electric array, stuff batteries, inverter, water pump, etc), you must begin with what your personal energy demands are going to be.

Contributing factors:

  • Whether you are using a grid tie-in system (you will still have access to electricity from the power company) or a stand-alone solar energy system (you will supply all your own power)
  • The availability of sunshine in your particular location
  • Your ability to use energy wisely–to conserve and exercise efficiency (especially in periods of extended cloudy weather). Using the most efficient appliances and using them when you really need them will keep the cost of your system down.

Make a list of all your electrical appliances, DC and AC if an inverter[link to explanation] is in your plans. Most AC appliances have their ratings on a tag in the back of the appliance. They are usually rated for the maximum potential of energy use, so the actual operating wattage may be only half as much.

Use the following chart of common household appliances to help estimate your loads. Note the differences between some appliances and their DC counterparts.
Energy amounts of household appliances

If you plan to use a stand-alone system

For success in using a dependable yet small amount of energy delivered free every day, you have to live within your means. The following is a review of common household appliances (courtesy of Alternative Energy Engineering). Read this before estimating your energy demand.

Cooking, heating and cooling:

Conventional electric cooking appliances, space heating and water heating equipment use a prohibitive amount of electricity. Electric ranges use 1500 watts or more per burner, so bottIed propane or natural gas is a popular alternative for cooking. A large microwave oven has about the same power draw, but since food cooks much more quickly, the number of kilowatt hours used may not be too large. Propane and wood are popular alternatives for space heating. Good passive solar design and proper insulation can reduce the need for heat. For home cooling, central air conditioning uses a prohibitive amount of energy, though in a large system a small AC window unit may suffice on a very limited basis. Evaporative cooling is a more reasonable load in locations with low-to-moderate humidity. Appropriately placed fans will keep air moving in the house. A solar cooling plus: the largest amount of solar energy is usually available when the temperature is the highest.

Lighting:

Lighting requires the most study, since so many options exist in type, size, voltage and placement. The type of lighting that is best for one system may not be right for another. The first decision is whether your lights will be run on low voltage DC or 120 AC. In a small home, an RV, or a boat, low voltage DC lighting is usually the best choice. In addition to conventional size medium base low voltage bulbs, the user can choose from a large selection of DC fluorescent lights, which have three to four times the light output per watt of power used compared with standard incandescent types. Halogen bulbs are approximately 30% more efficient and actually seem almost twice as bright as similar wattage standard incandescents, because of the spectrum of light that they produce. Twelve and 24 volt replacement ballasts are available to convert AC Fluorescent lights to DC.

In a very large installation or one with many lights, the use of an inverter to supply AC power for conventional lighting is cost effective. (The increasing local availability of efficient AC compact fluorescent lights at a fraction of the cost of their DC counterparts makes this option more desirable.) In a stand alone system with AC lighting, the user should have a back up inverter or a few low voltage DC lights in case the primary inverter fails. AC light dimmers will not function on AC power from inverters unless they have pure sine wave output.

Refrigeration:

Gas powered absorption refrigerators are a good choice in small systems if bottled gas is available. Modern absorption refrigerators consume 5 to 10 gallons of LP gas per month. If an electric refrigerator will be used, it should be a high efficiency type. Sun Frost refrigerators use 300 to 400 watt hours of electricity per day, while conventional AC refrigerators use 3000 to 4000 watt hours of electricity per day at an average room temperature of 70 degrees F. The higher cost of good quality DC refrigerators is made up many times over by the savings in the smaller number of solar modules and batteries required.

Major appliances:

Standard AC electric motors in washing machines, larger shop machinery and tools, “swamp coolers,” pumps, etc. (usually 1/4 to 3/4 horsepower) require a large inverter. Often, a 2000 watt or larger inverter will be required. The inverter will get warm or hot when running these loads, which may shorten its life. These electric motors are sometimes hard to start on inverter power, they consume relatively large amounts of electricity, and they are very wasteful compared to high-efficiency motors (rectified permanent magnet motors) that use 50% to 75% less electricity. A standard washing machine uses between 300 and 500 watt hours per load. (It would require a 2000 watt inverter or larger to run it.) If the appliance is used more than a few hours per week, it is often cheaper to pay more for a high effiiciency appliance rather that make your electrical system larger to support a low-efficiency load. For many belt-driven loads (washers, drill press, etc,), their standard electric motor can often be easily replaced with a high efficiency type. These motors are available in either AC or DC, come as separate units or as motor-replacement kits. (Inquire for sources)

Vacuum cleaners typically consume 600 to 1000 watts, depending on how powerful they are, but most vacuum cleaners operate on inverters larger than 1000 watts because they have low-surge motors.

Small appliances:

Many small appliances such as irons, toasters and hair dryers consume large amounts of power when they are used, but by their nature require very short or infrequent use periods. If the system inverter and batteries are large enough, they may be useable.

Electronic equipment such as stereos, televisions, VCRs, and computers, has a fairly small power draw. Many of these are available in low voltage DC as well as conventional AC versions, and in general DC models use less power than their AC counterparts.

The following calculation will determine the total amp hours per day used by all the AC and DC loads in your system:

  1. If you are going to use an inverter then list all the AC loads, wattage and hours of use per week. Add up all the watt hours per week to determine DC watt hours per week:
    Appliance Description — Watts x Hours/Week = Watt Hours/Week
    (If an appliance is rated in amps, multiply amps by voltage (120) to find watts.)
  2. Multiply your total Watt Hours per Week by 1.10 to correct for inverter loss. This is your actual DC Watt Hours Per Week.
  3. Divide your Total DC Watt Hours Per Week by your System Voltage (12 or 24). This sum is your total Amp Hours per Week used by your inverter for the AC loads.
  4. Now list all the DC loads and calculate all the Watt Hours per Week.
    Appliance Description — Watts x Hours/Week = Watt Hours/Week
  5. Divide your Total DC loads’ Watt Hours per Week by System Voltage (12 or 24). This is the total Amp hours per Week used by DC loads.
  6. 6. Now, add the total DC and AC Amp Hours per Week. This is the total Amp Hours per Week used in your system.
  7. 7. Finally, divide the Total from step 6 by 7 and this will be the Total Average Amp Hours per Day.

*This final number will be used to estimate the amount of solar modules needed for your system.
In order to choose the right elements for your solar energy system (your solar electric array, stuff batteries, inverter, water pump, etc), you must begin with what your personal energy demands are going to be.

Contributing factors:

  • Whether you are using a grid tie-in system (you will still have access to electricity from the power company) or a stand-alone solar energy system (you will supply all your own power)
  • The availability of sunshine in your particular location
  • Your ability to use energy wisely–to conserve and exercise efficiency (especially in periods of extended cloudy weather). Using the most efficient appliances and using them when you really need them will keep the cost of your system down.

Make a list of all your electrical appliances, DC and AC if an inverter[link to explanation] is in your plans. Most AC appliances have their ratings on a tag in the back of the appliance. They are usually rated for the maximum potential of energy use, so the actual operating wattage may be only half as much.

Use the following chart of common household appliances to help estimate your loads. Note the differences between some appliances and their DC counterparts.
Energy amounts of household appliances

If you plan to use a stand-alone system

For success in using a dependable yet small amount of energy delivered free every day, you have to live within your means. The following is a review of common household appliances (courtesy of Alternative Energy Engineering). Read this before estimating your energy demand.

Cooking, heating and cooling:

Conventional electric cooking appliances, space heating and water heating equipment use a prohibitive amount of electricity. Electric ranges use 1500 watts or more per burner, so bottIed propane or natural gas is a popular alternative for cooking. A large microwave oven has about the same power draw, but since food cooks much more quickly, the number of kilowatt hours used may not be too large. Propane and wood are popular alternatives for space heating. Good passive solar design and proper insulation can reduce the need for heat. For home cooling, central air conditioning uses a prohibitive amount of energy, though in a large system a small AC window unit may suffice on a very limited basis. Evaporative cooling is a more reasonable load in locations with low-to-moderate humidity. Appropriately placed fans will keep air moving in the house. A solar cooling plus: the largest amount of solar energy is usually available when the temperature is the highest.

Lighting:

Lighting requires the most study, since so many options exist in type, size, voltage and placement. The type of lighting that is best for one system may not be right for another. The first decision is whether your lights will be run on low voltage DC or 120 AC. In a small home, an RV, or a boat, low voltage DC lighting is usually the best choice. In addition to conventional size medium base low voltage bulbs, the user can choose from a large selection of DC fluorescent lights, which have three to four times the light output per watt of power used compared with standard incandescent types. Halogen bulbs are approximately 30% more efficient and actually seem almost twice as bright as similar wattage standard incandescents, because of the spectrum of light that they produce. Twelve and 24 volt replacement ballasts are available to convert AC Fluorescent lights to DC.

In a very large installation or one with many lights, the use of an inverter to supply AC power for conventional lighting is cost effective. (The increasing local availability of efficient AC compact fluorescent lights at a fraction of the cost of their DC counterparts makes this option more desirable.) In a stand alone system with AC lighting, the user should have a back up inverter or a few low voltage DC lights in case the primary inverter fails. AC light dimmers will not function on AC power from inverters unless they have pure sine wave output.

Refrigeration:

Gas powered absorption refrigerators are a good choice in small systems if bottled gas is available. Modern absorption refrigerators consume 5 to 10 gallons of LP gas per month. If an electric refrigerator will be used, it should be a high efficiency type. Sun Frost refrigerators use 300 to 400 watt hours of electricity per day, while conventional AC refrigerators use 3000 to 4000 watt hours of electricity per day at an average room temperature of 70 degrees F. The higher cost of good quality DC refrigerators is made up many times over by the savings in the smaller number of solar modules and batteries required.

Major appliances:

Standard AC electric motors in washing machines, larger shop machinery and tools, “swamp coolers,” pumps, etc. (usually 1/4 to 3/4 horsepower) require a large inverter. Often, a 2000 watt or larger inverter will be required. The inverter will get warm or hot when running these loads, which may shorten its life. These electric motors are sometimes hard to start on inverter power, they consume relatively large amounts of electricity, and they are very wasteful compared to high-efficiency motors (rectified permanent magnet motors) that use 50% to 75% less electricity. A standard washing machine uses between 300 and 500 watt hours per load. (It would require a 2000 watt inverter or larger to run it.) If the appliance is used more than a few hours per week, it is often cheaper to pay more for a high effiiciency appliance rather that make your electrical system larger to support a low-efficiency load. For many belt-driven loads (washers, drill press, etc,), their standard electric motor can often be easily replaced with a high efficiency type. These motors are available in either AC or DC, come as separate units or as motor-replacement kits. (Inquire for sources)

Vacuum cleaners typically consume 600 to 1000 watts, depending on how powerful they are, but most vacuum cleaners operate on inverters larger than 1000 watts because they have low-surge motors.

Small appliances:

Many small appliances such as irons, toasters and hair dryers consume large amounts of power when they are used, but by their nature require very short or infrequent use periods. If the system inverter and batteries are large enough, they may be useable.

Electronic equipment such as stereos, televisions, VCRs, and computers, has a fairly small power draw. Many of these are available in low voltage DC as well as conventional AC versions, and in general DC models use less power than their AC counterparts.

The following calculation will determine the total amp hours per day used by all the AC and DC loads in your system:

  1. If you are going to use an inverter then list all the AC loads, wattage and hours of use per week. Add up all the watt hours per week to determine DC watt hours per week:
    Appliance Description — Watts x Hours/Week = Watt Hours/Week
    (If an appliance is rated in amps, multiply amps by voltage (120) to find watts.)
  2. Multiply your total Watt Hours per Week by 1.10 to correct for inverter loss. This is your actual DC Watt Hours Per Week.
  3. Divide your Total DC Watt Hours Per Week by your System Voltage (12 or 24). This sum is your total Amp Hours per Week used by your inverter for the AC loads.
  4. Now list all the DC loads and calculate all the Watt Hours per Week.
    Appliance Description — Watts x Hours/Week = Watt Hours/Week
  5. Divide your Total DC loads’ Watt Hours per Week by System Voltage (12 or 24). This is the total Amp hours per Week used by DC loads.
  6. 6. Now, add the total DC and AC Amp Hours per Week. This is the total Amp Hours per Week used in your system.
  7. 7. Finally, divide the Total from step 6 by 7 and this will be the Total Average Amp Hours per Day.

*This final number will be used to estimate the amount of solar modules needed for your system.
Inverters

Inverters simplify the process of living on renewable energy, rx because they produce conventional electricity —Alternating Current (AC). This allows the use of conventional AC appliances from battery power. Inverters come in all shapes and sizes, online and can be specialized for certain applications. Today, ask most solar electric homes are primarily using inverters to power most of their electrical needs. In the early days of solar electricity, most homes used DC power and only used the inefficient inverters available for intermittent use of some AC appliances. But with the technological breakthroughs at the end of 20th century, inverters have become an efficient, powerful and above all, reliable component in renewable energy systems. Today’s inverters offer the home power resident most of the modern conveniences of the average grid-powered home.

Sine Wave vs. Modified Sine Wave

There are basically two types of inverter output. Sine wave is a relatively new development in inverters, while modified sine wave constitutes the output of most of the inverters on the market. Utility power companies and gas AC generators produce sine wave AC (alternating current) power. All conventional electric appliances are designed to operate on sine wave AC, so some appliances have problems accepting modified sine wave. Loads like copiers, laser printers, audio equipment, some computers, etc. have electronic circuitry that is very picky about the wave form of its power supply. But in every category there is an exception to the rule. In most cases modified sine wave inveTters are satisfactory running most household appliances. At this stage of development they are more efficient running smaller loads than sine wave inverters. Sine wave inverters will run loads more quietly, and motors more powerfully. Only sine wave inverters can be synchronous with grid or generator power. Trace Engineering’s sine wave inverters are capable of selling power to the grid.

Choosing an Inverter

With the exception of water pumping, refrigeration and fans, most household loads, because they are intermittent, will operate efficiently and cost-effectively on inverter power. Determine what your overall needs are in your system. What is the total continuous power that you might need from an inverter at peak demand times (powering two or more large appliances simultaneously, like a deep well pump and an automatic washer.) In choosing an inverter, continuous power ratings and surge power ratings (for starting motors), idle power consumption and their overall efficiency are the most important factors. If there are just too many choices for you, ask the friendly folks at Rocky Grove for a recommendation. (Tip: Trace Inverters have been around since ’86 and they quickly set the standards for the rest of the inverter market. We generally recommend Trace over the others in most full-time residential systems.)

Check out the inverters we offer…

System voltage is basically equal to the DC (direct current) voltage of your battery bank. The standard system voltages are 12, 24 and 48. 12 volt systems are the most common, largely because 12 volts is the standard for the automobile and RV industry and there are more 12 volt appliances available. 24 volt systems are probably the most used in medium to large systems. 48 volts is used in very large systems or systems where long-distance DC energy transmission is necessary.

12 volt vs 24 volt vs 48 volt systems:

In general,  12 volt systems are the most cost-effective for small to medium sized systems. 24 volt systems are best for larger systems where larger inverters, larger water pumps and larger DC motor-powered appliances are required. At 24 volts, smaller wire can be used to deliver power efficiently. Compared to 12 volts, it takes only 1/4 the size of wire to conduct the same amount of electricity at 24 volts. At 48 volts it takes only 1/8 the size.

Batteries:

A 12 volt battery requires six healthy (lead-acid) cells, a 24 volt system requires 12 and a 48 volt battery requires 24., it is ideal to have a parallel set of batteries, or two sets at the same voltage. Two parallel sets double the capacity, and in the event of an accident or cell failure, your system need not shut down while you find a cell or set replacement. It is preferable to have large cells in your battery bank versus a lot of small cells to provide an appropriate amount of storage. Large cells are more massive and less prone to structural failure. Also the fewer cell to cell connections the better. Large Cells, Less Cells, Less Trouble.

PV Array:

A single PV module is designed to charge a 12 volt battery. It takes two modules, wired in a series, to charge a 24 volt battery and it takes four in series at 48 volts. So buying an even number of like modules is required for both 24 and 48 volt systems (different size modules can be in the same array as long as the modules in each pair or foursome are identical). Because of the line loss advantage at 24 volts, the PV array can be located much further from the batteries if necessary to maximize solar exposure. An electronic device called a Linear Current Booster [link]can also be used to work like a DC step-down transformer  to allow wiring the array in series for higher voltage (higher than system voltage) so that transmission will be very efficient on smaller wire for greater distances and you can still efficiently charge a 12 or 24 volt battery.

Inverters:

On the low power end of the scale 50 watts to 2500 watts, there are more 12 volt inverters to choose from than 24 volt. For power inverters ranging over 2500 watts, 24, 36 or 48 volt batteries are required. Some inverters like Trace and Heart are stackable–meaning that two units can work in tandem to double the output. The Trace DR and SW Series inverters [link] also supply you with 240VAC when they are stacked. Most 12 and 24 volt inverters in the mid-range (700 to 2400 watts) are comparable in efficiency and price per watt. 24 volt inverters tend to have better surge ratings (motor starting capability) than 12 volt inverters of the same wattage. Most of the Trace line of inverters [link]include standard built-in battery chargers. This gives the option of using a gas generator or utility power as a back-up power source.

DC motors :

DC motors inherently use a lot less energy than equally sized AC motors. That’s why in a solar electric system long term loads that use motors should use DC motors. We offer DC fans and water pumps. That’s the way to go if possible. Inverters are practical for running short term AC motor loads (circular saws, drills vacuum cleaners, washing machines etc.). Long duration AC motor loads like the array of different AC fans (ceiling, attic, window, table, etc.) are significant energy hogs. If operating various stationary shop tools is a high priority in your system, then we would recommend going with a 24 volt system and running your bigger tools with 24 volt DC permanent magnet motors. PM motors are the most efficient of all the different types (series, compound, etc.). For converting tools like table saws, drill presses, planers, etc, with DC motors, you will find a greater selection at 24 volts than at 12–especially for 3/4 HP and up. Tools with pulley and belt drive are the easiest to convert since the RPM differences in the motors can be adjusted by changing the pulley size. Check out different mail order surplus catalogs for bargains on DC motors. (Surplus Center 800-488-3407) A general rule on motor conversions: A DC motor can generally replace an AC motor with 1.5 to 2 times the horsepower rating!

Upgrading:

Upgrading from 12 to 24 volts requires either adding 6 more cells to your battery bank or reconfiguring two 12 volt sets in a series; or starting over with a new 24 volt set. Two identical 12 volts sets in parallel will be equal in age and capacity and can be connected in series to make one 24 volt set. Mixing old and new batteries is not recommended, especially connecting them in series. One old, dying cell can be a constant drain on your whole bank. If you have an odd number of PV modules then another one will be necessary to make a pair for the series-parallel connection.  If two modules of different current ratings are wired in series, then, the output of that series will be equal to the output of the module with the lower current. Some charge controllers will work on 12 or 24 volt systems, otherwise to upgrade to 24 volt another controller will be necessary. Changing from 12 to 24 volts makes your 12-volt appliances obsolete, unless you employ an electronic converter (like the Vanner Battery Equalizer) that will allow you to discharge equally from both sides of a 24 volt battery. Careful consideration of your present and future electrical needs will aid in choosing the most appropriate system voltage.