How much electrical energy will I need?

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 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.


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.