How Many PV Modules?

With your estimated energy demand calculated in total average amp hours per day, you are ready to calculate the number of PV modules in your array. You need to have an idea of what make and size of module you will use in your system (For an overview see our PV Module section).

Here are some other considerations before you come up with an array size:

MODULE MOUNTING STRUCTURES
Tracking vs Stationary Mounts

PV modules can be mounted on structures that are stationary or track the sun. Stationary mounts may be seasonally adjustable so that they will tilt north or south to be perpendicular with the noon day sun. Trackers follow the sun keeping the plane of the modules perpendicular with the rays of the sun all day. The output of a solar array can be increased over 50% on a summer day on a tracker. On a yearly basis, depending on type of tracker and your geography, the PV output can range from 25% to 40% increase over stationary mounting.

Trackers are not considered cost-effective if the array is less that eight modules, unless they are part of a PV-direct water pumping system. Trackers are best if your energy demand is high in the summer months--refrigeration, water pumping, fans, etc. Also, your site needs to provide a low east-west horizon. If most of the early morning and late afternoon sun is blocked out by the terrain around your array, then a tracker is not good investment. A tracker will cost 2 to 3 times more than a quality stationary mount.

Stationary mounts can either be partially adjustable for seasonal angle of the sun or fixed in one angle for the whole year. The fixed angle is usually your latitude (36 degrees at Rocky Grove. Or plus or minus 15 degrees from your latitude if you want to favor the summer or winter months. Changing the season tilt angle four times a year will increase PV energy production by 5 to 7% over a fixed position. Usually roof mounted arrays are not adjustable due to the difficulty of access.

PRIORITY LOADS

Determine what your priority loads are in your system. If it is lighting, then your greatest demand is in the winter months, when days are short and lights go on early in the evening. If your priority loads are refrigeration or water pumping, then your greatest demand is summer months. Priority loads for the summer correlate with the abundance of solar energy available. Priority loads for the winter do not, since winter and the transition months before and after feature a lot of overcast days.

Sizing your array for the winter months can be costly, but it will leave you with a surplus of energy for most of the year. You can have the pleasant task of figuring out what to do with it. However, if you want to keep your investment in PV down, we recommend using the yearly average peak sun hours per day for your locale (See the solar insolation maps below).

Good conservation habits and using efficient fluorescent lighting can make up for the lack of sunshine. Also, having a back-up means of charging your batteries like a gas generator is highly recommended. One good quality gas generator/charger can serve as a back-up to one or more households.

Size your array for peak demand loads using yearly average peak sun hours. The following is a format to calculate your array size:

1. Multiply your Total Amp Hours Per Day (System Energy Demand) by 1.2. This compensates for energy loss in battery charging.
2. Divide the adjusted Amp Hours per Day by the Average Sun Hours per Day in your area. This will equal the Total PV Array Amps required.
3. Now, divide the peak amps of the solar module used (for example, most 50 watt modules will have 3.0 amps, 60 watt module 3.5 amps, 70 watt module 4 amps, 80 watt, 4.5 amps) into the Total PV Array Amps. If this number is not a whole number, round off to next highest whole number. This number will equal the Total PV Array Modules required for your system if it is a 12 volt system. If your system voltage is 24 volts, then you need to multiply this number by two.

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