How many solar panels (PV modules) will I need?

Our sample systems [link] range from two to sixteen panels, and a large public building may need many more. Determining the size of your array – how many solar panels you’ll need – is part of what we at Rocky Grove do in preparation for your installation, and the cost is included in the total price.

However, if you want to buy solar panels and do your own installation, you can calculate the number you should purchase, using the figure you’ve determined for how much energy you need [link].

If lighting is your highest priority, then your greatest demand will be 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, when there is more sunlight.

Sizing your array of solar panels 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).
average peak sun hours per day for your locale
Good conservation habits and using efficient fluorescent lighting can make up for the lack of sunshine. If you are not on the grid, 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.

The following is a formula 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.