How to calculate solar for a home

Written by hazel morgan
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How to calculate solar for a home
Solar energy is clean and saves money. (Solar Panel image by kuhar from

Calculating how many solar panels you need and, if required, how many storage batteries are needed is an exercise in math. However, the work is worth doing. Solar power reduces your electricity bill and offsets your carbon footprint. No polluting power stations are required for solar energy.

Skill level:

Things you need

  • Six to 12 months of electricity bills
  • Calculator

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  1. 1

    Study your electricity bills for the last twelve months. This will give you a sense of how much electricity you use. Electricity bills are figured in kilowatt-hours. One kilowatt-hour represents 1,000 watts (1kW) of power used for an hour. You can either add up all the kWh you have used for a year and divide by 12 to get a monthly value or simply pick the highest month and work with that figure instead.

  2. 2

    According to the U.S. Energy Information Administration, in 2007 a typical home used 936 kWh per month. Assume 900 kWh per month. Divide this figure by 30 to get an approximate daily usage rate. This would mean you were using 30 kWh per day (30 x 30 = 900). You now know that your solar panels need to generate 30 kWh per day.

  3. 3

    You may get approximately eight to 10 hours of usable daylight during a day. You now calculate how much your solar panels need to generate per hour. Assuming 10 hours of sunlight, divide the daily 30 kWh by 10 to get a figure of 3kW per hour: 3000 watts.

  4. 4

    Solar panels are supplied in sizes such as 200W and 400W. Their rating is the amount they can output per hour. To generate your 3kW of electricity, you would need 3000 watts' worth of solar panels. Because mismatched panels may produce different voltages, it is better to work with the same size and same brand of panels. Thus, if working with 200W panels you would need 15 (15 x 200 = 3000), but if you were working with 400W panels it would be better to go a little over or under and buy seven or eight rather than seven plus a smaller panel.

  1. 1

    Some solar electricity systems feed excess solar power into the grid so you can draw on the grid while offsetting your electricity bills. This is called a grid-tie system. However, if you are not on the grid, you will need to calculate the number of batteries needed.

  2. 2

    Using the figures above, during your 10 hours of daylight you would generate 3000W of electricity. In 10 hours, the solar panels must generate the electricity for a whole day. We now divide the daily usage of 30 kWh by 24 to get an hourly rate of approximately 1.25kW per hour. This is your hourly household usage.

    During daylight, your house uses 10 x 1.25kW or 12,500W of electricity. Your system generates 30,000 Watts (30kWh). You must find a way to store 17,500W, or 17.5kW of energy (30,000 - 12,500 = 17,500).

  3. 3

    Most home solar battery banks use 12-volt batteries, with the batteries rated in ampere-hours (Ah). To get the number of ampere-hours of battery power needed, divide 17.5 kWh by 12 to get a value of 1458.33 ampere-hours. Increase this 20 per cent (multiply by 1.2) to account for power loss. 1.2 x 1458.33 is 1749.996 ampere-hours. Round up to 1750.

  4. 4

    If you are using a 200 Ah battery, you will need nine 12-volt batteries to store your energy. (1750 / 200 = 8.75; round up to nine). However, there are two further issues. First, according to Northern Arizona Wind and Sun, deep cycle batteries should not be run down lower than 50 per cent to preserve their life. Thus, twice as many batteries (18) would be required. Second, it is common to wire four batteries together into a single 48-volt bank because this requires a smaller charge and less wiring and is thus a more efficient and economical approach.

    You may therefore wish to either downsize to 16 batteries or increase to 20.

Tips and warnings

  • A number of factors, including cloudy weather, damaged cells and even the transfer to batteries, can lead to a power loss. You may wish to add an extra solar panel to make up for those losses.
  • Households use the most electricity on heating and cooling. In most cases it is either prohibitively expensive or there simply isn't enough roof space to supply all a home's needs from solar electricity. You can always supply some of your electrical needs from solar while considering alternatives such as propane or a solar water heater for the heating.

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