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You need to decide how much of your energy you want supplied by your solar electric system. If you want to be entirely independent, you will have to buy a large solar PV array. The same goes if you want to provide the bulk of power to your home: a large solar panel array will be needed.

Step 1: Energy usage

Our handy solar calculator will help you determine your needs.

As you calculate, also consider the following factors:

  • Does your weekly energy usage change? Do you use more on weekends? In the case of a cabin or cottage, you may be able to store energy during the full week.
  • Does your seasonal energy usage change? Do you use more in summer or winter?
  • How much energy variation is there within a day? How about from one day to the next? Energy usage per appliance may vary widely.

Step 2: Solar energy availability

Determine the number of full sun hours in your area. This varies seasonally, of course, and you can find handy data for your area using a solar insulation map or chart.

Step 3:

Both of these figures will be rendered in watts or watt hours or kilowatt hours (kWh). Divide the figure from step 1 (the number of watt hours) by the figure in step 2 (the number of hours of available sunlight). This will tell you the amount of wattage needed.

Medium sized solar panels have a standard wattage of 50 watts. A large panel (about 10 feet2) will render about 190 watts.

Regardless of how independent you want to be, you should have a certified solar energy installer check your system size.

Our experts recommend: to account for the variables in a solar energy system such as wire loss, roof shading, poor weather conditions and battery drain, you should double the wattage calculated.

Cost

Figure out how many panels you will need by dividing the number of watts needed into the sized panels you wish to purchase.

Solar panels cost about $10 per watt and other system components add up to about the same. So, to install a system will cost about $20 per watt (remember, this is only a rough estimate to get you started). And don’t forget (US buyers) that state financing incentivescan cut this cost as much as half.