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Solar Electricity

System Sizing

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The size of your solar electric system depends on:

You or a system installer can review your utility bills to determine how much electricity you use–typically shown in kilowatt-hours (kWh). If the information is not on your bill, contact your electricity provider and ask for your average monthly use in kWh.

One method for approximating system size is using Daily Peak Sun Hours:Annual Average Daily Peak Sun Hours

  1. Determine your average monthly electricity use in kWh.
  2. Divide by 30 for average use per day.
  3. Find the Peak Sun Hours for your location on the map. Peak sun hours are the hours of direct sunlight that fall on a PV panel (not total hours of daylight).
  4. Divide the answer calculated in #2 by your Peak Sun Hours.

The answer is a rough estimate of the solar electric system size you will need (in kW) for 100% of your electricity.


  1. A Helena home’s monthly average electricity use is 856 kWh/month.
  2. 856 kWh ÷ 30 = 28.5 kWh average per day
  3. Helena is near the diagram’s “5” line = 5 annual average peak sun hours per day.
  4. 28.5 ÷ 5 = 5.7 kW PV system would be needed to produce 100 percent of this home’s electricity.

NOTE: This method provides a rough estimate and should not be used to size a system. The actual system will most likely be larger due to many system-related factors.

For more accurate sizing, use the worksheet below. It takes conservation and efficiency measures and system component inefficiencies into account.

Solar Electricity PV Panel System Worksheet

Worksheet Notes:

  1. Montana homes used an average of 856 kWh/month of electricity in 2009.
  2. 1 kilowatt-hour (kWh) = 1,000 watt-hours
  3. 365 days/year divided by 12 months = 30 average days in one month.
  4. Energy conservation and efficiency measures can reduce the size of your system.
  5. PV panels can be added to a system over time.
  6. The solar radiation value used to rate panels is 1 kW/m2 and thus the results are in watts.
  7. The amount of PV-produced electricity decreases as it flows through wires, the inverter, and other system components. NREL’s PV Watts on-line tool addresses this using a de-rate factor.
  8. Converts watts back to kilowatts (the units used to describe a system’s “size”).
  9. If you know what PV panel you will use, divide Box 7’s watts by the Peak Power (in watts) of the panel to determine the # of panels needed. Do you have enough space on your roof?

After the number of panels is determined, the remaining balance of system components (inverter, etc,) can be sized accordingly. While the kW output calculated in Step 8 above is specific to the PV panels/array, the kW number is also referred to/used as the “system size.”

To find your solar resource/radiation number, access the map at http://www.nrel.gov/gis/solar.html

Computer/On-Line Sizing Tools allow you to consider a combination of PV system options for specific locations using your address or zip code. Know how much electricity you use in one year (in kWh) and the approximate PV system size you are considering before you access the free programs described below.

Whether you use pencil and paper or a computer, these exercises provide a sense of what system size will work for you. A consultant or installer can provide a more detailed analysis and can advise on what will work best for your particular needs. Are you interested in solar electricity because you want to reduce your environmental footprint? To determine the amount of greenhouse gases you are not emitting into the atmosphere by installing a solar electric system, use Option 1 (insert the kWh provided by the solar electric system) with the U.S. EPA’s Greenhouse Calculator.


McCabe, J. (ed.). (2010, Fall/Winter). All About Photovoltaic Systems. Solar Today: Getting Started (Bonus Issue), 16-19.

National Renewable Energy Laboratory (produced) for the U.S. Department of Energy. (2002, August). How to Size a Grid-Connected Solar Electricity System. DOE/GO-102002-1607.

National Renewable Energy Laboratory (produced) for the U.S. Department of Energy. (1997, March). Photovoltaics: Basic Design Principles and Components. DOE/GO-10097-337.