How to Size Solar Panels in Excel

Putting together a solar power system for your home involves a bit of math and calculation up front, starting with how much power you actually consume and how much power you're likely to generate from a solar panel array. A few of these calculations use known "fudge factors" for their inputs, but they're commonly accepted practice in the solar panel industry and are easy to set up in Microsoft Excel.

Things You'll Need

• Excel 2000 or later

Instructions

1. Open up Excel with a blank worksheet.

2. Enter the kilowatt-hours for your three highest utility bills for the last year in cells A1, B1 and C1, respectively. In cell D1, enter the following formula: "=AVERAGE(A1:C1)". This will give you the average number of kilowatt-hours your home consumed for its peak power usage. This is your energy budget.

3. Enter the percentage of your home's energy budget you want to generate from solar power in cell E1; typical values range from 10 percent to 30 percent and depend largely on how much you're willing to spend and how much sunlight your home gets. Enter the formula "=D1*E1" in cell F1. This is how many kilowatt-hours your solar panels must generate to meet the percentage of the energy budget given.

4. Find the specifications of the solar panels you intend to buy. Most manufacturers give a range of watts per unit area for a given solar panel, such as 130 watts per square meter (roughly 12 watts per square foot) for most residential units. Enter the watts per unit area in cell A3.

5. Enter the number of "solar hours" for your location in cell B3. You can find a table of "solar hour" numbers at www.solar4power.com/solar-power-insolation-window.html. The average number (the third number for each city) is what you're looking for.

6. Enter the following formula in cell D3: "=A3*B3*(3,600/1,000)". The last part of the equation is the number of seconds per hour, divided by 1,000 to get kilowatt-hours. This will tell you how many kilowatt-hours one unit area (square meter or square foot) of solar panel will generate in your location.

7. Enter the formula "=F1/D3" in cell F3. This is how many unit areas (square feet or square meters) of solar panels you need.

References

• Vermont Department of Public Service: Calculating Solar Energy Requirements (PDF)
• Solarpanelstore: Watts Per Square Foot of Solar Panel
• Solar4Power: Average Solar Hours By City

Resources

• Solar4Power: Map of North America with Solar Hours Overlay