How To Estimate How Many Solar Panels You Need To Power Your Home

Thinking of installing solar panels? The first step is to estimate how many solar electric (or photovoltaic, PV) panels you will need to generate the electricity you need.

Step 1 – How Much Electricity Do You Use?

You will need to know how much energy your household currently consumes or plans to consume.

One way to do this is to take an inventory of the wattage requirements of all of your electrical equipment and fittings. Multiply the total wattage requirement by the total number of hours you use your electrical equipment ..

A much simpler alternative is to use copies of your electric bills for the past 12 months.

Add up the number of kilowatts used each month, or simply use the most recent cumulative year end total. If you do not have copies of your bills, your utility company will typically provide you with the figures, but may charge you ..

Divide total kilowatts used during the year by 365 to get average daily kilowatts used.

Step 2 – Convert Kilowatts To Watts

Then to convert kilowatts into watts, divide the daily kilowatts used by 1,000. Now you have the average number of watts your household uses daily.

Step 3 – How Much Electricity Do You Want From Solar?

Multiply the daily watts used by your household by the percentage of total electricity that you want to replace with solar.

So, if you want to replace 25% of your electrical needs with solar, then multiply by 0.25. If you want to be able to supply 100%, so that you use grid electricity only for backup, then multiply by 1.00. This number is the average daily total watts of electricity to want your solar energy system to deliver.

Step 4 – Adjust For Solar Insolation To Your Site

Divide by the solar insolation value. You will need the to find the "solar insolation values" for your geographical location.

The "solar insolation value" is a measure of the amount of sunlight that actually reaches your site. The angles of the Sun's rays hitting Seattle at noon are different from their angles hitting San Diego at noon. Correspondingly, the Sun divers different amounts of solar energy at each location.

To find insolation values ​​for the United States, use the maps by the National Renewable Energy Laboratory.Look for the PV Solar Radiation Static Maps that show monthly averages. Using a yearly average will give you incorrect values ​​for the summer and winter extreme temperatures.) For other countries, search for "solar maps", "insolation maps", or "solar radiation maps" + the country's name.

Use the minimum monthly insolation value – December's for the United States. Your estimate will then be based on the least amount of the Sun's energy available at your location.

Now divide the average daily total watts of electricity you want your solar energy system to deliver (the number from step 1), by the solar insolation value for your site.

This gives you a preliminary estimate of the number of watts of electricity your solar panels will need to generate.

Step 5 – Adjust For System Inefficiencies

Now multiply the number of watts from the last step (step 2) by 120%, or (1.2). This will give you a 20% allowance for power loss which will occur as the electricity from your solar panels makes its way through the system. No system is 100% efficient. This number is your final estimate of the total amount of energy in watts that your solar electric system will need to deliver.

Step 6 – Adjust For Your Solar Panel Specs

Now divide your final wattage requirements (from step 3) by the peak wattage of the solar panel you plan to buy.

You can get this number form the solar panel manufacturer. If you are shopping online, look for a spec labeled "maximum power" or something similar, and expressed in watts.

Step 7 – Round Up For Total

Round the result to the next highest number (for example, round 5.3 to 6 solar panels) to get the approximate number of solar panels you will need to buy.