How do you size a solar system?

    I am working on sizing a solar power system for a residential customer.  How do you calculate how many panels for a system?  How do you determine how large of a system to install given their electrical load?  What portion of a customers' electrical bill is typically covered with a given solar electrical system?


     In order to accurately size a solar system you need to first gather a few  pieces of key information from the home owner:

    1. How much energy does the home use on a daily, what is the electrical load?
    2. What is the solar resource/insolation? How much sun and what is the strength of that sun in your location?
    3. What portion of the home’s energy do you want your solar power system to provide (ie. 50%, 75%, 100%)

    How much energy is used in the home on a daily basis?

    The best way to determine this is by analyzing the electric bill and finding the total kWh consumed at the home on a monthly basis.  The bill will show daily usage and you just need to total it for the month and the year.    

    The EIA estimates that the average American family uses 936 kWh per month or 11,232 kWh/year.

    In California, a home energy audit is required to receive rebates.  Here are the links for the CA major utility's free audit resources.  You need to have a customer ID to log-in to these sites but they provide a really good picture of the power usage for a given home.   Ask your customer to pull this or pull it for them.   Other states have similar free services provided by your friends at your local utility.

    Here is a sample report from Southern California Edison. 

    SCE        PG&E       SDG&E

    You can also advise you customer to figure out your home’s energy use with a whole house energy monitor, like TED. (The Energy Detective.)

    Energy Monitor

    A whole-house monitor hooks into your home’s meter and will give them an accurate reading of how much energy their home is using every day. These meters are great because they will provide them with real-time information in a slick format.  If your customer is tech savvy, they will love this.  For example if you run a hair dryer they can see exactly how much it’s costing them.  

    What is the solar resource in your area? How many sun hours and what is the strength of the sun that hits the house.

    Insolation, or sunlight intensity is measured in equivalent full sun hours. One hour of maximum, or 100% sunshine received by a solar panel equals one equivalent full sun hour. Even though the sun may be above the horizon for 14 hours a day, this may only result in six hours of equivalent full sun. There are two main reasons. One is reflection due to a high angle of the sun in relationship to your solar array. The second is also due to the high angle and the amount of the earth's atmosphere the light is passing through. When the sun is straight overhead the light is passing through the least amount of atmosphere. Early or late in the day the sunlight is passing through much more of the atmosphere due to its position in the sky.

    The National Renewable Energy Laboratory (NREL) created a tool called PV Watts  to permit professional installers and non-experts to quickly obtain performance estimates for grid-connected PV systems.  The tool takes a few runs to get familiar with, but once you figure it will be critical for sizing your systems.  PV Watts

    In California the CEC created an EPBB calculator tool to determine the output of system based on the solar resource, equipment, direction of your panels and your tilt.    It is required that you run this tool in order to receive your rebate.

    A really simple but rule of thumb formula to determine the output of a system you are planning is:

    (PV array wattage) x (Ave. Hours of Sun) x 75% = Daily watt-hours   

    The fudge factor of 75 percent takes into account the real-world effects on a system.   It certainly can vary, but not much more than 5-7%.

    What portion of the home’s energy do you want your solar power system to provide (ie. 50%, 75%, 100%)

    In a grid-tied system vs. a off grid-system you don't need to cover 100% of the home's electricity load, in addition you will not produce power when the grid is down.     Based on your solar resource, your space limitations and your financing you can decide what percentage of the electrical load you want to cover for your clients.   In select states, you can sell excess electricity through a net metering program to your utility, but it is not available in all states. 

    12 years 3 months ago
    Asked by
    Jon Steel
    Support topics
    System sizing