How Many Solar Panels Do I Need?

There are two main steps to take when trying to determine how many photovoltaic panels are needed. First, we need to calculate how much energy you consume, and then we need to understand the total amount of solar energy that is available at your location. Knowing these numbers will make it possible for you to calculate how many solar panels you will needed to power your home.

Step 1 - What are the energy needs of the house and how much of the energy do you want to provide from solar? 
The first step in determining the total number of solar panels need, is to determine your needs.  To do this, you need to know how many watt-hours (Wh) your house consumes in a month, and there are several ways to arrive at this number.  

The easiest way to get started is to look at your energy bills, which will list how much energy you have consumed.  It's important to keep in mind that one month of information isn't enough to gain a clear understanding of your energy use, since energy needs can vary greatly from month to month. You will want a full years worth of information to generate an accurate monthly average.

Another method is to determine the energy consumed by all of the appliances, lights, and electronics in your home.   Most electronic items and appliances list their wattage on their labels, and light bulbs are pretty easy to find the wattage listed (also note that Watts = Volts x Amps).  For instance, to generate the monthly usage for a light bulb, multiply it's wattage by how many hours the light will be on each day, and multiply by 30 to arrive at the total number of watt-hours needed each month for that item.  Do this for every device that consumes energy, add them all up, and you will have a total energy usage for your house in watt-hours.

Step 2 - How much sunshine will reach the solar panels?
A variety of factors will affect how much energy producing light will actually reach the solar panels.  Your city's latitude and weather will play a large role, as well as the orientation, placement, and tilt of the solar panel.

Sun hours, or insolation, is the total amount light that strikes your location over the course of the day. For instance, during the month of January when the sun is at a low angle in the sky, the photovoltaic panel won't be able to capture it's maximum capacity for much of the day. You can still gain electricity during these times, but it is greatly reduced from when the sun is high overhead. If the sun is shining for 10 hours, the total accumulation of light that hits the solar panel may only amount to three hours of full energy over the course of the day. This is referred to as sun hours, and it is a very important to factor in when calculating how many photovoltaic panels you will need. Luckily, there are easy to use tables and maps that give us these numbers, so it's as easy as looking up your geographic location and noting the average sun hours.

Insolation values can be found on the NASA website.  A user name and password is required, but once you create your account, you can find the insolation value for your specific location.

To arrive at the necessary amount of energy output needed to be created by the entire solar system, divide the amount of watt-hours that you need to produce (derived in step 1) by the insolation value (found in step 2). Because there are always inefficiencies, it's also necessary to include a safety factor. Multiplying by 1.3 (30%) should provide additional buffer to ensure that you are meeting your energy requirements. The resulting number is the total number of watts needing to be produced by your system. You can now find the photovoltaic panels that you would like to use, and know how many you need.

You can now size your system based on the ratings of the solar panels that you will be using.