How much energy will my solar array produce and what will that do to my electric bill?

This is one of the most common questions that we hear and often times there are misconceptions about just how much energy your residential solar system will produce. A good place to start for any home or business owner is with your electric bill from your utility provider. In most cases there is a bar graph that shows your yearly electric usage in units of  kilowatt-hrs(kwh)/day.

In January, for example, this home used on average 144 kwh/day. Multiplying this number by 31 days and we can calculate that 4464 kwh were used in the month of January. Going month by month and adding up all of the kwh usage, you can calculate the usage for the entire year. For this particular graph the yearly usage is about 36,840 kwh.

What exactly is a kwh you may ask?

Think of a standard 60W incandescent light bulb. 60W is the power requirement for this bulb. If you leave this light bulb on for one hour this bulb has used 60 watt-hours of energy. If the light is on for 10 hours the bulb has used 600 watt-hours of energy. Dividing watt-hours by one thousand gives you kilowatt-hours. So, leaving a 60W light bulb on for ten hours uses 0.6 kwh of electricity.

How many kwh will my solar energy system produce?

Now that we know how to determine our yearly kwh usage from one electric bill statement, the next step is to understand how many kwh a solar electric system will produce. Solar energy system production is based on many factors including system size, shading, roof pitch, and roof orientation. The ideal roof pitch and orientation for a system in Portland, OR is 32 degrees at 190 degrees.

As a general rule of thumb, you can assume that for every 1kW of solar energy your system in Portland will produce 1000 kwh/year of electricity.

For example, if you install a 3kW solar electric system it will produce approximately 3000 kwh/year of electricity. If you calculate from your electric bill that your yearly usage is 10000kwh then you know your system will offset 30% of your yearly bill.