Sizing Grid-Tied Solar – Or, What Makes a Kilowatt a Kilowatt
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We’ll often say that the only limiting factors of a grid-tied solar electric system’s size are available roof space and budget.
If you have a huge roof with great southern exposure, you can install enough solar to produce more electricity than you’ll ever need.
Most homeowners, however, choose to offset a portion (say 30-50%) of their overall use.
Here are a few steps on how we calculate what you use and how to size a system accordingly:
Find Out What You Use
One of the first things to do is look at your electric bills, which will give you can get a good handle on what your energy needs are each month.
You’re billed based on how many kilowatt hours (kWh) of electricity you use. A kilowatt hour is simply a measurement of energy – 1kWh = 1,000 watts * 1 hour.
Every appliance in your home has a wattage rating which, multiplied by the number of hours you use it per day, will give you an idea of how much it contributes to your electric bill.
Conveniently, most electric bills include several months of data so you can get a rough average of what you use over a longer period of time. It’s also helpful to compare winter vs. summer data as your needs might vary widely – for instance, if you use electric space heat your use might spike in the winter; on the other hand, if you use air conditioning your use might spike in the summer.
Once you have a basic handle on how much you consume, you can see how much impact a solar electric system will have on your bill.
How Grid Tied Solar Cuts Down Your Electric Bill
The beauty of a grid-tied solar electric system is that we use the electric grid as a battery.
When you produce more electricity than you consume, that energy goes out to the grid and is credited to your account. When you are using more electricity than you are producing, you simply use grid power as normal, and deduct accumulated credits off of your bill.
This means two things:
- You can produce more than you use in the summer and benefit from that overproduction during the dark winter months.
- You can install a system that provides less than 100% of your needs and still benefit from all of the electricity it generates.
A 1 kW solar electric system will produce 1,000 watts of electricity when in full sun. If you get an hour of full sun on that system, you’ll have generated 1 kWh.
In New England, we’ve found that each kilowatt of installed solar will generate roughly 1,300 kWh of electricity each year, varying a bit depending on where you are in Maine or New Hampshire.
By taking the amount of kWh you consume, and dividing them by 1,300, you can get a ballpark estimate of how many kilowatts of solar you would need to completely offset your electric usage.
Solar Energy at Work
Let’s look at an example based on the typical American home. The US Energy Information Agency estimates that the average home uses about 10,656kWh each year (Source: End-Use Consumption of Electricity 2001).
That means the average American home would require a 7.6kw solar electric system to completely offset their electric needs (10,656/1,400 = 8.2).
Realistically, most homeowners opt for a 3-5kw system to offset a portion, but not 100%, of their electric needs, and either work towards a second solar system sometime in the future or work towards reducing their load to match their solar production as closely as possible.
We’ve covered the economics of solar electricity before – by purchasing solar you’re basically locking in your electric cost at below market rates for the next 40-50 years. A great buy in an increasingly uncertain energy climate!