In order to determine the best size for your solar electric system, you must first decide how much electricity you want it to generate. For example, you may want a system to meet 50% of your household need. You can work with the installation contractor and examine past electric bills to determine your household electricity requirements and the system size necessary to achieve that goal. Your utility company can also supply a 12-month report of your household’s total electricity consumption, measured in kilowatt-hours.

Ask your contractor how many kilowatt-hours the system will produce annually. Compare that number to your annual electricity usage to estimate the potential savings on your electric bills each year. (More tips on estimating savings can be found in the next section.)

Installation costs may also impact your cost per kilowatt-hour. For example, the labor costs to install a larger system may be almost as much as for a smaller one. Your installation contractor is likely to offer a better price to install a two-kilowatt system all at once, rather than installing a one-kilowatt system this year, and another similar-sized system next year.

In addition, inverters that convert DC current to AC current are sized for systems in certain increments, from one-to-five kilowatts. If your system is in the middle of this range (i.e., three kilowatts), purchasing a larger inverter (such as a five-kilowatt inverter) than your system requires would increase your overall system cost. It’s more cost-effective to match the inverter to your system size, unless you intend to increase the size of your system at a later date.

The cost of operating your electric appliances

To help you assess your electricity costs, below are examples of monthly kilowatt-hour consumption levels and the associated costs for several common home appliances. Your actual use may differ; older appliances tend to be more expensive to operate and less efficient than newer appliances. Reducing your overall electricity usage can significantly lower your electricity bills and, in turn, reduce the size of the solar electric system required to meet your household energy needs.

*Twenty of these light bulbs would use about 300 kilowatt-hours per month — or nearly half of an average home’s monthly electricity usage.