How much do I need?

The best way to determine your home's maximum solar array size is to call our office and have us determine the best size by looking at your home's electric usage data. You can get a rough idea of what size solar you can install by using the formula below. Keep in mind, the formula just gives you a "back of the napkin" estimate of your best solar installation size.

Maximum solar size estimate = minimum kWh divided by 720 hours divided by load factorIn this formula, Minimum kilowatt-hours (kWh) is your home's lowest monthly kWh usage from the previous 12 months. This month is frequently April, May, September or October, when the weather is nice and HVAC systems run the least. Divide this number by 720 (which is the number of hours in a month). Then divide this number by your home's load factor. For most homes, the load factor is .2 but for businesses with more consistent power use the load factor will be .5 or even .8 if a large electrical load is turned on and runs most of the time.

Please call our office for an examination of your electrical use, and a more accurate calculation of your ideal solar array size.

For members who install renewable generation significantly more than they expect to use during a typical month, an alternate parallel generation rider can be chosen which pays members for their power generated at a rate of 150% of the cooperative's monthly system average cost of energy.

In the "Properly sized vs oversized solar" graph shown here, we show a home's monthly kWh consumption (blue bars), along with the monthly output of two possible solar arrays. The 6kW solar installation (orange bars) is properly sized. The 10kW array (gray bars) is too big, and overproduces more electricity than the home needs on seven months of the year. Under our net metering policy, Heartland does not purchase this excess generation.

Graph showing comparison between monthly kilowatt-hour usage, a 6 kilowatt solar array, and a 10 kilowatt solar array
An example of 2 alternate solar arrays, one sized properly and one too big.