Estimating solar production in kWh

You do not need modeling software for a solid first estimate of solar production. With your system size, local sun data, and a realistic loss factor, you can get within range of what an array will actually make. Here is the method and what the numbers mean.

The core formula

Annual kWh ≈ system kW × peak sun hours/day × 365 × derate

Every term is something you can look up or reasonably estimate. Work through each one and the estimate falls out.

Step 1: System size in kW

This is the array's DC nameplate — module wattage times the number of modules, divided by 1,000. Twenty 400-watt panels is 8,000 W, or 8 kW.

Step 2: Peak sun hours

Peak sun hours describe how much usable sunlight your location gets per day, averaged over the year. A place with 4.5 peak sun hours receives the daily energy equivalent of 4.5 hours of full noon sun. This value depends on latitude, climate, and your array's tilt and azimuth. National solar resource maps publish typical figures by region.

Step 3: The derate factor

Nameplate assumes ideal lab conditions your roof never sees. The derate rolls all the losses into one number:

Stacked together, these commonly land the whole-system derate near 0.80. A clean, cool, unshaded site trends higher; a hot, dusty, or shaded one trends lower.

A worked example

An 8 kW array at 4.5 peak sun hours with a 0.80 derate:

8 × 4.5 × 365 × 0.80 ≈ 10,500 kWh/year

That is roughly 875 kWh a month on average — higher in summer, lower in winter.

Turning production into offset

Once you have annual kWh, compare it to your yearly usage to get your system offset. That single ratio tells you how much of your bill the array is designed to cover and whether to size up or down.

Monthly and seasonal spread

The annual figure hides big swings. A summer month can produce well above the yearly average while a short, cloudy winter month falls well below it, driven by both shorter days and lower sun angles. If you are trying to zero out a specific season's bill — not just the year as a whole — look at monthly estimates, which resource maps and modeling tools also provide. This matters most where net metering does not roll surplus credit across seasons, or where a rate charges more in the months your array produces least. For a simple annual-offset goal, the single yearly number is enough; for cash-flow planning, the monthly shape is worth a look.

Keep the estimate honest

This method is a planning tool, not a guarantee. Weather varies year to year, and detailed modeling that accounts for hourly shade and orientation will refine it. Use the estimate to size the system and set expectations, then confirm equipment and interconnection details with your AHJ and utility. A licensed electrician or PE must review and stamp the final design.

Educational reference, reviewed 2026-07. A design aid, not a substitute for a licensed electrician or PE. Confirm the enforced NEC edition and local amendments with your AHJ.