The short version
- Size follows annual kWh consumption, not roof area and not house size.
- A modern residential panel is around 400–450 W, so most homes need somewhere between 12 and 30 of them.
- Sun hours vary roughly two-fold across the US — the same array produces far more in Arizona than in Maine.
- Sizing much beyond 100% of your usage usually pays poorly, because exports are credited below retail.
Start with kilowatt-hours, not dollars
Your bill shows both a dollar amount and a consumption figure in kilowatt-hours. The kWh number is the one that sizes a system, because dollar amounts move with rate changes and seasonal tariffs.
Add up twelve consecutive months. A typical US household lands somewhere around 10,000–12,000 kWh a year, but the spread is enormous — a small efficient home might use 5,000 and an all-electric house with two EVs might use 25,000. Use your own number, not the average.
Divide by your local sun hours
"Peak sun hours" is the industry shorthand for how many hours per day your location delivers full-strength sunlight equivalent. It bundles latitude, climate and typical cloud cover into one number.
| Region | Approx. peak sun hours/day | Annual kWh per 1 kW installed |
|---|---|---|
| Southwest (AZ, NV, NM) | 5.5 – 6.5 | ~1,600 – 1,900 |
| California, Texas, Florida | 4.5 – 5.5 | ~1,400 – 1,700 |
| Mid-Atlantic, Midwest | 4.0 – 4.5 | ~1,200 – 1,400 |
| Pacific Northwest, New England | 3.5 – 4.0 | ~1,000 – 1,250 |
Work out the system size
The arithmetic, using a home in Texas that uses 12,000 kWh a year:
- Annual usage ÷ annual yield per kW12,000 ÷ 1,500 = 8 kW of capacity to cover roughly 100% of usage.
- Capacity ÷ panel wattage8,000 W ÷ 400 W = 20 panels.
- Check it fitsAt roughly 18 sq ft per panel, 20 panels need about 360 sq ft of unshaded, well-oriented roof.
- Adjust for realityShade, a north-facing plane or an awkward roof shape can push the panel count up or the offset target down.
That is the same calculation a designer runs, with better shading data. If an installer's proposal is wildly different from your own estimate, ask them to explain the gap — there is often a good reason, and occasionally there is not.
Why 100% offset is usually the ceiling
It is tempting to oversize and "never pay for power again." That rarely pays, for one reason: the power you export is almost never credited at the same rate as the power you avoid buying. Under most modern tariffs, self-consumed energy is worth full retail while exports are worth noticeably less.
So the last few panels on an oversized array earn at the lower export rate, stretching their payback well beyond the rest of the system. Many utilities also cap system size relative to historical consumption, making the question moot.
If you expect consumption to rise — an EV, a heat pump, air conditioning you have been avoiding using — size for the future load rather than oversizing against today's. And read how net metering works before you decide, because your utility's export rate is the whole argument.
What shade actually does
Shade is not proportional. Depending on the system architecture, a shadow across part of one panel can drag down a whole string, not just that panel. Microinverters and power optimisers exist to contain that damage, which is why partially shaded roofs are often quoted with them.
Morning shade matters less than midday shade. A tree that clips the array until 9am costs you relatively little; one that covers it from noon to 3pm is a genuine problem. A proper site survey measures this rather than eyeballing it.