How Many Solar Panels Do I Need?

Overview

The question is usually answered with a house size, which is the wrong starting point. Two 2,000 sq ft houses can differ by a factor of three in electricity use depending on heating, cooling, an EV and occupancy. The right chain is: annual kWh → target offset → required kW → panel count → roof area. This guide walks that chain with real 2026 state rates and shows where the roof becomes the binding constraint.

Step 1: get annual kWh

Take twelve months of bills and add the kWh. If you only have dollars, divide the annual spend by your average rate. At the August 2026 U.S. residential average of 18.16 ¢/kWh, a $150 monthly bill is about 9,912 kWh a year. The same $150 bill in Hawaii at 47.18 ¢ is about 3,816 kWh — less than half the energy, because the rate is 2.6 times higher. That single conversion explains most of the state-to-state difference in recommended system size.

Step 2: choose an offset

A 90% offset is a common starting point. Under full-retail net metering, going higher is often harmless. Under net billing, going higher usually means buying modules whose output is sold at 4–8 ¢, so a lower offset with a battery or load shifting can be the better design. Decide this before sizing, not after.

Step 3: divide by specific yield

Specific yield = peak sun hours × 365 × system derate × roof factor. At 4.8 peak sun hours, a 0.84 derate and a perfect south roof, that is about 1,471 kWh per kW per year. A 9,912 kWh household at 90% offset needs about 6.06 kW. In Arizona's 6.3-hour band the same household needs about 4.6 kW; in the Pacific Northwest's 4.0-hour band, about 7.3 kW.

Step 4: convert to panels and area

At 450 W per module, 6.06 kW is about 13–14 panels. At 55 sq ft per kW, it needs roughly 333 sq ft of clear, unshaded, code-compliant roof. If you only have 250 sq ft, the cap is 4.55 kW and the offset you can actually reach is lower. The roof, not the bill, becomes the design constraint.

Key figures worth keeping

• 9,912 kWh — the annual usage implied by a $150 monthly bill at 18.16 cents/kWh.
• 1,471 kWh per kW per year — specific yield at 4.8 peak sun hours, a 0.84 derate and a south roof.
• 6.06 kW — the 90%-offset size at those national-average defaults.
• 13–14 — 450 W modules needed for that array.
• 333 sq ft — usable roof area at 55 sq ft per kW.

How this shows up in a real estimate

Two households with the same monthly bill can need very different arrays because the bill is a dollar amount and the array is sized in energy. At Hawaii's 47.18 cents per kWh, a $150 bill is about 3,816 kWh a year; at Louisiana's 12.87 cents it is about 13,988 kWh. Same spend, nearly four times the energy. This is the most common reason an online 'average system size' is useless for an individual house. Start from kWh, then let the state data set the rate and the resource band.

Checklist before you decide

• Add twelve months of kWh, not twelve months of dollars alone.
• Confirm the module wattage on the quote before counting panels.
• Subtract setbacks and access pathways from usable roof area.
• Decide the offset target before sizing, especially under net billing.
• Use PVWatts for an address-level production check.