Solar Panel Size Calculator

What this estimate does

Sizing is where a solar proposal can quietly go wrong. A system that is too small leaves you buying expensive evening power; one that is too large pushes midday surplus onto an export tariff worth a few cents. This calculator starts from energy, not from a target spend: it converts your bill into annual kWh, applies a state-level solar resource band, derates for inverter, wiring, soiling and temperature losses, then adjusts for orientation, pitch and shade before returning the array size in kW, a 450 W panel count and the roof area required. Because 2026 no longer offers a homeowner federal credit, the size that makes financial sense is often smaller than the size a salesperson would have proposed in 2024 — especially in net-billing states.

How this is calculated

The governing relation is kW = annual kWh × target offset ÷ (peak sun hours × 365 × derate × roof factor). The roof factor is the product of an orientation multiplier (south 1.00, east/west 0.90, north 0.78), a tilt multiplier and a shading multiplier. We use 55 sq ft of usable roof per kW, which is typical for modern modules at roughly 20% efficiency. If you enter a usable roof area, the array is capped at area ÷ 55 and the shortfall is reported. Production is then the capped size multiplied back through the same specific yield, so a constrained roof can never produce more than its area allows.

A worked example

A Texas household with a $150 monthly bill at 15.91 ¢/kWh uses about 11,313 kWh a year. With the Southeast band's 5.2 peak sun hours, a 0.84 derate and a south-facing 20° roof, the specific yield is about 1,594 kWh per kW per year. A 90% offset therefore needs roughly 6.39 kW — about fourteen 450 W panels and 351 sq ft of usable roof. In Arizona, with 6.3 peak sun hours and a similar bill, the same offset needs closer to 5.2 kW. Same house, same bill, noticeably different hardware.

What this tool does not do

It cannot see your roof planes, setbacks, fire-code pathways, obstructions or structural capacity. It does not perform an hourly simulation, so it cannot capture time-of-use self-consumption or battery dispatch. For an address-level production estimate use NREL PVWatts; for a structural answer use a licensed contractor or engineer.

Practical checks before you compare quotes

• Use twelve months of bills, not one summer month.
• Measure only roof planes that are unshaded between roughly 9 am and 3 pm.
• Ask for the module wattage on the quote — panel count alone means nothing.
• Check whether your utility caps system size for net-metering eligibility.

Common mistakes

• Sizing from home square footage instead of kWh.
• Counting shaded or north-facing roof as usable area.
• Forgetting that a battery does not create energy — it shifts it.
• Assuming production never declines; we model 0.5% degradation per year.