How to size a residential solar system
Sizing a home solar system isn't complicated once you start from the right number: your annual electricity use in kWh (on your bill, or twelve months of monthly kWh added up). Everything else follows.
The formula
System size (kW) = annual kWh ÷ (peak sun hours × 365 × derate)
- Annual kWh — from your utility bill. A typical US home is ~10,000–12,000 kWh/yr.
- Peak sun hours — the equivalent hours of full sun your location gets per day. Roughly 3.5–4.0 in the cloudy north, 4.5 average, 5.0–5.8 in the Southwest.
- Derate ≈ 0.80 — real-world losses (temperature, wiring, inverter, soiling, orientation).
Worked example: 12,000 kWh/yr ÷ (4.6 × 365 × 0.80) = 12,000 ÷ 1,343 ≈ 8.9 kW.
How many panels?
Divide system watts by panel watts: an 8.9 kW system with 430 W panels needs ⌈8,900 ÷ 430⌉ = 21 panels. Roof space check: each panel is ~18 sq ft, so ~380 sq ft of usable, unshaded roof.
Offset, and whether to oversize
The formula above targets ~100% offset. Reasons to go a bit bigger: an EV or heat pump on the horizon, a battery to charge, or a utility that pays little for exports (self-consumption matters more). Reasons not to: net-metering caps, roof space, and interconnection limits — many utilities cap system size at ~100–120% of historical use.
Batteries, briefly
Battery sizing is about backup, not offset. Size it to the loads you want to ride through an outage, times the hours you want them: a fridge, some lights, internet and a few outlets is ~3–5 kWh/day of essentials, so a 10–15 kWh battery covers a day-plus. Whole-home backup with AC and a well pump needs much more (and careful load management).
Important: this sizes the system, not the install
These numbers get you to a plan. The actual conductor sizing, string voltage, interconnection and permit set need real engineering, and a licensed electrician or PE must review and connect the system. Use this to walk in prepared.
FAQ
How many solar panels does a house need?
Most US homes need roughly 15 to 25 panels. Divide your target system size in watts by the panel wattage — an 8.9 kW system with 430 W panels needs about 21 panels.
How do I size solar from my electric bill?
Take your annual kWh use and divide by (peak sun hours × 365 × 0.80). Peak sun hours are about 4.0 in the north, 4.6 average and 5.0 to 5.8 in the Southwest.
Should I oversize my solar system?
Slightly, if you expect an EV, heat pump or battery, or your utility pays little for exports. But many utilities cap interconnection at about 100 to 120% of past use, so check before oversizing.
Related: Design & sizing
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.
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