What size solar system do I need?
"What size do I need?" has a real, repeatable answer — you don't have to guess or take a salesperson's word. It comes from your own electricity use plus your local sunshine. Here's the homeowner walkthrough, step by step.
Step 1: Find your annual kWh
Pull twelve months of kWh off your electric bill and add them up (or use the annual total). This is your energy appetite, and it drives the whole calculation. A typical US home uses 10,000 to 12,000 kWh per year. New to your bill? See how to read your electric bill for solar.
Step 2: Apply the sizing formula
System size (kW) = annual kWh ÷ (peak sun hours × 365 × 0.80)
- Peak sun hours — equivalent hours of full sun per day: about 3.5 to 4.0 in the cloudy north, 4.5 average, 5.0 to 5.8 in the Southwest.
- 0.80 — a real-world derate for heat, wiring, and inverter losses.
Worked example: 11,000 kWh ÷ (4.5 × 365 × 0.80) = 11,000 ÷ 1,314 ≈ 8.4 kW.
Step 3: Convert to panels
Divide system watts by panel watts. At 430 W: 8,400 ÷ 430 = 20 panels, needing roughly 360 sq ft of usable roof. More on the count in how many panels to power a house.
Step 4: Adjust for your goals
The formula targets about 100% offset. Tweak from there:
| Situation | Adjust size |
|---|---|
| EV or heat pump coming | Size up |
| Adding a battery | Size up slightly |
| Low export credit | Match daytime use, don't overbuild |
| Limited roof space | Higher-wattage panels or a smaller target |
Step 5: Check the ceilings
Two things can cap your size no matter what the math says:
- Utility interconnection limits — many cap systems near 100 to 120% of your historical usage.
- Your electrical panel — busbar capacity and the 120% rule may limit the inverter you can connect. See solar and your electrical panel.
From size to a real system
This gets you a well-grounded target to shop and budget with. But it's a plan — the final conductor sizing, string design, and interconnection are engineering work, and a licensed electrician or PE must review and connect the system. Nail the size first, and every later step gets easier.
FAQ
What size solar system does an average home need?
Most US homes land between 6 and 11 kW. The exact size depends on your annual kWh use, your local sun hours, and how much of your bill you want to offset.
How do I calculate the solar system size I need?
Divide your annual kWh by (peak sun hours times 365 times 0.80). That gives your system size in kilowatts, which you then convert to a panel count.
Is a bigger solar system always better?
No. Many utilities cap system size near 100 to 120 percent of your past usage, and extra panels you can't use offset little, so match size to your real needs.
Related: For homeowners & DIY
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.
Draft this design in minutes — free
Describe a residential solar + storage system in plain English. OneLine Studio drafts the one-line diagram, sizes the conductors, runs these NEC checks live, and builds a permit-ready package.
Start free →