Sizing

How Many Solar Panels Do I Need?

By Shehzad Hussain

How many solar panels do I need for my house? Enter your monthly usage and get your panel count, system size and roof area in seconds.

Your electricity use

Monthly electricity use (kWh)

Your roof & system

Peak sun hours per day

System losses (derate, 0–1)

Panel wattage (W)

Estimates only. The panel count always rounds up so the system is never under-sized, and it assumes about 18 sq ft of roof per panel including spacing. Real panel counts depend on your roof, shading and panel choice — get a quote from a licensed local installer before deciding.

How the panel count is calculated

Daily use ÷ sun hours ÷ derate, then ÷ panel wattage. The calculator takes your monthly kWh, divides by 30 for daily use, divides by your peak sun hours and by a 0.8 system derate, then divides by your panel wattage and rounds up. A home using 900 kWh a month at 4.5 peak sun hours with 400W panels needs 21 panels — an 8.4 kW system.

Panel wattage matters: the same home needs 24 panels at 350W but only 19 at 450W. Higher-wattage panels help on small roofs, where every square foot counts. Roof area is about 18 sq ft per panel including spacing — 21 panels need about 378 sq ft.

Bedrooms do not decide the number — usage does. A typical 3-bedroom US home uses 700 to 1,000 kWh a month, which usually means 16 to 24 panels at 400W. Your bill is the only figure that matters.

Want the full sizing, cost and payback? Use the solar panel calculator, or check how much your panels will produce.

How to use the How Many Solar Panels Do I Need?

Enter your monthly electricity usage in kWh — it is on your bill (the average US home uses about 880 kWh a month).
Set your peak sun hours (4.5 is a typical US average).
Choose your panel wattage — 400W is standard for new panels in 2026.
Read off your panel count, system size in kW, and roof area.

Frequently asked questions

How many solar panels do I need for a 900 kWh a month home?

About 21 panels at 400W — an 8.4 kW system covering about 378 sq ft of roof — assuming 4.5 peak sun hours and a 0.8 system derate. A home using half as much power needs about half as many panels.

How many solar panels do I need for a 3-bedroom house?

It depends on usage, not bedrooms. A typical 3-bedroom US home uses 700 to 1,000 kWh a month, which usually means 16 to 24 panels at 400W. Check your bill for your real number — it is the only figure that matters.

What size solar panel is best for a home?

Most new residential panels are 400W to 450W. Higher-wattage panels mean fewer panels for the same system size, which helps on small roofs. The calculator lets you compare 350W, 400W, 450W and 550W panels.

Do I need more panels if my roof faces east or west?

Yes, usually — an east- or west-facing roof produces less per panel than a south-facing roof, so you may need 10% to 20% more panels for the same yearly output. Shade matters even more: a shaded panel can drag down a whole string.

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How the panel count is calculated

How to use the How Many Solar Panels Do I Need?

Frequently asked questions

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