How Many Solar Panels Do You Need? (Simple Sizing Method)

How Many Solar Panels Do You Need?

By TrimWatt Editorial Team — who we areNot yet independently reviewed — how we reviewPublished July 3, 2026Figures last verified July 22, 2026

8 min read

QUICK ANSWER

Divide your annual kWh use by your area's daily peak sun hours × 365 to get system size in kW, then divide by your panel wattage for the panel count. A typical U.S. home lands around 15–22 modern panels.

Sizing a solar system sounds technical, but the core method is three numbers: how much electricity you use, how much sun your roof gets, and how much power each panel makes.

The method

Find your annual electricity use in kWh (add up 12 months of bills).
Divide by daily peak sun hours × 365 to get system size in kW.
Divide the system size by your panel wattage to get the number of panels.

A worked example

If your home uses 10,800 kWh a year and you get about 4.5 peak sun hours a day: 10,800 ÷ (4.5 × 365) ≈ 6.6 kW. With 400W panels that's about 17 panels; add a buffer for real-world losses and you're looking at roughly 18–19.

Example 6.6 kW system (illustrative)

Upfront cost

~$19,100

installed, no federal credit in 2026

Annual savings

~$1,450/yr

at $0.17/kWh

Payback

~13.2 yrs

vs 25-yr panel life

Illustrative estimate — actual figures vary by home, local rates, and region.

What changes the number

Roof orientation and tilt — south-facing roofs need fewer panels for the same output.
Shading from trees or neighbouring buildings.
Panel efficiency — higher-wattage panels mean fewer of them.
Whether you offset 100% of usage or only part.
Future loads like an EV or a heat pump, which raise your target.

Size for tomorrow, not just today

If a heat pump or EV is on your horizon, size for that higher usage now. Adding panels later often costs more per watt.

What these numbers assume — and where they stop

Assumptions

Panel output of about 400W per modern residential module.
An installed cost near $2.90/W — a national ballpark, not a quote. No federal credit is applied: the 25D credit ended for expenditures after 31 December 2025.
Roughly 4.5 daily peak sun hours in the worked example; your region will differ.
Net metering credits exports at or near retail rate, which is not true everywhere.

Limitations

This is a sizing method, not a substitute for an installer's site assessment or an NREL PVWatts run for your address.
Shading, roof orientation, roof age, and local permitting costs can change the result significantly.
We have not installed or monitored a system ourselves; figures come from public modelling data.

Frequently asked questions

How many solar panels does an average house need?

Most U.S. homes land around 15–22 modern panels (roughly 6–9 kW), but the right number depends on your usage, roof orientation, and local sun hours.

Can I install fewer panels and still save?

Yes. Many homeowners offset part of their usage, which lowers upfront cost and still cuts the bill. Local net-metering rules affect how well partial systems pay off.

Do solar panels work on cloudy days?

Yes, at reduced output. Annual production is what matters for sizing, and regional sun-hour figures already account for typical cloud cover.