How to Calculate What Size Solar System You Need (2026)

Most Southern California homes need between 6 and 12 kW of solar to cover their annual electricity use. You can calculate your own number in three steps using nothing more than your last 12 utility bills and the production data for your roof's orientation.

Last verified: September 2026 by Helios Energy Global.

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The three-step formula

Step 1 — Pull your annual kWh from a real bill

Log into your utility account (SCE, LADWP, SDG&E, or your muni) and download 12 months of usage. Add up the total kilowatt-hours consumed. Do not average a single month — seasonal swings are large, and one summer month on SCE can be 3× a winter month.

The average California home uses about 503 kWh per month — roughly 42% less than the U.S. average of 863 kWh, thanks to the mild climate and decades of efficiency standards. That's roughly 6,000 kWh/year statewide, but the average Southern California home lands between 7,000 and 18,000 kWh/year depending on square footage, whether you have a pool or EV, and how aggressively you run air conditioning.

Where to find it on your SCE bill: Look for the "Usage History" section or the 12-month bar chart on the back of any paper bill. The number labeled "kWh used this period" is what you need — add all 12.

Step 2 — Divide by your production factor (and derate for your roof)

A production factor (also called specific yield) tells you how many kilowatt-hours each installed kilowatt of panels will produce in a year at your location. Inland desert areas of Southern California yield roughly 1,870 kWh/kW/year; the LA coast comes in around 1,572 kWh/kW/year , per physics-based pvlib modeling using Open-Meteo ERA5 historical weather data. For most Southern California planning purposes, 1,500–1,700 kWh/kW is a solid range.

Next, apply a derate factor for anything that reduces your roof's real-world output versus a perfect south-facing, unshaded installation:

Roof condition	Derate factor	Effect on system size
South-facing, 15–30° tilt, no shade	1.00	No adjustment needed
West- or east-facing (±45° of south)	0.85–0.95	Size up ~5–15%
Significant morning or afternoon shade	0.75–0.85	Size up ~15–25%
Flat roof (ballasted, adjustable tilt)	0.95–1.00	Minimal adjustment
Steep tile roof (>35°)	0.90–0.95	Slight reduction in production

Coastal areas in Southern California also experience June Gloom — marine layer that reduces June production by 8–15% compared to inland locations at the same latitude. If you're in Santa Monica, Malibu, or Manhattan Beach, use the lower end of the production factor range.

The formula in plain math: