How many solar panels do I need?

Divide your yearly kWh by about 1,650, then divide that by your panel size in kilowatts. That's your panel count. A Southern California home using 800 kWh a month comes out to about 14 panels of 430 watts. The 1,650 is how many kWh one kilowatt of panels makes in a year on a good roof here, and the rest of this guide shows where that number comes from, when to adjust it, and why you shouldn't go bigger than you need.

Step 1: find your yearly kWh

Your bill shows kWh, the unit everything else is built on. Use a full 12 months, not one bill. In Southern California, an August bill can be double an April one, and sizing off either gets it wrong. Most SCE, SDG&E and LADWP bills include a small 12-month usage chart, and your online account has the full history.

For a sense of scale, the utilities' own typical-bill examples use 500 kWh a month for SCE (SCE) and 400 kWh a month for SDG&E (SDG&E). Homes with central AC, a pool or an EV often use a lot more.

If you only have dollars, divide each month's energy charges by your average rate to get close. SCE averaged 34.4¢ per kWh in June 2026, so a $279 SCE bill is roughly 810 kWh. This is rough, because fixed charges and time-of-use prices blur it, so use the kWh line when you can.

Step 2: how much one kilowatt of panels makes here

This is where location matters. We ran NLR's PVWatts calculator, the federal solar model formerly run by NREL, for a 1 kW system on a 20° roof with standard panels and 14% system losses (PVWatts, model inputs). Facing south, it estimates 1,677 kWh a year in Los Angeles, 1,694 in Riverside and 1,675 in San Diego.

We use 1,650 as a round, slightly cautious number for Southern California. It leaves a little room for roofs that aren't perfectly aimed. Of the three cities, inland Riverside came out a little ahead, and a shaded roof will do worse than any of them. A real design uses your exact roof.

West is worth a second look. It makes less in a year, but more of it lands in late afternoon, closer to the 4 to 9 p.m. window when SCE and SDG&E power costs the most. With a battery doing the evening work, south usually wins. Without one, a west-facing plane can be a sensible choice. Ask your installer to model both.

Step 3: divide by the panel size

Panels are rated in watts. We use 430 watts, the rating of common current models such as the Qcells Q.TRON BLK M-G2+ 430 (spec sheet). Divide your system size in watts by the panel rating and round up. A higher-wattage panel means fewer panels for the same system, and a lower one means more. The system size, in kW, is the number that matters. The panel count follows from it.

Why bigger isn't better under NEM 3.0

Under the old rules, extra panels earned close to full retail credit, so oversizing a bit was cheap insurance. That changed. New SCE and SDG&E solar homes are on the Net Billing Tariff, often called NEM 3.0, where exported power earns an average of roughly 4.6¢ per kWh on SCE and 3.7¢ on SDG&E (installer estimates). Every panel beyond what you use earns a few cents per kWh while costing the same as the others.

LADWP isn't on NEM 3.0, but it has its own reason to size to use: extra credit carries forward from bill to bill and is set to zero if you close the account, and its guidelines describe no cash payout (LADWP). Our NEM 3.0 guide covers the billing details, and LADWP solar covers the city's rules.

The practical target is to cover most or all of your yearly use, not to fill the roof. On SCE and SDG&E, the money is in using your own solar, either right away or later from a battery. A battery doesn't change the yearly kWh you need much, but the panels do need enough midday surplus to fill it, so the two get sized together.

Will it fit on your roof?

A 430-watt panel measures 1,722 by 1,134 millimeters, about 68 by 45 inches, or roughly 21 square feet (spec sheet). Fourteen panels need about 300 square feet of open, usable roof.

Usable is the key word. California's residential code requires clear 36-inch pathways from the roof edge to the ridge for firefighters, plus setbacks along the ridge (CALSSA summary of CRC R324.6). Vents, skylights, chimneys and shade from trees or a neighbor's second story eat more space. A roof that looks big from the street can hold fewer panels than you'd think, especially a hip roof with lots of small planes.

If the roof can't fit the full count, a smaller system can still make sense, since it covers part of your most expensive use. Higher-wattage panels can squeeze a bit more onto the same space.

Shade, the quiet panel thief

Shade does more damage than its size suggests, especially in the afternoon. A tree that shades part of the array from 3 p.m. on cuts output right when it's worth the most. Ask the installer for a shade report for your roof and check that the production estimate uses it. The CPUC's consumer guide covers what a solar sales packet should include (CPUC).

Plan for the next few years, not last year

Your past 12 months are the starting point, not the finish line. If an EV or a heat pump is coming, add it now. A Kansas Legislative Research memo using model-year 2023 EPA figures found all-electric vehicles ranged from 24 to 69 kWh per 100 miles, averaging 37.5 (KLRD). At about 35 kWh per 100 miles, an EV driven 10,000 miles a year adds about 3,500 kWh.

In the worked example, that takes the home from 9,600 to 13,100 kWh a year. Divided by 1,650 that's 7.9 kW, or 19 panels instead of 14. A heat pump replacing a gas furnace or water heater adds kWh too, and your installer can estimate it from your home's size and current gas use.

Going the other way, if you're about to cut use with new windows, insulation or a more efficient AC, size for the home you'll have afterward. Our guide to why electric bills are so high lists the free fixes worth doing first.

Questions to ask the installer about size

• What yearly kWh did you design to, and did you use my 12 months of interval data?
• What production per kW does your estimate assume for my roof, and with what shade loss?
• What share of my yearly use will this system cover?
• If I add an EV next year, does the inverter have room for more panels?

To try your own numbers, the solar calculator does the steps above in one go.