- The formulaAnnual kWh ÷ 1,600 = system kW
- Sacramento peak sun~5.5 hours/day average
- Per kW output here~1,500–1,700 kWh/year
- Typical household7,000–14,000 kWh/year
Work it out yourself in ten minutes
You do not need an installer to get within a kilowatt or two of the right answer. Knowing your own approximate number before anyone quotes you is the single best protection against being oversold — and against being undersold, which happens too.
Step 1: find your annual kilowatt-hours
Your bill shows monthly usage in kWh. Twelve months of it is what you want, because Sacramento usage is intensely seasonal: a household running central air through a Valley July can use three times what it uses in April.
Both SMUD and PG&E let you download twelve months of usage data from your online account, and it takes about two minutes. Roseville Electric and Lodi Electric customers can request the same from their utility. If you only have one bill, multiply the monthly figure by twelve as a crude start, but get the real data before you sign anything.
A typical Sacramento household uses somewhere between 7,000 and 14,000 kWh a year. Large homes with pools, EVs or well pumps run well above that.
Step 2: divide by your local production factor
Sacramento averages roughly 5.5 peak sun hours per day. Real-world systems lose some of that to inverter efficiency, wiring, soiling, heat and shade — typically around 14–20% in total.
Putting those together, one kilowatt of installed capacity in the Sacramento area produces broadly 1,500–1,700 kWh a year on a decent unshaded roof.
The arithmetic
System size (kW) = annual kWh ÷ 1,600
So 12,000 kWh a year ÷ 1,600 = roughly a 7.5 kW system.
Step 3: convert to panels
Modern residential panels are typically 400–450 W each. A 7.5 kW system at 425 W per panel is about 18 panels. Each occupies roughly 21–22 square feet including spacing, so 18 panels needs somewhere in the region of 380–400 square feet of usable, unshaded roof.
| Annual usage | System size | Panels (~425 W) | Roof area needed |
|---|---|---|---|
| 6,000 kWh | 3.8 kW | 9 | ~200 sq ft |
| 9,000 kWh | 5.6 kW | 13 | ~290 sq ft |
| 12,000 kWh | 7.5 kW | 18 | ~390 sq ft |
| 16,000 kWh | 10 kW | 24 | ~520 sq ft |
| 20,000 kWh | 12.5 kW | 30 | ~650 sq ft |
Step 4: adjust for the things the arithmetic misses
Shade
Reduce the production factor. Significant shade can move a roof from 1,600 kWh per kW down to 1,200 or lower, which means you need proportionally more capacity for the same output. Only a measured shade study gives you the real figure.
Orientation
A west-facing array produces somewhat less total energy than a south-facing one, but produces it later in the day — which under a time-of-use rate can be worth more per kilowatt-hour than the raw total suggests.
Future load
This is the adjustment most people forget, and the most expensive to fix later. An electric vehicle adds roughly 3,000–4,000 kWh a year for typical mileage. A heat pump water heater, a pool pump, a heat pump replacing a gas furnace, or an ADU each add meaningfully. If any of those are plausible within five years, size for them now — adding capacity later costs far more per watt than including it at the start.
Do not oversize past your usage
The opposite error, and a real one. Once you generate more than you consume, each surplus kilowatt-hour is worth only whatever your utility credits exports at — which under most current structures is well below retail. Building an array substantially larger than your annual consumption is one of the commonest ways to pay for capacity that never pays back.
Step 5: sanity-check it independently
Run your address and proposed system size through NREL’s PVWatts calculator. It is free, it is from the National Renewable Energy Laboratory, and it is the honest way to check any installer’s production claim — including ours. If a quote shows production materially above what PVWatts gives for the same system and location, ask what assumptions produced the difference.
A note on “100% offset”
Many proposals lead with a percentage offset of your bill. Treat it carefully. Offsetting 100% of your consumption does not mean eliminating your bill: fixed charges, connection fees and the difference between what you are credited for exports and what you pay for imports all remain. A proposal promising a zero bill is describing something the tariff structure generally does not permit.
Written by the Sacramento Solar team for Sacramento-area homeowners. We install solar; we are not tax advisers, accountants or financial advisers. Figures are ranges current as of early 2026 and are offered as orientation, not as advice about your own situation. Verify current programme rules with your utility, the relevant agency or a qualified professional.