Most Las Vegas homes need a 6–9 kW solar system; most California coastal homes need 4–7 kW; most California inland homes need 6–10 kW. The right size for your specific home depends on three numbers: your annual kWh usage, your local peak sun hours, and a realistic derate factor. Multiply, divide, done. Below is the same sizing math I walk every client through on the first call.
The One Equation That Actually Sizes a System
The basic formula:
System size (kW DC) = Annual kWh usage ÷ (Peak sun hours/day × 365 × Derate factor)
That's it. Everything else is plugging in numbers honestly. The derate factor accounts for heat, soiling, wiring loss, inverter loss, panel mismatch, and age. For Las Vegas I use 0.78. For California coastal I use 0.82. For California inland (Bakersfield, Fresno, Sacramento Valley) I use 0.79.
Step 1: Find Your Annual kWh Usage
Pull 12 months of utility bills. Add up the kWh used. Don't use the dollar amount — rates change. Don't use one summer bill — that's misleading. The full 12-month total is the only number that matters.
Typical annual usage by household:
| Profile | Annual kWh |
|---|---|
| Small home, no pool, gas heat | 6,000–8,500 |
| Average Las Vegas / Henderson home | 10,000–13,500 |
| Larger home with pool | 14,000–18,000 |
| Home with pool + EV + electric heat | 18,000–25,000+ |
If you're planning to add an EV, a pool, or to electrify gas appliances, size for the future load — not today's bill. I've watched too many homeowners install a 6 kW system, then buy a Tesla six months later and realize they're shy. See my Las Vegas solar overview for typical local usage patterns.
Step 2: Look Up Peak Sun Hours for Your Location
Peak sun hours (PSH) is the number of hours per day equivalent to 1,000 W/m² of solar irradiance. NREL publishes this data at nrel.gov/gis/solar-resource-maps.html.
| Location | Annual avg peak sun hours/day |
|---|---|
| Las Vegas, NV | 6.4 |
| Henderson, NV | 6.4 |
| Reno, NV | 5.7 |
| Bakersfield, CA | 5.9 |
| Sacramento, CA | 5.5 |
| San Diego, CA | 5.4 |
| San Francisco, CA | 4.7 |
| Eureka, CA | 4.0 |
Step 3: Apply a Realistic Derate
The "system loss" or derate factor is where most installers cheat. They'll plug in 12% loss to make a smaller, cheaper system look like enough to cover your bill. Reality:
- Las Vegas / Henderson / Phoenix: 18–22% loss → derate 0.78–0.82
- California inland: 17–20% loss → derate 0.80–0.83
- California coastal: 15–18% loss → derate 0.82–0.85
If you see a quote modeled at 8% loss for a Las Vegas roof, ask them to re-run it at 18%. The system that looked perfect on paper now produces 11% less than promised.
Worked Example: Henderson Home, 12,000 kWh/year
Plug it in: 12,000 ÷ (6.4 × 365 × 0.78) = 6.6 kW DC.
So a 6.6 kW system covers this home at exactly 100% offset. I'd round up slightly to 6.8 or 7.0 kW to give a small buffer for tree growth, panel degradation over 25 years, and one bad weather year — but I wouldn't push to 9 kW. That extra production gets cashed out at the avoided-cost wholesale rate at NV Energy true-up. Wasted money.
Worked Example: Sacramento Home, 9,500 kWh/year (NEM 3.0)
Plug it in: 9,500 ÷ (5.5 × 365 × 0.80) = 5.9 kW DC.
But under California's NEM 3.0 rules, exports are paid at the avoided-cost rate (typically $0.05–$0.08/kWh) instead of retail. So oversizing to push exports is a losing bet. For NEM 3.0 homes I size the panel array slightly smaller — say 5.4 kW — and add a battery to capture the midday surplus and discharge it during 4–9pm peak hours. The battery turns 75% of what would have been low-value exports into 100% retail offset. See my California solar page for the full NEM 3.0 strategy.
Roof Constraints: Sometimes You Can't Hit the Math
A 6.8 kW system needs roughly 380–420 sq ft of usable roof at modern panel densities (~22% efficiency, ~440W panels). South, southwest, and west roofs all work in Vegas. East works at about 88–92% of south output. North you skip unless desperate. Shading from trees, dormers, or HVAC penetrations carves into usable area fast. On a recent Summerlin install I had to fit a 7.5 kW design across two roof planes because a single plane only had room for 5.2 kW.
Inverter Sizing: AC vs. DC and Why Both Numbers Show Up
You'll see system sizes quoted as "kW DC" (sum of panel ratings) and "kW AC" (inverter output limit). The AC rating is usually 0.80–0.92 of DC. A 7.0 kW DC system might pair with a 6.0 kW AC inverter — that's intentional and called "DC-to-AC clipping ratio." It costs you about 1–2% of summer peak energy but saves real money on a smaller inverter. Anything above 1.25 DC-to-AC starts clipping too much. Below 1.10 and you're overpaying for inverter capacity that never gets used.
Don't Let an Installer Talk You Into a Bigger System
Bigger systems mean bigger commissions. Common pitches to push past your actual need:
- "You'll be glad you sized for an EV later." (Maybe — but only if you're actually buying an EV.)
- "Panels degrade — better to oversize." (True at ~0.5%/year, but a 5% buffer covers 10 years.)
- "Your usage will go up." (Statistically, household usage is flat or declining.)
- "Fill the roof." (No reason to, unless you're planning future loads or running a small business from home.)
Right-sizing means 95–105% of your honest annual usage — adjusted up only for documented future loads.
The Bottom Line
Sizing isn't black magic. Twelve months of bills, a peak sun hours value, and an honest derate. If your installer's number doesn't match what falls out of that math, ask them to show their work. If you want me to run the numbers on your house, send me a year of bills via the quote form and I'll come back with three sizing options and the math behind each.