What size solar system for 20 kWh per day?
Quick answer: NREL PVWatts v8 shows 1 kW of south-facing panels tilted at latitude making 1,787 kWh a year in Phoenix, 1,308 in Chicago and 1,071 in Seattle. Using 20 kWh a day is 7,300 kWh a year, so plan on about 4.1 kW of panels in Phoenix, 5.6 kW in Chicago and 6.8 kW in Seattle.
Pick your city
Array size for 20 kWh a day by city
| City | Peak sun hours (kWh/m²/day) | Annual AC kWh per 1 kW | kW of panels for 7,300 kWh/yr (derived) |
|---|---|---|---|
| Phoenix, AZ | 6.57 | 1,787 | 4.1 |
| Las Vegas, NV | 6.47 | 1,791 | 4.1 |
| Los Angeles, CA | 6.09 | 1,699 | 4.3 |
| Denver, CO | 5.74 | 1,635 | 4.5 |
| Albuquerque, NM | 6.48 | 1,817 | 4.0 |
| Dallas, TX | 5.41 | 1,513 | 4.8 |
| Houston, TX | 5.15 | 1,422 | 5.1 |
| Miami, FL | 5.57 | 1,552 | 4.7 |
| Atlanta, GA | 5.09 | 1,412 | 5.2 |
| Nashville, TN | 4.77 | 1,342 | 5.4 |
| Kansas City, MO | 5.05 | 1,451 | 5.0 |
| Chicago, IL | 4.46 | 1,308 | 5.6 |
| Minneapolis, MN | 4.66 | 1,358 | 5.4 |
| Milwaukee, WI | 4.55 | 1,337 | 5.5 |
| Detroit, MI | 4.42 | 1,287 | 5.7 |
| New York, NY | 4.45 | 1,293 | 5.6 |
| Boston, MA | 4.71 | 1,329 | 5.5 |
| Seattle, WA | 3.83 | 1,071 | 6.8 |
| Portland, OR | 3.98 | 1,113 | 6.6 |
| Anchorage, AK | 3.03 | 886 | 8.2 |
PVWatts columns: NREL PVWatts v8, fixed open rack, tilt = latitude, facing south, 14% system losses, standard module (full rows and request links on the peak sun hours data page). Last column = 7,300 ÷ annual kWh per kW, rounded.
How the math works
20 kWh a day × 365 days = 7,300 kWh a year. Divide that by how much 1 kW of panels makes in your city each year and you get the kW of panels. Divide kW × 1,000 by your panel's rated watts for a panel count, rounded up. PVWatts already includes its 14% loss assumption; shading, a different tilt or a different direction changes the result, so run your own address in PVWatts before you buy.