Peak Sun Hours in Washington, District of Columbia, United States
- Solar zone
- Zone 3 (good)
- Best month
- June (6.2)
- Worst month
- December (1.9)
- Climate
- Temperate · 13.1°C
Monthly solar breakdown
| Month | GHI | Clear-sky | DNI | DHI | Temp °C | Humid % |
|---|---|---|---|---|---|---|
| Jan | 2.17 | 3.12 | 3.38 | 0.93 | 0.1 | 82 |
| Feb | 2.97 | 4.23 | 3.76 | 1.26 | 1.3 | 80 |
| Mar | 3.97 | 5.64 | 4.17 | 1.63 | 5.8 | 79 |
| Apr | 5.00 | 6.93 | 4.59 | 2.00 | 12.2 | 76 |
| May | 5.57 | 7.67 | 4.39 | 2.39 | 17.6 | 78 |
| Jun | 6.20 | 7.89 | 4.89 | 2.57 | 22.8 | 74 |
| Jul | 6.04 | 7.55 | 4.62 | 2.57 | 25.8 | 69 |
| Aug | 5.36 | 6.78 | 4.28 | 2.30 | 25.1 | 70 |
| Sep | 4.39 | 5.78 | 4.40 | 1.72 | 21.3 | 72 |
| Oct | 3.32 | 4.49 | 4.23 | 1.23 | 14.4 | 76 |
| Nov | 2.47 | 3.30 | 3.85 | 0.94 | 7.8 | 79 |
| Dec | 1.91 | 2.73 | 3.18 | 0.83 | 2.8 | 83 |
GHI, Clear-sky, DNI, DHI in kWh/m²/day. Data: NASA POWER climatology (long-term monthly averages).
Off-grid calculator
Add appliances
- Add an appliance to size a system.
Sizing
- Daily load
- 0.00 kWh
- Panel wattage
- 0 W
- Panel count (400W modules)
- 0
- Battery bank
- 0.0 kWh
Sizing against worst-month PSH of 1.91 kWh/m²/day, 0.77 system efficiency.
About solar in Washington
Washington, United States has a temperate climate with meaningful seasonal variation in solar resource. Its annual peak sun hours average 4.11 kWh/m²/day, a good solar resource by global standards.
The strongest month in Washington is June (summer) at 6.2 kWh/m²/day, and the weakest is December (winter) at 1.91 kWh/m²/day. When sizing a year-round off-grid system, it's standard practice to design against the December value rather than the annual average — otherwise the battery bank runs low during the darkest weeks.
Washington's solar conditions are well within the range where off-grid PV is a straightforward engineering exercise with standard-sized arrays and lithium batteries.
FAQ
- What are the peak sun hours in Washington?
- Washington averages 4.11 peak sun hours per day annually, ranging from 1.91 in December to 6.2 in June.
- How many solar panels do I need in Washington?
- Panel count depends on your daily load. At Washington's annual average of 4.11 kWh/m²/day, a 5 kWh/day load needs roughly 4 × 400 W panels. Use the calculator above for your actual load.
- What size battery do I need in Washington?
- Sizing against Washington's worst month (December, 1.91 kWh/m²/day) with 2 days of autonomy at 80% depth of discharge, a 5 kWh/day load needs about a 12.5 kWh battery bank.
- How does Washington's solar resource compare globally?
- Washington sits in solar zone 3 out of 5 (where 5 is strongest) at 4.11 kWh/m²/day — good by global standards. For reference, top-tier desert sites average ~6.5 and high-latitude cities around 2.5 kWh/m²/day.
- How much does winter reduce solar output in Washington?
- Washington's worst month (December) delivers 1.91 kWh/m²/day — about 31% of the summer peak of 6.2. Off-grid systems here typically oversize 1.5–2× or pair with a backup source.
- Do cloudy days significantly affect Washington's solar?
- Partly. Diffuse light still produces 10–25% of clear-sky output. Washington's 4.11 kWh/m²/day already reflects average cloudiness, so no further derating is needed for sizing.
- What panel tilt works best in Washington?
- At Washington's latitude (39°), a fixed tilt near 39° balances year-round yield. Winter-heavy loads favor latitude + 15°; summer-heavy, latitude − 15°.