Quick estimate
A single 400W solar panel in a typical US location produces about 1,100–1,600 watt-hours (Wh) per day — roughly 33–48 kWh per month. That’s enough to run a small chest freezer, recharge phones and laptops several times over, or run LED lights for hours. Actual output depends on your peak sun hours and system efficiency; use the calculator below for your exact setup.
Solar panel output calculator
The formula
Daily Wh = Panel watts × Peak sun hours × System efficiency × Number of panels
Example: 400 W × 4.5 hours × 0.80 = 1,440 Wh/day (about 43 kWh/month).
System efficiency accounts for real-world losses: heat, wiring voltage drop, inverter conversion, dust, shading, and mismatch. Use 0.75–0.85 for planning; drop to 0.70 for conservative off-grid estimates.
Choose an efficiency factor
| Condition | Efficiency factor | When to use |
|---|---|---|
| Cool climate, clean array, quality MPPT | 0.85 | Best-case estimate |
| Typical US residential install | 0.80 | Default planning value |
| Hot climate, some shading, PWM controller | 0.75 | Realistic warm-climate value |
| Off-grid winter estimate or poor conditions | 0.70 | Conservative sizing |
Quick-reference output table
Don’t want to calculate? Here’s realistic daily output for common panel sizes at different peak sun hours, using 0.80 system efficiency (typical US install):
| Panel size | 3.5 sun hrs (cloudy/poor) | 4.5 sun hrs (typical) | 5.5 sun hrs (good/sunny) | 6.5 sun hrs (Southwest) |
|---|---|---|---|---|
| 100W | 280 Wh/day | 360 Wh/day | 440 Wh/day | 520 Wh/day |
| 200W | 560 Wh/day | 720 Wh/day | 880 Wh/day | 1,040 Wh/day |
| 400W | 1,120 Wh/day | 1,440 Wh/day | 1,760 Wh/day | 2,080 Wh/day |
| 600W | 1,680 Wh/day | 2,160 Wh/day | 2,640 Wh/day | 3,120 Wh/day |
| 800W | 2,240 Wh/day | 2,880 Wh/day | 3,520 Wh/day | 4,160 Wh/day |
How to read this: A 400W panel in Phoenix (6+ sun hours) produces nearly double what the same panel produces in Seattle (3.5 sun hours). Location matters more than panel brand.
What can this actually power?
Numbers are abstract. Here’s what common daily outputs translate to in real appliance runtime:
At 400 Wh/day (one 100W panel, typical conditions):
- Phone charges: ~25–30 full smartphone charges
- LED lighting: 10W LED bulb for ~40 hours
- Laptop: 2–3 full charges of a 15" laptop
- Camping fridge (12V compressor): About 3–4 hours of runtime (not enough on its own)
At 1,440 Wh/day (one 400W panel, typical conditions):
- Full-size refrigerator: ~10–12 hours of runtime (covers a full day with a decent battery buffer)
- CPAP machine: All night (8+ hours) with humidifier
- TV + streaming stick: ~6 hours of viewing
- Microwave: ~1 hour of continuous use (realistically, plenty for daily meals)
At 2,880 Wh/day (two 400W panels):
- Off-grid cabin basics: Lights, fridge, laptop charging, water pump, and a fan — all day, with surplus to spare
- RV daily use: Full off-grid capability including microwave and coffee maker
For more on matching your production to your loads, see how to size a solar system and battery capacity calculator.
Seasonal variation: expect 30–50% less in winter
Solar output swings dramatically between summer and winter. Plan for your worst-producing month, not your average:
| Season | Output vs. peak | What causes it |
|---|---|---|
| Summer | 100% (peak) | Long days, high sun angle |
| Fall/Spring | 70–85% | Shorter days, lower angle |
| Winter | 40–60% | Short days, low angle, more clouds |
Practical example: A 400W panel in the US Midwest might produce 2,000+ Wh/day in July but only 700–900 Wh/day in December. If you need reliable winter power, oversize your array by 40–60% or add a generator backup.
Latitude matters most. The further north you are, the steeper the winter drop. Arizona winters are mild; Minnesota winters cut output nearly in half. If you’re sizing an off-grid system for year-round use, always calculate using your December sun hours.
Common mistakes
- Using total daylight instead of peak sun hours. A location with “14 hours of daylight” may only get 3.5 peak sun hours. The sun isn’t at full intensity all day — it ramps up and down. Always use peak sun hour data.
- Forgetting system losses. A “400W” panel rarely delivers 400W. After heat, wiring, inverter, and controller losses, you’ll see 280–340W actual. The 0.80 efficiency factor accounts for this.
- Ignoring heat derating. Panels lose ~0.4% efficiency per °C above 25°C (77°F). A roof-mounted panel at 150°F produces 10–15% less than the same panel in cool air. See solar panel efficiency for the full breakdown.
- Planning around summer numbers. If you size your system using July output, you’ll be dark by December. Always size for your worst month.
- Assuming “rated watts” equals “actual watts.” A 400W panel produces 400W only at Standard Test Conditions (STC) — 25°C, 1000W/m², perfect angle. Real conditions rarely match STC.
Why this number matters
- Battery sizing: a 1,440 Wh/day load needs enough usable battery capacity to cover cloudy days. Use the battery capacity calculator.
- Load planning: knowing daily production lets you match appliances to available energy. Start with system sizing.
- Troubleshooting: if real output is much lower than this estimate, check shading, tilt, soiling, or controller issues in low output troubleshooting and panel cleaning basics.
Estimated output is theory; a meter is truth. A 1000V-rated auto-ranging multimeter lets you confirm panel Voc and string voltage against spec-sheet numbers.
Check price on Amazon Price & availability shown on Amazon.com — we may earn a commission.Next logical reads
How to size a solar system Battery capacity calculator Solar system cost breakdown