Solar Panel Output Calculator (Watts to Watt-hours)

Key takeaways

Daily Wh = panel watts × peak sun hours × efficiency factor.
Peak sun hours are different from daylight hours.
Losses from heat, wiring, and inverter efficiency reduce output.
Use conservative inputs for winter or cloudy seasons.

Formula

Daily Wh = Panel watts × Peak sun hours × System efficiency

Example: 400W × 4.5 hours × 0.8 ≈ 1,440Wh/day

Use peak sun hours for your location (not daylight hours). Pair this estimate with your daily load plan when sizing batteries.

How to use this calculator

Find your panel watt rating (W) from the label or datasheet.
Choose peak sun hours for your location and season.
Select an efficiency factor to account for losses.
Multiply to estimate daily Wh output.

If your system includes an inverter, remember that AC loads typically introduce additional losses.

Choosing peak sun hours

Peak sun hours represent the average daily solar energy at your location. They are not the same as daylight hours. A sunny location might average 5–6 peak sun hours, while a cloudy region might average 3–4.

If you need year-round reliability, use the lowest seasonal value instead of the annual average.

If you do not know local sun hours, start with a conservative estimate and adjust after comparing with real production data.

Choose an efficiency factor

Use 0.75 to 0.85 to account for losses from heat, wiring, charge controllers, and inverter efficiency. The more complex the system, the more conservative you should be.

0.85: clean panels, short wiring, efficient inverter.
0.80: typical mixed conditions.
0.75: hot climates, longer wiring, or older equipment.

If you have monitoring data, update the factor after a few weeks so the estimate reflects your real-world system.

Example scenarios

Small RV setup: 300W panels, 4 peak sun hours, 0.8 efficiency.

300 × 4 × 0.8 = 960Wh/day

Cabin weekend setup: 800W panels, 3.5 peak sun hours, 0.75 efficiency.

800 × 3.5 × 0.75 = 2,100Wh/day

Use your own panel size and location for a more accurate estimate over time.

AC vs DC loads

If most of your loads are AC, the inverter introduces losses. A 90 percent efficient inverter turns 1,000Wh of DC into about 900Wh of AC. For DC-only loads, you can often skip the inverter and reduce losses.

If you are sizing batteries, use the battery capacity calculator and include inverter efficiency in your assumptions.

Assumptions and limitations

Output assumes panels are correctly oriented and not shaded.
Temperature and weather can reduce output below the estimate.
Dust or snow can cause significant short-term losses.
Real systems often perform 10–25 percent below nameplate ratings.

If your output stays low, use low solar output troubleshooting.

Seasonal adjustments

Summer averages can be much higher than winter averages. If you are building an off-grid system that must work year-round, use winter sun hours and a conservative efficiency factor. For grid-tied systems, you can often use the annual average and accept seasonal variation.

Quick seasonal reminder

Expect winter output to be meaningfully lower. Use conservative assumptions if you depend on solar for daily energy, especially off-grid.

Common mistakes

Using daylight hours instead of peak sun hours.
Skipping efficiency losses in the formula.
Assuming all panels always hit their max rating.
Ignoring shade or partial cloud cover.

Connect output to your loads

Compare daily output to your daily Wh use. If your panels produce 1,400Wh/day and your loads are 1,200Wh/day, you have a small buffer. If your loads are higher, you need more panels, better sun exposure, or lower usage.

Use the sizing guide to translate these estimates into a full system plan.

For budgeting, see the solar system cost breakdown.

When output seems low

If your output is lower than expected, compare actual production to weather conditions, then check shading, panel cleanliness, and inverter alerts. Use the low output troubleshooting guide for a structured checklist.

For cleaning costs and expectations, see panel cleaning basics.

Multiple arrays and orientations

If your panels face different directions or have different tilt angles, calculate each group separately and add the results. East- and west-facing panels spread production across the day but usually reduce total daily output compared to a south-facing array. Use the most conservative sun hours if shading varies throughout the day.

Solar panel output calculator