Solar panels are rated at STC — Standard Test Conditions: 1,000 W/m² of irradiance, a 25 °C cell temperature, and a specific light spectrum. It is a laboratory benchmark that makes panels comparable to each other.
It is not a forecast. You will not see STC conditions on a camping trip, and the shortfall is significant.
An honest note about our own assumption
Everywhere on this site that we calculate a solar recharge time, we assume panels deliver 75% of nominal in clear conditions. That is a common planning figure and it is what our published assumptions state.
The one independent measurement we have been able to find recorded closer to 51%.
Where the missing watts go
Temperature. This is the biggest single factor and the least intuitive. Panels are rated at a 25 °C cell temperature, but a panel in direct sun reaches 45–65 °C. Output falls roughly 0.3–0.5% per degree above 25 °C, so a hot panel on a bright day can lose 10–20% purely to heat. Bright, cool days out-produce bright, hot ones.
Angle. Output follows the cosine of the angle between the panel and the sun. Flat on the ground at midday in summer is reasonable; flat on the ground in winter, or at either end of the day, is very poor. Tilting a panel toward the sun and repositioning it two or three times a day is the cheapest performance gain available.
Spectrum and atmosphere. STC assumes a specific air mass. Haze, humidity, altitude and time of day all change what actually reaches the cells.
Cable and controller losses. Long or thin cables cost you volts. An MPPT charge controller — which every unit on this site uses — recovers a good deal of what a cheaper PWM controller would waste, but nothing is free.
Dirt and shading. Partial shade is disproportionately damaging. A single shaded cell can drag down an entire string, so a branch shadow across one corner costs far more than a proportional share.
What this means for planning
Work in watt-hours per day, not watts.
A realistic useful solar window is about five to seven hours — not sunrise to sunset, because early and late sun arrives at a hopeless angle.
| Array | At 75% (our assumption) | At 55% (conservative) | Realistic daily Wh |
|---|---|---|---|
| 1 × 100 W | 75 W | 55 W | 275–525 Wh |
| 2 × 100 W | 150 W | 110 W | 550–1,050 Wh |
| 4 × 100 W | 300 W | 220 W | 1,100–2,100 Wh |
| 6 × 100 W | 450 W | 330 W | 1,650–3,150 Wh |
- At 75% (our assumption)
- 75 W
- At 55% (conservative)
- 55 W
- Realistic daily Wh
- 275–525 Wh
- At 75% (our assumption)
- 150 W
- At 55% (conservative)
- 110 W
- Realistic daily Wh
- 550–1,050 Wh
- At 75% (our assumption)
- 300 W
- At 55% (conservative)
- 220 W
- Realistic daily Wh
- 1,100–2,100 Wh
- At 75% (our assumption)
- 450 W
- At 55% (conservative)
- 330 W
- Realistic daily Wh
- 1,650–3,150 Wh
The daily range spans the conservative figure over five hours to the optimistic figure over seven. Reality on a given day sits somewhere inside it, and on an overcast day well below.
Set that against a typical van's consumption of roughly 1,500 Wh a day and the conclusion is uncomfortable: two panels do not sustain a van. Four might, on good days. This is the calculation people skip.
The ceiling that wastes half your array
Every power station has a maximum solar input. Exceed it and the surplus is simply discarded.
| Unit | Max solar input | Useful array |
|---|---|---|
| EcoFlow Delta Pro 3 | 2,600 W | Effectively unlimited for portable panels |
| Bluetti Elite 100 V2 | 1,000 W | Up to 10 × 100 W |
| Bluetti Elite 200 V2 | 1,000 W | Up to 10 × 100 W |
| Anker SOLIX C1000 Gen 2 | 600 W | Up to 6 × 100 W |
| EcoFlow Delta 3 | 500 W | Up to 5 × 100 W |
| Jackery Explorer 1000 v2 | 400 W | 4 × 100 W maximum |
| Jackery Explorer 2000 v2 | 400 W | 4 × 100 W maximum |
| EcoFlow River 2 Pro | 220 W | 2 × 100 W maximum |
- Max solar input
- 2,600 W
- Useful array
- Effectively unlimited for portable panels
- Max solar input
- 1,000 W
- Useful array
- Up to 10 × 100 W
- Max solar input
- 1,000 W
- Useful array
- Up to 10 × 100 W
- Max solar input
- 600 W
- Useful array
- Up to 6 × 100 W
- Max solar input
- 500 W
- Useful array
- Up to 5 × 100 W
- Max solar input
- 400 W
- Useful array
- 4 × 100 W maximum
- Max solar input
- 400 W
- Useful array
- 4 × 100 W maximum
- Max solar input
- 220 W
- Useful array
- 2 × 100 W maximum
Note also that the ceiling matters most exactly when conditions are worst. On a perfect day a 400 W ceiling and a 400 W array are matched. On a hazy day your array delivers 200 W and the ceiling is irrelevant. Headroom is what lets a bigger array claw back a bad day — which is the real argument for the Bluetti units' 1,000 W input.
Getting more from what you have
Tilt, and reposition. The single highest-return habit. Aim the panel at the sun rather than the sky, and move it two or three times a day.
Keep panels cool. A few inches of airflow underneath is worth real watts. Panels lying flat on hot ground run hottest and perform worst.
Avoid partial shade completely. Not "mostly avoid". One shaded corner can cost far more than its share.
Use short, thick cables. Voltage drop over a long thin run is pure loss.
Charge in the morning. Panels are cooler and more efficient before the day heats up, and it gives you the whole day to recover from a poor start.
FAQ
How much power does a 100 W solar panel actually produce?
About 50–75 W in good conditions. One independent test of a Jackery SolarSaga 100 W recorded a peak average of 50.7 W and 57.4 Wh over an hour in full sun, falling to 17.4 Wh in an hour of overcast.[1]
How long to charge a power station from solar?
Divide the unit's rated capacity by your realistic input. A 1,024 Wh unit with two 100 W panels delivering about 150 W takes roughly seven hours — which is most of a usable solar day. At a more conservative 110 W it is over nine hours, meaning two days.
Why is my solar input lower than the panel rating?
Panel ratings are measured at 25 °C cell temperature and 1,000 W/m² irradiance. Real panels run at 45–65 °C in sun, which alone costs 10–20%, and angle, haze, cable losses and dirt take more.
How many solar panels do I need?
Work out daily consumption in watt-hours, then divide by a realistic daily yield per panel — roughly 275–525 Wh for a 100 W panel. A van using 1,500 Wh a day needs three to five 100 W panels, and a power station that will accept them.
Does a higher solar input ceiling matter if I only have two panels?
Not today, but it decides whether you can expand. Two 100 W panels deliver about 150 W of real input, which every unit here accepts. The ceiling becomes the binding constraint the moment you add a third and fourth panel — and headroom is what lets a larger array compensate on poor days.
Next: work out your daily consumption with the sizing guide, or see which units have the highest solar ceilings.
Sources
- OutdoorGearLab. Jackery SolarSaga 100 W panel — measured output. Independent lab test · accessed 5 September 2026
- Jackery. Explorer 2000 v2 — specifications. Manufacturer spec · accessed 5 September 2026
- EcoFlow. RIVER 2 Pro Portable Power Station — specifications. Manufacturer spec · accessed 5 September 2026