☀️ Solar Charge Time Calculator
Estimate how long solar charging actually takes — using real-world panel output, not the optimistic rated wattage on the box.
How to Calculate Solar Charge Time
Charging time from a solar panel comes down to one division: the energy the battery still needs, divided by the power the panel actually delivers. The calculator above runs exactly this.
- Work out the watt-hours still needed. Battery capacity in Wh × (100 − current charge %) ÷ 100.
- Derate the panel. Multiply its rated wattage by a real-world factor — see below. This is the step people leave out, and it is the reason estimates come out two to five times too optimistic.
- Divide. Watt-hours needed ÷ derated watts = hours of usable sun, not hours of daylight.
The answer is in hours of direct sun on the panel. A winter day may offer four or five of those, so a figure above roughly five hours means more than one day of charging.
Why a Rated 400W Panel Never Delivers 400W
Panel wattage is measured under standard test conditions — a set irradiance, a set cell temperature, and the panel square-on to the light. Outdoors you get none of those at once, so the honest planning figure is a fraction of the number on the box. These are the factors the calculator uses:
| Conditions | Share of rated output | A 400W panel delivers |
|---|---|---|
| Full sun, panel well angled | ~70% | 280 W |
| Partly cloudy or a poor angle | ~40% | 160 W |
| Overcast | ~20% | 80 W |
These are planning estimates for sizing, not measured figures for any specific panel.
Worked Example
Take the EcoFlow DELTA 2 from this dataset — 1024 Wh, accepting up to 500 W of solar input — at 20% charge, with a 400 W panel in full sun:
- Still needed: 1024 Wh × 80% = 819 Wh
- Panel actually delivering: 400 W × 0.7 = 280 W
- 819 ÷ 280 = about 2.9 hours of direct sun
Change nothing but the weather and the same job takes 12.8 hours from empty under overcast skies. That gap, not the capacity figure, is what decides whether solar is a realistic way to refill a unit on a given trip.
Solar Charging: Common Questions
How many solar panels do I need to charge a power station in one day?
Divide the capacity you need to replace by about four to five hours of usable sun, then divide again by 0.7 to allow for real-world panel derating. Refilling 1,000Wh in a single day needs roughly 300W of rated panel — but never more than the unit's maximum solar input, since the charge controller caps intake at its limit.
Can I charge a power station on a cloudy day?
Yes, but slowly. Output drops to roughly 40% of rated wattage under partial cloud and about 20% under heavy overcast, so a job that takes four hours in full sun can take most of a week. The calculator above lets you pick the condition rather than assuming full sun.
Will more solar panels charge it faster?
Only up to the unit's rated maximum solar input — beyond that, extra panels add nothing. Maximum input voltage matters more: wiring panels in series raises voltage, and exceeding the controller's voltage window can damage the unit, so check the voltage range on the spec sheet, not just the watts.
Why won't my power station charge in freezing weather?
Most units refuse to charge a lithium battery below about 0°C, because charging a cold cell causes lithium plating and permanent capacity loss. The battery management system blocks it deliberately, so panels producing power while the unit reports no charging is usually protection working, not a fault.
Methodology: charge time = remaining Wh needed ÷ (panel's rated watts × a real-world derating factor). The derating factor accounts for panel angle, temperature, and the fact that manufacturer wattage ratings are measured under lab-ideal test conditions rarely seen outdoors. This is a general estimate, not a per-product verified figure — always cross-check against the manufacturer's own charge-time spec when available.