Solar Charge Time Calculator
Estimate how many solar days your panels need to recharge a battery. Include daily sunlight, charging limits, losses and the loads that keep running.
Your recharge estimate
1.2 estimated solar days.
That is 4.8 peak-sun hours, spread over days with 4 peak-sun hours each. The estimate uses daily energy totals, so the actual finish time will depend on when sunlight is available.
Energy available at each stage
| Stage | To add | Net per day |
|---|---|---|
| Before taper | 768 Wh | 640 Wh |
| Slower finish | 0 Wh | 320 Wh |
Sunlight and loads are averaged across each day. Clouds, shading and battery drain overnight can change the result. Check controller compatibility separately.
How solar charging time is calculated
A 100 Ah battery at 12.8 V stores a nominal 1,280 Wh. Moving from 20% to 80% adds 768 Wh. With a 200 W array, 80% retained energy and four peak-sun hours per day, an idle battery receives 640 Wh per day: about 1.2 solar days.
Battery Wh = Ah × nominal V
Charging power = smaller of (panel W × retained fraction) and battery-side W limit
Days in each stage = energy to add ÷ (stage power × peak-sun hours/day − load Wh/day)
We calculate each charging stage separately, using less power during the slower finish, then add the times. The retained-energy percentage accounts for losses once, before applying the charging limit. Daily loads are spread evenly over the calculated time, including partial days. For changes within a day, you would need an hourly model.
What peak-sun hours mean
Four peak-sun hours means the day provides as much sunlight as four hours at full reference intensity. Weaker morning and evening light also contributes. The total does not show when your panels reach the charging limit. If the controller often caps panel output, or the panels spend time in shade, try a lower panel rating to see how the estimate changes.
Charging assumptions and sources
Use a compatible solar charge controller and your battery’s charge limits. Victron’s solar charger operating guide describes bulk, absorption and float stages and the effect of solar power being shared with loads. The efficiency and slower-finish settings above are examples you can adjust; they do not come from that guide.
For location-specific hourly solar and battery modelling, NREL’s System Advisor Model is a more detailed next step. This calculator uses the sunlight estimate you enter.