Solar Charge Time Calculator: How long does the solar system take to charge the battery?
200 W charge a 100 Ah LiFePO4 from 30% to a good 80% on a summer day with five peak sun hours – if nothing is consumed on the side. With the fridge running it takes twice as long. This calculator shows how many sunny days the battery needs to reach the desired state of charge.
Result
Sunny days to target state of charge
1.6 days
at 4 h peak sun hours per day
- Energy to recharge
- 924 Wh
- Solar output per day
- 560 Wh
- Pure charge time in peak sun hours
- 6 h 36 min
- Average charging current in full sun
- 10.9 A
Show calculation
- 1To recharge: 100 Ah × 12.8 V × (100% − 30%) ÷ 97% = 924 Wh
- 2Daily output: 200 W × 4 h × 70% = 560 Wh
- 3Sunny days to target: 924 Wh ÷ 560 Wh = 1.6 days
How it's calculated
to recharge [Wh] = capacity [Ah] × voltage [V] × (target − start) ÷ charge efficiency
daily output [Wh] = solar power [W] × peak sun hours × performance ratio
sunny days = to recharge ÷ (daily output − daily consumption)The pure charge time in peak sun hours shows how many hours of full sun would be needed; since a day only has a few peak sun hours, sunny days are the more practical figure. If a load runs at the same time, only the net output is available for charging.
Worked example
After two days parked, the 200 Ah LiFePO4 is at 40%. There are 300 W on the roof, it's May with four peak sun hours, and the fridge uses 400 Wh a day.
Inputs
- Solar power: 300 W
- Peak sun hours per day: 4 h
- Simultaneous daily consumption (optional): 400 Wh
- Battery capacity: 200 Ah
- Battery type: LiFePO4 (lithium)
- State of charge at start: 40 %
- Target state of charge: 100 %
- Performance ratio: 70 %
- Battery charge efficiency: 97 %
Result
3.6 days
Sunny days to target state of charge
- Energy to recharge
- 1,584 Wh
- Solar output per day
- 840 Wh
- Pure charge time in peak sun hours
- 7 h 32 min
- Average charging current in full sun
- 16.4 A
Calculation
- To recharge: 200 Ah × 12.8 V × (100% − 40%) ÷ 97% = 1,584 Wh
- Daily output: 300 W × 4 h × 70% = 840 Wh
- Net after consumption: 840 Wh − 400 Wh = 440 Wh
- Sunny days to target: 1,584 Wh ÷ 440 Wh = 3.6 days
The variables explained
- Solar power (W)
- Sum of the panels' rated power.
- Peak sun hours
- Daily irradiation in kWh/m² – values per region and month in the solar output calculator, or from NREL PVWatts for your site.
- Simultaneous daily consumption (Wh)
- What keeps running while charging; reduces the net output.
- Charge efficiency (%)
- LiFePO4 about 97%, lead-acid 80–90%.
Common mistakes
- Calculating rated power × hours: flat-mounted panels deliver only 60–75% of their rated power on average during peak sun hours.
- Forgetting consumption: a fridge quickly eats half the daily output.
- Planning a lead-acid battery to 100%: the absorption phase drags on – often the sun is gone before.
Assumptions and limits
- Every day has the stated peak sun hours (no weather variation).
- The charge controller (ideally MPPT) is included in the performance ratio.
- Constant charge efficiency, no charging current limit by the BMS.