The calculation
energy to replace = capacity Ah × voltage × depth to recover
real charge power = array W × (1 − system losses) × charge efficiency
peak sun hours needed = energy to replace ÷ real charge power
days = peak sun hours needed ÷ your site’s peak sun hours
Nameplate array watts are measured at 1000 W/m² on a 25 °C cell in a laboratory. Between the module and the battery you lose some to soiling, shading, mismatch and wiring, and then the battery loses some of what it accepts. Both matter, and a calculator that skips them will tell you a system charges faster than it does.
The number that actually matters
Charge time in hours is interesting. Whether your array can replace one full cycle in one day of sun is the number that determines whether the system works.
If it cannot, and the bank is cycled daily, it falls progressively further behind. On lead-acid that is not a mild inconvenience: a bank that never reaches a full absorption stage sulfates, and the capacity loss is permanent. The calculator flags this case explicitly rather than reporting “1.4 days” as though it were a neutral fact.
Why the last 20 % takes so long
This is a bulk-charge estimate, and bulk is the phase where a controller pushes maximum current at rising voltage. It holds well to roughly 80 % state of charge.
After that the controller switches to absorption: it holds voltage constant and lets current fall away as the battery’s internal resistance rises. Charging becomes asymptotic. On flooded lead-acid, absorption can take as long again as bulk did, which is why manufacturers specify absorption times in hours rather than as a percentage. LiFePO4 has a much flatter charge curve and far less taper, which is a genuine practical advantage in a solar system where daylight is finite.
Charge rate, and when it becomes a problem
The C-rate is charge current divided by capacity in amp-hours. 50 A into a 100 Ah bank is 0.5C.
- Lead-acid generally wants 0.1C to 0.3C. Too fast causes gassing and heat; too slow — below about 0.1C — can mean the bank never reaches a proper absorption stage at all, regardless of how long it is left.
- LiFePO4 commonly accepts up to 0.5C, some packs more, and does not object to slow charging. Below freezing, charging must stop entirely, which a good BMS enforces.
The calculator reports your C-rate and warns at both ends. In practice the charge controller is what limits current, which is one more reason to size it properly rather than buying the cheapest unit that fits.
If the answer is too slow
Three levers, in order of cost:
- More array. Directly proportional. Doubling array watts halves the charge time.
- MPPT instead of PWM. A PWM controller pulls the array down to battery voltage and discards the difference. MPPT converts it, typically recovering 15 to 30 % more energy from the same panels.
- Less depth. A bank cycled to 30 % instead of 80 % needs far less back each day, and lasts longer for it. Sometimes the right answer is a bigger bank shallow-cycled rather than a bigger array.