Solar Battery Charge Time Calculator

Your charge controller is the gatekeeper between panels and batteries, and the type you use changes real-world charge speed dramatically.

PWM (Pulse Width Modulation) controllers are simple and cheap. They connect panels directly to batteries, which means panel voltage gets dragged down to battery voltage. Victron's paper "Which solar charge controller: PWM or MPPT?" works one example: a 100W, 36-cell monocrystalline panel at 25°C cell temperature charging a discharged battery at 13V. The paper adds 0.5V for the loss in the cabling and the controller, so the PWM controller holds the panel at 13.5V. There it harvests 13.5V x 6A = 81W, 19% less than the 100W harvested with an MPPT controller. At 75°C cell temperature the same paper prints the difference as negligible. PWM works acceptably only when panel voltage closely matches battery voltage.

MPPT (Maximum Power Point Tracking) controllers convert the higher panel voltage down to battery voltage while boosting current. They harvest more than PWM controllers when the cells are cool, in low light and when panel voltage is much higher than battery voltage. Victron's paper calls the option to wire more panels in series, and so cut the current, a compelling reason to install an MPPT controller as soon as the array exceeds a few hundred watts on a 12V battery.

This calculator applies a 98% MPPT controller efficiency: Victron's printed maximum for its SmartSolar MPPT 100/30 and 100/50. That is the maker's printed maximum, and your own controller's datasheet prints its own figure. It also applies the cell-temperature factor 302 / 320 = 0.944 from Trina's printed 302W at NOCT against 320W. If you are using a PWM controller, raise the system loss input by what your own comparison shows: Victron's 19% is one panel at one battery voltage and one cell temperature.

Setup: 400W of solar panels, 5 peak sun hours per day, 200Ah 12V LiFePO4 battery bank drained to 20% state of charge (80% DoD), 14% system losses (the PVWatts default), 92% charge efficiency (the default: Victron's printed round-trip efficiency for LiFePO4).

Energy needed to refill: 200Ah x 0.80 = 160Ah. At 12.8V (Victron's printed LiFePO4 nominal for a 12V bank), that is 2,048Wh to put back. At 92% charge efficiency the array must deliver 2,048 / 0.92 = 2,226Wh.

Effective solar input per day: 400W x 5 hours x 0.86 (14% losses) x 0.98 (MPPT) x 302 / 320 (cell temperature) = 1,591Wh per day.

Charge time: 2,226 / 1,591 = 1.40 days. A little under a day and a half of sun to go from 20% back to 100%.

In practice, the last part of a charge is slower because the charge controller enters absorption phase, reducing current as the battery approaches full. The calculator computes the bulk energy only. For lead-acid, Battery University's BU-403 prints that the battery charges to about 70 percent in 5-8 hours and that the remaining 30 percent takes another 7-10 hours. For a LiFePO4 battery, how much longer the last part takes is for the battery's own datasheet to say.

All figures assume a LiFePO4 bank (12.8V nominal per 12V block) at 92% charge efficiency, 14% system losses (the PVWatts default), Victron's printed 98% maximum MPPT efficiency (your controller's datasheet prints its own) and the cell-temperature factor 302 / 320 = 0.944, and every row is the calculator's own output at those inputs. Every row is the bulk energy only: the last part of a charge is slower, and by how much the battery's own datasheet says. The calculator also assumes the battery accepts all the power the array delivers. Victron prints a recommended charge current of 50A for its 12.8V 100Ah LiFePO4 battery, so an array that can deliver more than that charges it no faster than that current allows. Your battery's datasheet prints its own recommended charge current. To convert these wattages to the amps your charge controller must handle, use the solar watts to amps calculator.