Short answer: about 400W to fully recharge a 12V 100Ah LiFePO4 battery in a single good day of sun. If you have lead-acid or AGM, it is closer to 200W — not because those batteries are easier to charge, but because you can only safely use half of one.
The reason this question gets so many different answers online is that most of them skip at least one of the three steps below.
Step 1: convert amp-hours to watt-hours
Amp-hours alone tell you nothing until you know the voltage. Wh = Ah × V. And the voltage to use is the chemistry's nominal voltage, not the label on the box — a "12V" LiFePO4 battery actually runs at 12.8V nominal.
| Battery | Nominal V | Total capacity | Usable capacity |
|---|---|---|---|
| 12V LiFePO4 100Ah | 12.8V | 1,280 Wh | ~1,220 Wh (95% DoD) |
| 12V AGM / lead-acid 100Ah | 12.0V | 1,200 Wh | ~600 Wh (50% DoD) |
| 24V LiFePO4 100Ah | 25.6V | 2,560 Wh | ~2,430 Wh |
| 48V LiFePO4 100Ah | 51.2V | 5,120 Wh | ~4,860 Wh |
That lead-acid row is the one people miss. Routinely discharging a flooded or AGM battery below 50% dramatically shortens its life, so a 100Ah lead-acid battery is functionally a 50Ah battery. LiFePO4 can be cycled to 90-100% without the same penalty, which is why it costs more per amp-hour and still usually wins on cost per usable kilowatt-hour over the battery's life.
Step 2: account for the losses
A panel rated 400W is rated under standard test conditions — 25°C cell temperature, perfect perpendicular light. In the real world, on a hot roof, at an imperfect angle, expect 75-80% of nameplate. Then the charge controller takes its own cut.
So: Panel watts = usable Wh needed ÷ (peak sun hours × 0.75)
Step 3: run it for your situation
| Scenario | Sun hours | Controller | Panel watts needed | Buy |
|---|---|---|---|---|
| 12V LiFePO4, full recharge, good sun | 5 | MPPT | 325W | 400W |
| 12V LiFePO4, winter or overcast | 3 | MPPT | 542W | 600W |
| 12V LiFePO4, good sun, PWM controller | 5 | PWM | ~430W | 500W |
| 12V AGM (50% usable), good sun | 5 | MPPT | 160W | 200W |
| 24V LiFePO4, good sun | 5 | MPPT | 648W | 800W |
Round up rather than down. A slightly oversized array costs a little more and finishes charging earlier in the day; an undersized one never quite gets the battery full, which for lead-acid specifically causes sulfation and permanent capacity loss over time.
Your charge controller matters more than you would expect
A PWM controller simply connects the panel to the battery, dragging the panel's operating voltage down to battery voltage and wasting the difference. A MPPT controller converts that excess voltage into extra current, typically harvesting 20-30% more energy from the same panel — and the advantage grows in cold weather and low light, exactly when you need it most.
On anything above roughly 200W of array, MPPT pays for itself. It also lets you wire higher-voltage panels (a 24V or 36V panel into a 12V battery), which means thinner, cheaper wire for the same power. Size the controller to both ends: its current rating must exceed your array's output amps, and its input voltage window must accept the panels' open-circuit voltage at your coldest expected temperature, since panel voltage rises as it gets colder. Exceeding that limit on a cold clear morning is a common way to destroy a controller.
Sizing a whole system, not just one battery
If this battery is part of a backup or off-grid system rather than a standalone project, size the loads first and the battery second — our free wattage calculator handles the appliance side, and solar and battery sizing for a 72-hour outage works through a complete multi-day build.