TheGreenWatt

What Size Solar Panel To Charge A 100Ah Battery? (LFP vs Lead-Acid, 2026)

To charge a 12V 100Ah LiFePO4 battery in one sunny day (5 peak sun hours), you need about 200W of solar with an MPPT controller. For lead-acid, plan on 250–300W because of the slow absorption stage. The formula: panel watts = (Ah × volts × DoD) / (PSH × η_controller × η_battery). This guide gives the right panel size for every battery voltage (12V, 24V, 48V), both chemistries, and the MPPT/PWM split — plus a practical rule of thumb (2–3 W per Ah) for the real world.

I started solar with a single 100W panel and a 12V 100Ah deep-cycle AGM on a camper van. That panel was too small — it never fully charged the battery in one day, and the chronic undercharging killed the AGM in 18 months. If I had started with a 200W panel, the battery would have lasted 4–5 years. Panel sizing matters. If you need help sizing the whole battery bank, try the solar battery sizing calculator.

The Formula

Panel watts = (Ah × V × DoD) / (PSH × η_controller × η_battery)
SymbolMeaning12V 100Ah LFP, MPPT12V 100Ah AGM, PWM
AhBattery amp-hours100100
VBattery voltage1212
DoDDepth of discharge0.800.50
PSHDaily peak sun hours55
η_controllerController efficiency0.95 (MPPT)0.78 (PWM)
η_batteryCharge efficiency0.98 (LFP)0.85 (lead-acid)
ResultPanel watts needed206 W272 W

So: 200W for LFP + MPPT, 300W for lead-acid + PWM (rounding up to the nearest standard panel size). The efficiency gap between controller types is significant — see MPPT vs PWM charge controllers for the full comparison.

Panel Size For Every Common 100Ah Battery Setup

All for one-day charging (5 PSH), 80 % DoD for LFP, 50 % DoD for lead-acid:

BatteryChemistry + ControllerEnergy to replace (Wh)Panel watts (theoretical)Recommended panel
12V 100AhLFP + MPPT960206200 W
12V 100AhLFP + PWM960251300 W
12V 100AhAGM lead-acid + MPPT600148200 W (needs extra for absorption)
12V 100AhAGM lead-acid + PWM600181200 W (needs extra for absorption)
12V 100AhFlooded lead-acid + MPPT600148200 W (needs extra for absorption)
24V 100AhLFP + MPPT1,920412400 W
24V 100AhAGM + MPPT1,200296300 W (+absorption margin)
48V 100AhLFP + MPPT3,840824800 W (typically 2 × 400W panels)

Important note on lead-acid: the formula above covers the bulk charge (0–80 % SOC). The absorption stage (80–100 %) takes an additional 3–5 hours at reduced current, and the panel cannot accelerate this — the battery limits the charge rate. So even if the formula says 200W is enough, in practice you need 250–300W for lead-acid to have any hope of reaching 100 % in a single sunny day.

The 2–3 W Per Ah Rule Of Thumb

For daily cycling (discharge each evening, charge each day) at 12 V:

Panel watts = 2 to 3 × battery Ah
RatioWhat it means
1 W/Ah (100 Ah → 100 W)Too small — 2+ days to charge, chronic undercharging in lead-acid
2 W/Ah (100 Ah → 200 W)Minimum for LFP + MPPT — charges in one sunny day, no margin for clouds
2.5 W/Ah (100 Ah → 250 W)Good practical target — margin for clouds and imperfect angle
3 W/Ah (100 Ah → 300 W)Recommended for lead-acid or if you're in a cloudy location (3–4 PSH)
4+ W/Ah (100 Ah → 400+ W)Overkill for a 100 Ah battery — excess goes into float and is wasted

This is the rule of thumb I recommend to anyone building an off-grid system for the first time. It is simple, it works, and it has enough margin for real weather.

How PSH Changes The Answer

The formula scales linearly with peak sun hours. If you live in a cloudy location, you need more panel:

LocationPSHPanel watts for 12V 100Ah LFP + MPPT
Anchorage, AK3.17325 W
Seattle, WA3.95261 W
Chicago, IL4.27241 W
Boston, MA4.70219 W
U.S. average4.98207 W
Austin, TX5.30194 W
Los Angeles, CA5.61184 W
Phoenix, AZ6.54158 W

In Phoenix, a single 200W panel is more than enough. In Seattle, you need 300W to reliably charge in one day.

What Panel Size If You Don't Charge Daily?

If you only discharge and recharge every 2 or 3 days (weekend camper, seasonal cabin), you can use a smaller panel:

Charge cyclePanel watts (12V 100Ah LFP + MPPT, 5 PSH)
Every day200 W
Every 2 days100 W
Every 3 days70 W
Every 5 days42 W
Trickle maintenance only10–20 W

A 100W panel is perfect for a battery that cycles every other day. A 10–20W trickle panel keeps a parked RV battery topped off indefinitely. For the wiring side, see how to connect solar panels to a battery.

Common Misreadings

  1. "100W is enough for a 100Ah battery." Only if you charge every 2 days and use LFP + MPPT. For daily cycling, 200W is the minimum.
  2. "I need the same wattage for lead-acid and LFP." No — lead-acid has 50 % usable DoD and a slow absorption stage. You need 25–50 % more panel wattage for lead-acid than for LFP.
  3. "The formula gives the exact panel size." The formula gives a theoretical minimum under perfect conditions. Add 20–30 % margin for clouds, soiling, and imperfect panel angle. The 2–3 W/Ah rule of thumb builds this margin in.
  4. "A 400W panel charges twice as fast as a 200W." True in bulk stage, but once the battery enters absorption (lead-acid) or reaches its acceptance limit, the extra panel power goes unused. The benefit of oversizing is reliability (charges on cloudy days), not speed (past a certain point the battery is the bottleneck).
  5. "24V battery needs the same panel as 12V." No — a 24V 100Ah battery holds 2× the energy of a 12V 100Ah battery (2,400 Wh vs 1,200 Wh). You need 2× the panel wattage.

Bottom Line

For a 12V 100Ah LiFePO4 battery with MPPT controller at U.S. average sun: 200W minimum, 250–300W recommended. For lead-acid, 250–300W minimum. The practical rule of thumb: 2–3 watts of panel per amp-hour of 12V battery covers daily cycling with margin for real weather.

Solar panel charging a battery through a charge controllerA solar panel at the top connected to a charge controller in the middle and a battery at the bottom, with energy flowing downward.
Ah
hrs
Required solar panel size
0W
To charge a 100Ah 12V Lithium (LiFePO4) battery in 5 hours
Energy to charge
1.26kWh
If you use 100W panels
3
panels needed
If you use 200W panels
2
panels needed
171 kg
CO₂ avoided per year
0.04
equivalent US homes powered
8
trees planted equivalent
$74
estimated annual savings
ChemistryEfficiencyCycle LifePanel Watts
Lithium (LiFePO4)95%3,000–5,000252 W
Deep Cycle AGM85%500–1,000283 W
Lead-Acid Flooded80%300–500300 W
Tap to see sensitivity analysis
202 W-20%252 W302 W+20%
Sensitivity range
ScenarioValue
Low (-20%)202 W
Expected252 W
High (+20%)302 W

Battery chemistry has the biggest effect \u2014 switching from lead-acid to lithium reduces required panel watts by ~20%.

What size battery do I need?See production for your state

Keep Reading

Frequently Asked Questions

What size solar panel do I need to charge a 100Ah battery in one day?
For a 12V 100Ah LiFePO4 battery at 80 % DoD with an MPPT controller and 5 peak sun hours: about 200W. Formula: panel watts = (100 Ah × 12 V × 0.80) / (5 PSH × 0.95 × 0.98) = 960 / 4.655 = 206W. Round up to a 200W panel. For lead-acid (50 % DoD, slower absorption): 250–300W to reliably reach 100 % in one day.
Can a 100W panel charge a 100Ah battery?
Yes, but it takes about 2 days of full sun (MPPT + LFP). A 100W panel with MPPT delivers ~93W of effective charge — so 960 Wh of energy (the 80 % DoD portion of a 100Ah 12V LFP) takes 10.3 peak sun hours, or 2.1 days at 5 PSH. If you need one-day charging, use 200W.
What size panel for a 24V 100Ah battery?
Double the 12V answer: about 400W for one-day charging (LFP + MPPT at 5 PSH). The energy content doubles (100 Ah × 24 V × 0.80 = 1,920 Wh vs 960 Wh for 12V), so the panel wattage doubles to match.
What size panel for a 48V 100Ah battery?
About 800W for one-day charging (LFP + MPPT at 5 PSH). Energy = 100 Ah × 48 V × 0.80 = 3,840 Wh. That is a large system — most 48V 100Ah battery banks are paired with 800–1,200W of solar.
Does MPPT vs PWM affect the panel size I need?
Yes. With MPPT, a 200W panel delivers ~190W of usable charge to the battery. With PWM, the same 200W panel delivers only ~156W (because PWM forces the panel off its Vmp). So to charge in one day with PWM, you need about 250W instead of 200W — roughly 25 % more panel.
Should I oversize the panel or undersize it?
Always oversize slightly. Real-world conditions include clouds, haze, suboptimal angle, and the battery's own acceptance limits. A panel that is 20–30 % bigger than the theoretical minimum gives you enough margin to still reach a full charge on partly cloudy days. For a 100Ah 12V LFP battery, 250–300W is a comfortable real-world size — not 200W theoretical.
What is the best panel-to-battery ratio?
For daily cycling (discharge each evening, charge each day), aim for 2–3 W of panel per Ah of 12V battery. So a 100Ah 12V battery pairs best with 200–300W of solar. Less than 2 W/Ah and you risk chronic undercharging; more than 3 W/Ah and the excess is wasted during the float stage.

Sources

Marko Visic
Physicist and solar energy enthusiast. After installing solar panels on my own house, I built TheGreenWatt to share what I learned. All calculators use NREL PVWatts v8 data and peer-reviewed formulas.