How Long To Charge A 12V Battery With A 100 Watt Solar Panel? (MPPT vs PWM, LFP vs Lead-Acid)

This is the most common beginner solar question because a 100W panel + a 12V battery is the universal entry-level off-grid setup — camper vans, sheds, RV trickle charging, and trail cameras. I started solar with exactly this combination years before building the 6 kW grid-tie array on my house.

The Right Formula

Peak sun hours needed = (Ah × 12 V × DoD) / (100 W × η_controller × η_battery)
Days to charge = Peak sun hours needed / Daily PSH

Where:

• Ah = battery nameplate amp-hours
• 12 V = nominal battery voltage
• DoD = depth of discharge (the fraction actually discharged — 0.80 for LFP, 0.50 for lead-acid)
• η_controller = charge controller efficiency (0.95 MPPT, 0.78 PWM)
• η_battery = charge efficiency (0.98 LFP, 0.85 lead-acid)

Worked Examples For The Most Common Battery Sizes

Example 1: 100W Panel → 12V 100Ah LiFePO4 (MPPT, 5 PSH)

Energy to replace = 100 Ah × 12 V × 0.80 = 960 Wh
Effective charge rate = 100 W × 0.95 × 0.98 = 93.1 W
Peak sun hours = 960 / 93.1 = 10.3 hours
Days = 10.3 / 5 = 2.1 days

Example 2: 100W Panel → 12V 100Ah AGM Lead-Acid (PWM, 5 PSH)

Energy to replace = 100 Ah × 12 V × 0.50 = 600 Wh
Effective charge rate = 100 W × 0.78 × 0.85 = 66.3 W
Bulk hours = 600 / 66.3 = 9.0 hours → gets to ~80 % SOC
Absorption stage = 4–6 additional hours at constant voltage
Total = ~15 hours of peak sun → 3.0 days

Lead-acid takes almost 50 % longer despite having less usable energy to replace, because of the double penalty: lower controller efficiency (PWM) and the slow absorption stage.

Example 3: 100W Panel → 12V 50Ah LiFePO4 (MPPT, 5 PSH)

Energy = 50 × 12 × 0.80 = 480 Wh
Rate = 93.1 W
Hours = 480 / 93.1 = 5.2 hours
Days = 5.2 / 5 = 1.0 day

This is the sweet spot for a single 100W panel: a 50 Ah LFP battery charges from 20 % to full in one sunny day.

Complete Charge Time Table — 100W Panel With MPPT + LiFePO4

For 80 % DoD, η_controller = 0.95, η_battery = 0.98, 5 PSH/day:

For PWM controller instead of MPPT, multiply days by 1.22.

For lead-acid AGM instead of LFP, multiply days by approximately 1.4× (bulk) to 1.8× (including absorption).

The Real-World Advice

Looking at this table, the practical takeaways are:

1. A 100W panel is well-matched to a 50 Ah LFP battery. One sunny day, one full charge. This is the camper van sweet spot.
2. For a 100 Ah battery, a 100W panel is on the slow side. Two days to recharge means you can't fully cycle the battery daily from a single panel. Either add a second 100W panel in parallel (halves the charge time) or accept that you can only draw ~40 Ah per day.
3. For 200+ Ah batteries, a 100W panel is too small. 4+ days to charge means you will chronically undercharge in real use. Use 200–400 W of solar for a 200 Ah bank.
4. MPPT is worth the price. The $35–55 premium over PWM saves ~22 % of your charge time every single day. On a 100W panel charging a 100Ah battery, that is the difference between 2.1 and 2.6 days.
5. LFP is worth the price. 2× the usable capacity per nameplate Ah and no slow absorption stage means faster, more complete daily recharges.

Why Older Articles Get This Wrong

Most "how long to charge" articles divide the battery Wh by a raw panel Wh per day number with a 0.75 "loss factor" and call it done. That skips:

• Controller type. PWM wastes 17+ % of panel power. MPPT does not.
• Battery chemistry. Lead-acid absorption takes 4–6 hours on top of bulk. LFP has no meaningful absorption stage.
• Depth of discharge. Lead-acid should never go below 50 % SOC; LFP goes to 80–95 %. A "100 Ah" lead-acid only gives you 50 Ah of usable energy; a "100 Ah" LFP gives you 80–95 Ah.

The combined error from ignoring these three factors can be 50+ % off in either direction.

Common Misreadings

1. "100W panel produces 375 Wh per day, a 100Ah battery holds 1,200 Wh, so 1,200/375 = 3.2 days." This skips controller efficiency and depth of discharge. The real answer with MPPT + LFP is 2.1 days.
2. "It takes 19.2 hours to charge a 50 Ah battery." That is the older article's number using 0.75 derate and no controller efficiency. The real answer with MPPT + LFP is 5.2 peak sun hours (about 1 day).
3. "I can charge a 200 Ah battery in two days with a 100W panel." No — it takes 4.1 days with MPPT + LFP. A 200W panel does it in 2 days.
4. "MPPT and PWM don't matter for a small 100W panel." They matter the most for small panels. The 22 % efficiency gap turns "barely charged by sunset" into "fully charged by sunset" on a daily cycle.
5. "I can charge a car battery with a 100W panel." Don't. Car (cranking) batteries are not designed for deep cycling and will die in weeks. Use a deep-cycle lead-acid or LiFePO4 battery.

Bottom Line

A 100W solar panel with an MPPT controller charges a 12V 50 Ah LiFePO4 battery in one sunny day and a 12V 100 Ah LFP in about two days. With PWM or lead-acid, add 25–80 %. For anything over 100 Ah, consider a larger panel.

Keep Reading

If you found this useful, these guides go deeper into related topics:

• Solar Panel Charge Time Calculator (Any Panel + Battery)
• What Size Solar Panel To Charge A 100Ah Battery
• How Many Amps Does A 100 Watt Solar Panel Produce
• How Many Amp-Hours Is A Tesla Powerwall
• Solar Panel Output Voltage Explained
• Average Peak Sun Hours By State
• Solar Panel Calculator — Full Energy Estimate