How this is calculated
Amp-hours times system voltage times the fraction you discharged gives the watt-hours to put back. Divide that by your peak sun hours and you have the panel power the day must sustain. Everything after is loss.
Three losses stack. NREL's PVWatts model carries a default total system loss of 14 percent, built from soiling 2, shading 3, mismatch 2, wiring 2, connections 0.5, light-induced degradation 1.5, nameplate tolerance 1 and availability 3 percent, combined multiplicatively rather than added. An MPPT controller takes about 2 percent more at its best; Victron rates the SmartSolar 100/30 at 98 percent maximum efficiency. The battery takes the last bite: Victron puts round-trip efficiency at 80 percent for lead-acid and 92 percent for lithium iron phosphate.
Multiply them and 100 watts of nameplate panel stores about 77.5 watt-hours per peak sun hour in a lithium bank. That factor turns a battery into a panel size.
Worked example. A 12 V 100 Ah lithium battery run down to half holds 600 Wh to replace. At 4.2 peak sun hours each watt of panel stores 4.2 × 0.775 = 3.26 Wh, so 600 ÷ 3.26 = 184 watts. That is why one 200 W module is the standard answer.
The 4.2 default is deliberately cautious. Annual averages at optimum tilt run 6.7 hours in Phoenix and 4.1 in Seattle, so most sites beat it in summer and miss it badly in December.
The 3.0 default is deliberately cautious. Annual averages at optimum tilt run 5.8 hours in Madrid and 3.5 in Amsterdam, so most sites beat it in summer and miss it badly in December.
Quick answers
What size solar panel do I need to charge a 100 Ah battery?
About 185 W where the day averages 4.2 peak sun hours, 260 W at 3.0, to refill a half-discharged 12 V pack in a day. The same 100 Ah at 24 V holds twice the energy and needs twice the panel.
How long does a 100 W panel take to charge a 100 Ah battery?
Roughly two days at 4.2 peak sun hours, nearer three at 3.0, from half discharged. A 100 W module delivers about 325 Wh a day once the losses above come out.
Can a solar panel be too big for a battery?
Yes. A flooded 100 Ah battery accepts 10 to 13 A, so panel past roughly 190 W is clipped. The same 100 Ah in lithium accepts 50 A, or around 735 W of panel.
Should I use MPPT or PWM?
MPPT above roughly 100 W, or whenever the panel's operating voltage sits well above the battery. A PWM controller drags the panel down to battery voltage and throws the surplus away.
How many peak sun hours should I use?
Your own site, from PVGIS or a national solar map, and the December figure if the system must carry winter. An annual average hides a fourfold seasonal swing up north.
Panel size for every battery from 50 to 400 Ah
Assumes 50 percent discharged, the 14 percent PVWatts loss default, an MPPT controller and a lithium bank at 92 percent round trip. Click a heading to sort; for lead-acid, multiply the panel figures by 1.15.
| Battery | System | Wh to replace | Panel W at 4.2 h | Panel W at 3.0 h | Amps into the battery at 4.2 h | Controller |
|---|---|---|---|---|---|---|
| 50 Ah | 12 V | 300 | 90 | 130 | 6 | 10 A |
| 50 Ah | 24 V | 600 | 185 | 260 | 6 | 10 A |
| 50 Ah | 48 V | 1,200 | 370 | 515 | 6 | 10 A |
| 100 Ah | 12 V | 600 | 185 | 260 | 13 | 15 A |
| 100 Ah | 24 V | 1,200 | 370 | 515 | 13 | 15 A |
| 100 Ah | 48 V | 2,400 | 735 | 1030 | 13 | 15 A |
| 200 Ah | 12 V | 1,200 | 370 | 515 | 25 | 30 A |
| 200 Ah | 24 V | 2,400 | 735 | 1030 | 25 | 30 A |
| 200 Ah | 48 V | 4,800 | 1475 | 2065 | 25 | 30 A |
| 300 Ah | 12 V | 1,800 | 555 | 775 | 38 | 40 A |
| 300 Ah | 24 V | 3,600 | 1105 | 1550 | 38 | 40 A |
| 300 Ah | 48 V | 7,200 | 2210 | 3095 | 38 | 40 A |
| 400 Ah | 12 V | 2,400 | 735 | 1030 | 50 | 60 A |
| 400 Ah | 24 V | 4,800 | 1475 | 2065 | 50 | 60 A |
| 400 Ah | 48 V | 9,600 | 2950 | 4125 | 50 | 60 A |
The 4.2 hour column is a year-round design figure for most of the continental United States; 3.0 is closer to a Pacific Northwest winter.
The 3.0 hour column applies across northern and central Europe; 4.2 suits Iberia, southern Italy and Greece year-round.
Two patterns fall out. Panel watts track amp-hours times voltage, so a 48 V bank of the same amp-hours needs four times the array. Charging current tracks amp-hours alone, because the extra watts arrive at a higher voltage, which is why the controller column repeats down each group of three.
The ceiling the battery sets
Panel size has an upper bound with nothing to do with sunshine. Every battery has a maximum charge current, and past it the controller holds back.
| Chemistry | Charge current limit | On a 100 Ah bank | Panel that saturates it, 12 V | Source |
|---|---|---|---|---|
| Flooded lead-acid | 0.10 to 0.13 C | 10 to 13 A | 145 to 190 W | Trojan Battery |
| AGM and gel | 0.2 C preferred maximum | 20 A | 295 W | Victron Energy |
| LiFePO4 | 0.5 C recommended | 50 A | 735 W | Battle Born |
Trojan asks for a charger rated at 10 to 13 percent of the 20-hour capacity on its flooded batteries. Victron says charge current for AGM and gel should preferably not exceed 0.2 C, because above that the battery warms by more than 10 °C. Battle Born gives 0.5 C on its 100 Ah lithium module.
Lead-acid refuses to charge quickly however much roof you cover. Lithium takes roughly four times the current, which is why a small lithium bank keeps up with a big array.
Charge controller amps, MPPT against PWM
An MPPT controller is rated by its output current into the battery, so panel watts divided by charging voltage gives the number. Victron's SmartSolar MPPT 100/30, a 30 A unit, is specified for 440 W of array on a 12 V battery and 880 W on a 24 V one. Same amps, double the watts.
A PWM controller has no converter. It closes a switch between array and battery, so the panel is dragged down to battery voltage. Take Renogy's RNG-100D-SS, the 36-cell revision of its 100 W 12 V module: 20.4 V at maximum power, 4.91 A, 5.21 A short circuit. Charging at 14.4 V, PWM delivers 4.91 × 14.4 = 71 W. An MPPT turns the spare 6 V into current and delivers about 98 W.
That 38 percent gap on paper shrinks outdoors, because a hot panel's operating voltage sags toward the battery. Laguado-Serrano and colleagues built one of each controller and ran them side by side on a 30 W panel and a 12 V 18 Ah battery: the MPPT unit averaged 14.9 percent better efficiency, and it did so while the PWM unit had the kinder weather (Scientia et Technica, Vol. 24 No. 1, March 2019). The advantage widens in cold and narrows in heat.
- PWM only works when the array's nominal voltage matches the battery.
- MPPT is what lets a cheap 60- or 72-cell roof module, whose maximum-power voltage sits in the thirties, feed a 12 V battery at all.
- Size either controller past the array: NFPA 70 (National Electrical Code) 690.8(A)(1) takes 125 percent of the combined short-circuit current.
- A long cable run is not in the 14 percent figure — check it on the DC wire size calculator.
Peak sun hours where you actually live
Peak sun hours is a day's irradiation expressed as hours at 1,000 W per square meter. These come from the European Commission's PVGIS tool at optimum tilt, as annual daily averages, so design a winter-critical system on the December column.
| Location | Annual average | December | June | Optimum tilt |
|---|---|---|---|---|
| Phoenix, Arizona | 6.7 | 5.2 | 7.4 | 33° |
| Los Angeles, California | 6.2 | 4.6 | 6.8 | 32° |
| Denver, Colorado | 5.7 | 4.2 | 6.7 | 38° |
| Austin, Texas | 5.5 | 4.1 | 6.4 | 28° |
| Atlanta, Georgia | 5.3 | 3.7 | 6.3 | 32° |
| Boston, Massachusetts | 4.7 | 2.8 | 5.4 | 39° |
| Chicago, Illinois | 4.3 | 1.8 | 6.1 | 35° |
| Seattle, Washington | 4.1 | 1.4 | 5.7 | 37° |
| Madrid, Spain | 5.8 | 4.1 | 7.0 | 37° |
| Rome, Italy | 5.3 | 3.3 | 6.7 | 37° |
| Paris, France | 3.9 | 1.8 | 5.5 | 39° |
| Munich, Germany | 3.9 | 1.9 | 5.2 | 39° |
| Berlin, Germany | 3.6 | 1.2 | 5.6 | 40° |
| Amsterdam, Netherlands | 3.5 | 1.1 | 5.3 | 40° |
| Stockholm, Sweden | 3.4 | 0.6 | 5.8 | 44° |
| Dublin, Ireland | 3.3 | 1.4 | 4.8 | 41° |
Chicago drops from 6.1 hours in June to 1.8 in December, Stockholm from 5.8 to 0.6. A panel sized on the annual average refills a battery in a day in July and takes a week in January. Where winter is that brutal, carry more panel than summer needs, or a generator.
Sources
Loss set: NREL PVWatts Version 5 Manual, NREL/TP-6A20-62641, September 2014, Table 6 (soiling 2, shading 3, mismatch 2, wiring 2, connections 0.5, light-induced degradation 1.5, nameplate 1, availability 3 percent, combined by Equation 9 to 14 percent). Peak sun hours: European Commission Joint Research Center PVGIS 5.2 grid-connected PV tool, optimum fixed tilt, retrieved September 2026 — PVGIS-NSRDB for North America, PVGIS-SARAH2 for Europe. Charge current limits: Trojan Battery charger-selection guidance (10 to 13 percent of 20-hour capacity); Victron Energy GEL and AGM Batteries datasheet, section 15 (0,2 C); Battle Born BB10012 datasheet, revision 12.26.2024 (0.5 C recommended, 50 A maximum). Efficiency: Victron 12,8 and 25,6 Volt Lithium-Iron-Phosphate Smart datasheet (92 percent round trip for LFP, 80 percent for lead-acid). Controller: Victron SmartSolar MPPT 100/30 technical specifications (440 W of array at 12 V, 880 W at 24 V). Panel: Renogy RNG-100D-SS G3 datasheet, 36-cell revision. Code: NFPA 70 690.8(A)(1). Controller comparison: M. A. Laguado-Serrano and others, “Performance comparison between PWM and MPPT charge controllers”, Scientia et Technica, Vol. 24 No. 1, March 2019, Universidad Tecnológica de Pereira. Figures checked September 2026.
Go deeper
With the panel settled, size the whole system on the off-grid solar sizing calculator, which works from daily loads and days of autonomy. Size storage with the battery bank calculator, read the chemistry trade in LiFePO4 against lead-acid sizing, and check state of charge on the LiFePO4 voltage chart. Portable packs are covered in the solar panels for power stations guide.