What Size Solar Panel to Charge a 12V Battery (Wattage Guide)

Quick sizing rules that work

Use these practical shortcuts if you don’t want to calculate everything:

• Maintenance / float charging (stored battery): 10–30W per battery (keeps up with self-discharge and small parasitic loads).
• Light use (a few LED lights, occasional phone charging): 50–100W.
• Typical weekend/van/camping loads: 100–200W (common sweet spot for 12V systems).
• Daily heavier use (fridge, long runtimes, cloudy margin): 200–400W+.

These ranges assume you want the panel to do real charging work (not only maintain) and that you experience typical losses from heat, wiring, and the charge controller.

The simple formula for “what size solar panel to charge a 12V battery”

To size a solar panel correctly, start from energy you need to put back into the battery each day.

Step 1: Convert daily usage to watt-hours (Wh)

If you track usage in amp-hours (Ah) on a 12V system:
Daily Wh ≈ Daily Ah × 12

Step 2: Account for real-world losses

Solar systems don’t deliver nameplate power all day. A practical overall efficiency factor is 0.65–0.80 (includes temperature derating, controller losses, wiring, and non-ideal sun angle). If you want a safe default, use 0.70.

Step 3: Use peak sun hours (PSH)

Peak sun hours are the “equivalent full-sun” hours per day. Common planning values:

• 2–3 PSH: winter, cloudy regions, shaded installs
• 4 PSH: conservative annual average for many locations
• 5–6 PSH: sunny seasons / good tilt / clear skies

Final sizing equation

Panel Watts ≈ Daily Wh ÷ (PSH × Efficiency)
Using defaults (12V system, 4 PSH, 0.70 efficiency):
Panel Watts ≈ (Daily Ah × 12) ÷ (4 × 0.70) ≈ Daily Ah × 4.3

Worked examples (real numbers)

Example A: 12V 100Ah battery, replacing 20Ah/day

Daily energy to replace: 20Ah × 12V = 240Wh
Assume 4 PSH and 0.70 efficiency:
Panel watts ≈ 240 ÷ (4 × 0.70) = 240 ÷ 2.8 ≈ 86W
Practical choice: 100W (or 150W if you want margin for clouds, shade, and winter).

Example B: 12V 100Ah battery, replacing 40Ah/day (heavier daily use)

Daily energy: 40Ah × 12V = 480Wh
Panel watts ≈ 480 ÷ 2.8 ≈ 171W
Practical choice: 200W (often the point where a 100Ah battery feels “solar-supported” daily).

Example C: Maintenance only for a stored 12V battery

If the battery is mostly idle and you only need to offset self-discharge and small standby loads, 10–30W is commonly sufficient, especially with a proper controller. Use the higher end if the battery sits hot, you have parasitic draws, or you get limited sun.

Panel size table for common 12V battery setups

The “Typical Daily Use” column assumes you want the battery to recover daily in average conditions. If you’re in winter sun, have shading, or run higher loads, move up one bracket.

How long will it take to charge a 12V battery with solar?

Charging time depends on how depleted the battery is, plus solar conditions. A useful approximation:
Charge hours ≈ (Ah to replace × 12) ÷ (Panel Watts × Efficiency)

Example: Replace 50Ah on a 12V battery using a 200W panel

Energy to replace: 50Ah × 12V = 600Wh
Effective panel output: 200W × 0.70 ≈ 140W
Time in strong sun: 600Wh ÷ 140W ≈ 4.3 hours of “good” sun (not clock time—sun quality matters).

Real charging slows near the top because the controller reduces current during absorption. So even if the math says 4 hours, plan extra time if you need a true 100% charge.

Charge controller choice affects panel sizing

The controller doesn’t change the sun, but it affects how efficiently panel power becomes battery charge—especially in cold weather or when panel voltage is higher than battery voltage.

PWM vs MPPT (practical impact)

• PWM: Simple and cheaper, but often leaves power on the table when panel voltage is much higher than battery voltage.
• MPPT: More efficient energy harvest, especially in colder temps and with higher-voltage panels; commonly yields noticeably more usable charge per day.

If you’re sizing tightly (you “must” recharge daily), MPPT gives you more margin and can justify a slightly smaller panel for the same outcome—though many people still oversize panels for cloudy days.

Critical sizing details people miss

Battery chemistry changes your charging expectations

• Lead-acid (AGM/Flooded/Gel): Needs absorption time; slow top-off is normal. Avoid chronic undercharging.
• LiFePO4: Accepts higher current efficiently and reaches near-full faster; still needs correct charge profile.

Panel “12V” labeling can be misleading

A “12V solar panel” usually has a Vmp around ~18V (to charge a 12V battery properly). That’s normal. What matters most for sizing is wattage, not the marketing voltage label.

Shading can cut output dramatically

Even partial shade can slash production. If your panel will see shade from roof racks, trees, or vents, consider oversizing by 25–50% versus the calculated minimum.

You can’t “force” a faster charge than safe limits

Panel size should also respect charge current limits. A common planning guideline:
Lead-acid comfortable bulk charge current is often around 0.1C–0.2C (10–20A for a 100Ah battery), though specifics vary by model.
At ~14V charging voltage, 20A is ~280W into the battery (before losses). This is why 200W on a 100Ah 12V system is usually “strong but reasonable”.

Practical checklist to choose the right panel size

If you want a dependable choice without overthinking:

1. Estimate your daily draw in Ah (or Wh). If unsure, start with a measured average from a monitor.
2. Pick PSH: use 4 for a conservative general estimate, 3 if winter/shade is common.
3. Use an efficiency factor of 0.70 unless your install is premium and unshaded.
4. Compute watts: Panel W ≈ (Daily Ah × 12) ÷ (PSH × 0.70).
5. Round up to the next common panel size (100W, 200W, 300W) and add margin if you expect clouds or shade.
6. Match a charge controller with adequate current rating (panel watts ÷ ~14V gives a quick controller-amps estimate).

If you only remember one takeaway: for a typical 12V battery system that needs daily recovery, oversizing to 100–200W is usually the difference between “sometimes charges” and “consistently charges.”