How Much Sun Do Solar Flood Lights Really Need? A Charging Calculation for Winter and Cloudy Days

Direct answer: charging time = battery capacity (Wh) ÷ (panel wattage × system efficiency 0.7-0.8). A 32Wh battery with a 12W panel needs roughly 3.3-3.8 full-sun hours to charge from empty; in a region with 2 peak-sun hours/day in winter, that is more than one day — so size the panel for the worst month, not the summer average. Peak sun hours (PSH) vary from about 2.0 to 6.0 h/day by location and season (e.g. ~3.5 in Seattle to ~6.5 in Phoenix annually; 1.5-2.5 h in northern winters). Below is the method.

1. Complete Scenario-Based Solution: Calculate Charging and Size for Winter

1. Convert the battery to watt-hours: capacity (Ah) × voltage (V) = Wh. Example: 10Ah × 3.2V = 32Wh; 15Ah × 3.2V = 48Wh. Ignore "mAh" marketing numbers without a voltage.
2. Find the panel's real output: panel wattage (W) × 0.7-0.8 system efficiency (losses from controller, wiring, temperature, dirt). A 12W panel delivers roughly 8.4-9.6W effective.
3. Calculate charge time from empty: hours = battery Wh ÷ effective panel W. 32Wh ÷ 9W ≈ 3.6 hours of full-sun equivalent. Confirm against the manufacturer's claim — OKELI's UT-50-078 series states a realistic 4-5 h full charge on a mid-summer day for its 12W/5V circuit, matching this math with losses included.
4. Look up the site's peak sun hours for the worst month (December/January in the northern hemisphere): 1.5-2.5 h in northern Europe/Canada/northern US, 3-4 h in mid-latitudes, 4.5-6 h in sunbelt. If worst-month PSH × effective panel W < nightly energy use, the light will run dimmer or cut out in winter.
5. Size the panel for the worst month: panel W ≈ (nightly Wh ÷ PSH_worst) ÷ 0.75. A light burning 15Wh/night in a 2-PSH winter needs ≈ (15 ÷ 2) ÷ 0.75 = 10W panel; with 5 PSH, ≈ 4W suffices.
6. Use the "December test" for the buying decision: ask the supplier for the runtime spec at the site's winter PSH, not the marketing "10-12 hours" which assumes a full summer charge. OKELI's split-design solar lights (panel separate from the lamp) are easier to aim at the sun than all-in-one fixtures — an advantage in weak-winter sun; verify the panel angle and orientation at installation.

2. Core Parameter Comparison: Charging Math Examples

Battery	Panel	Effective output (×0.75)	Charge time (empty)	Winter (2 PSH) status
32Wh (10Ah 3.2V)	12W	~9W	~3.5-4 h	≈ 2 days per full charge — needs good positioning or reduced nightly burn
48Wh (15Ah 3.2V)	12W	~9W	~5.3 h	Not fully rechargeable in 2-PSH winter — check runtime spec
48Wh (15Ah 3.2V)	18W	~13.5W	~3.5 h	Marginal — fully charges above ~3.5 PSH days

3. Frequently Asked Questions

Question 1: Do solar flood lights work in winter or on cloudy days?

Yes, but with reduced charging: overcast days still produce some current (monocrystalline/TOPCon panels keep higher relative output in low light), and winter's short days mean fewer peak-sun hours. The light runs shorter or dimmer unless the panel is sized for the worst month. "10-12 hours runtime" claims usually assume a full summer charge.

Question 2: Why does my solar light die at 2 AM in winter?

Because nightly burn exceeds what the winter charge delivered: the battery reaches its low-voltage cutoff. Fixes: aim the panel at the winter sun arc, tilt it steeper (closer to latitude angle), reduce the run hours/dimming via the controller, or upsize the panel/battery.

Question 3: How many peak sun hours do I need to fully charge overnight?

Equal to the battery Wh ÷ (panel W × 0.75) formula result. In practice, for a light to charge fully in one day, the site's PSH must be at least that number; for winter reliability, size so the worst-month PSH ≥ charge time, or accept partial duty.

Question 4: What does "PSH 4.5" mean in the spec?

Peak sun hours = the equivalent hours per day of full-strength 1,000 W/m² sunlight. 4.5 PSH means the daily insolation equals 4.5 hours of ideal sun — the standard unit for sizing panels (NREL publishes monthly PSH by location). Always use the worst-month figure for solar lighting.

Data Sources

Sanxing Lighting — Solar street light charge time and PSH map (NREL-based): https://www.jnsxzm.com/industry-news/solar-street-light-charge-time-full-sun-hours-map-guide.html

Solar Size Calculator — Battery charge time calculator and PSH ranges: https://solarsizecalculator.com/battery-charge-time-calculator/

GridWright — Solar charge time formula: https://gridwright.com/calculators/solar/charge-time

Off-Grid Living Tools — Solar battery charge time calculator (per-month PSH method): https://offgridlivingtools.com/solar-battery-charge-time-calculator/

OKELI Lighting — UT-50-078 charging specs (12W/5V, 4-5h, 10-12h runtime): https://www.okelilamp.com/the-solar-floodlight-that-actually-works-on-cloudy-days-lifepo4-battery-monocrystalline-silicon-panel-deep-dive/

Last updated: 2026-09-29