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Solar Panel Charge Time Calculator

Free solar panel charge time calculator. Find out how many sun hours or days your solar array needs to recharge your battery from any depth of discharge.

Solar Panel Charge Time Calculator

Energy needed
600 Wh
Sun hours to full
7.5
peak sun-hours
Days to full
1.5
at 5 sun-hr/day

How to use this calculator

Enter six numbers and the calculator returns charge time in hours or days plus a verdict on whether your array is sized correctly:

  1. Battery capacity (Ah) — printed on the case. A typical RV deep-cycle is 100 Ah; an off-grid bank might be 400–800 Ah.
  2. Battery voltage — usually 12 V for vehicles/small systems, 24 V or 48 V for cabins and whole-house setups.
  3. Depth of discharge (%) — how empty the battery is right now. 50% is the typical lead-acid daily target; LiFePO₄ tolerates 80–100%.
  4. Panel total wattage — the sum of every panel’s STC nameplate rating (e.g. four 200W panels = 800W).
  5. Peak sun hours per day — for your location and season (see the FAQ).
  6. System efficiency (%) — leave at 75% unless you have a clean MPPT + LiFePO₄ setup, in which case 85% is reasonable.

The formula

The calculator uses the standard energy-balance equation that every off-grid solar designer applies:

energyNeeded (Wh) = batteryAh × batteryV × (depthOfDischarge / 100)
dailyEnergy (Wh)  = panelW × peakSunHours × (efficiency / 100)
days              = energyNeeded / dailyEnergy

A worked example for a typical RV setup:

  • 100 Ah × 12 V × 0.50 = 600 Wh to recover from 50% DoD
  • 200 W × 5 h × 0.75 = 750 Wh delivered per sunny day
  • 600 ÷ 750 = 0.80 days (about 6.4 peak-sun hours)

And for an off-grid cabin pulling deeper from a larger bank:

  • 400 Ah × 24 V × 0.60 = 5,760 Wh to recover from 60% DoD
  • 800 W × 4 h × 0.75 = 2,400 Wh delivered per sunny day
  • 5,760 ÷ 2,400 = 2.4 days of clear sun

If the result exceeds 1 day for typical daily use, the array is undersized for the battery — add more panels or accept that the bank won’t recover between draws.

Charge time reference table

Common scenarios using 5 peak sun hours per day, 75% system efficiency, and a 50% depth of discharge starting point:

BatteryPanel arrayEnergy neededDaily outputCharge time
12V / 50 Ah100 W300 Wh375 Wh0.8 day (6.4 hrs)
12V / 100 Ah100 W600 Wh375 Wh1.6 days
12V / 100 Ah200 W600 Wh750 Wh0.8 day (6.4 hrs)
12V / 200 Ah400 W1,200 Wh1,500 Wh0.8 day (6.4 hrs)
24V / 200 Ah600 W2,400 Wh2,250 Wh1.07 days
48V / 400 Ah2,000 W9,600 Wh7,500 Wh1.28 days

The pattern: panel watts roughly equal to battery Wh × 0.5 gives a “charges in one sunny day from 50% DoD” result. That’s a reasonable design target for daily-use off-grid systems.

Common scenarios

RV or van life

A 200–400 W array on the roof and a single 100 Ah lithium battery is the standard configuration. With a 50% nightly draw, a 200 W array recovers in about a day of decent sun; 400 W gives you headroom for cloudy days.

Off-grid cabin (weekend use)

Battery sized for two days of autonomy, panel array sized to recharge in two days of average sun. A 400 Ah / 12 V bank with 800 W of panels is typical for a weekend cabin running lights, fridge, and electronics. See the solar wire size calculator when planning the run from rooftop array to interior battery bank — the lengths add up fast on a cabin.

Whole-house off-grid

48 V system voltage, 400+ Ah battery, 4–8 kW of panels, MPPT charge controllers. At this scale you’re typically sizing the array for one-day-of-autonomy recharge under worst-case (winter) sun hours, which means much larger summer overproduction.

Backup battery for grid-tied (charge-from-grid + solar)

You usually don’t size the panel array for battery recharge in this case — you size it for daily home consumption, and the battery cycles in the background. Charge time only matters during outages, which is when the solar panel orientation and tilt make the most difference (winter sun is lowest, outages are most common in storm season).

What the calculator deliberately ignores

  • Solar irradiance variation across the day. Real production curves are a bell shape, not a flat block. The “peak sun hours” abstraction handles this for energy totals — but if your charge controller can’t accept the midday peak current (undersized for array Imp), you’ll lose more than the 75% default suggests.
  • Battery state-of-charge taper. The last 10–20% of a lead-acid charge cycle takes the same time as the first 80% because the battery accepts current more slowly as it fills. The calculator models the bulk phase only — add 1–2 hours for absorption + float on lead-acid systems.
  • Charge-rate limits. Lithium batteries accept up to 1 C charge rate (a 100 Ah battery can take 100 A). Lead-acid is typically 0.1–0.2 C (10–20 A for 100 Ah). If your array delivers more current than the battery accepts, the surplus is wasted. Check your battery’s max charge current spec before trusting “fast” charge times.
  • Temperature derating. Cold batteries accept charge slowly. Below 0°C, lithium charging should be disabled; below -10°C, lead-acid acceptance drops by half.

Sizing rule of thumb

If you want a system that fully recovers from one normal day’s discharge by sunset:

  • Panel watts ≈ Battery Wh × 0.5 (for 5 peak sun hours)
  • Panel watts ≈ Battery Wh × 0.7 (for 4 peak sun hours, northern US winter)
  • Panel watts ≈ Battery Wh × 0.4 (for 6 peak sun hours, desert southwest)

This sizes for a clear day. For dependable off-grid power, multiply panel watts by 1.5–2× to handle 2–3 day cloudy stretches without running the bank flat.

Sources

Frequently asked questions

How long does a 100W solar panel take to charge a 100Ah battery?
From 50% depth of discharge: roughly 1.6 days assuming 5 peak sun hours and 75% system efficiency. The math: a 12V/100Ah battery at 50% DoD needs 600 Wh; a 100W panel produces about 375 Wh of usable energy per day (100W × 5h × 0.75). 600 ÷ 375 = 1.6 days. From a fully discharged battery (100% DoD) it takes about 3.2 days, although discharging lead-acid batteries that deeply repeatedly will shorten their life.
How long does a 200W solar panel take to charge a 100Ah battery?
About 0.8 days (roughly 6.4 hours of peak sun) to recover from 50% depth of discharge. A 200W panel doubles daily production from a 100W setup to about 750 Wh/day, so the same 600 Wh battery deficit fills in just over half a day. In practice you'd want a battery to charge inside one solar day, which is the rule of thumb most off-grid sizing follows.
What is 'peak sun hours' and what value should I use?
Peak sun hours are the equivalent number of hours per day your location receives 1,000 W/m² of solar irradiance — roughly the intensity of bright midday sun. Most of the continental US sits between 4 and 5.5 peak sun hours per day on annual average, with the desert southwest reaching 6+ and the Pacific Northwest dropping to 3.5–4. NREL publishes location-specific values in its National Solar Radiation Database; for a quick estimate use 5 if you're at southern US latitudes and 4 if northern.
Why use 75% system efficiency instead of 100%?
Real solar charging loses energy at four points: charge controller (PWM ≈ 70%, MPPT ≈ 92%), wiring resistance (2–4% drop on a properly sized run), panel temperature derating (panels rated at 25°C lose ~0.4% per °C above that), and battery round-trip efficiency (lead-acid ≈ 80–85%, LiFePO₄ ≈ 92–96%). Compounded, these knock total system efficiency to 70–80% in typical conditions. The 75% default matches NREL PVWatts for off-grid lead-acid systems.
Can I charge a 12V battery faster by using a higher-voltage panel array?
Yes, with an MPPT charge controller. Wiring panels in series doubles or triples the array's voltage while keeping current the same, which dramatically reduces wire losses on long runs (drop is proportional to current squared). The MPPT controller then converts the high-voltage DC down to battery-charging voltage with about 92–96% efficiency. PWM controllers can't do this conversion — they pin panel voltage to battery voltage and waste the extra capacity.

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