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Solar Panel Charge Time Calculator (UK)

Free UK solar panel charge time calculator. Estimate how many sun hours or days your solar array needs to recharge a leisure or off-grid 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 values and the calculator returns charge time in hours and days plus a verdict on whether your array is sized correctly:

  1. Battery capacity (Ah) — printed on the case. A typical UK leisure battery is 75–110 Ah; a static caravan or off-grid cabin bank might be 200–450 Ah.
  2. Battery voltage — usually 12 V for caravans, motorhomes and small systems, 24 V or 48 V for stand-alone cabins and whole-property off-grid setups.
  3. Depth of discharge (%) — how empty the battery currently is. 50% is the standard lead-acid daily target; LiFePO₄ tolerates 80–100%.
  4. Panel total wattage — the sum of every panel’s STC nameplate rating (e.g. two 200W panels on a campervan roof = 400W).
  5. Peak sun hours per day — for your location and season (see the FAQ for UK regional averages).
  6. System efficiency (%) — leave at 75% unless you have a clean MPPT plus 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, including those certified under MCS Standard 3005:

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

A worked example for a typical UK motorhome:

  • 100 Ah × 12 V × 0.50 = 600 Wh to recover from 50% DoD
  • 200 W × 4 h × 0.75 = 600 Wh delivered per UK summer day
  • 600 ÷ 600 = 1.0 day of clear summer sun

And for a Welsh off-grid cabin in winter:

  • 400 Ah × 24 V × 0.60 = 5,760 Wh to recover from 60% DoD
  • 800 W × 1.2 h × 0.75 = 720 Wh delivered per UK December day
  • 5,760 ÷ 720 = 8 days of clear winter sun — which is why winter generators are standard in Welsh and Scottish off-grid setups.

Charge time reference table (UK)

Common scenarios using 4 peak sun hours (UK summer average) and 75% system efficiency, starting from 50% depth of discharge:

BatteryPanel arrayEnergy neededDaily outputCharge time
12V / 75 Ah100 W450 Wh300 Wh1.5 days
12V / 100 Ah100 W600 Wh300 Wh2.0 days
12V / 100 Ah200 W600 Wh600 Wh1.0 day
12V / 200 Ah400 W1,200 Wh1,200 Wh1.0 day
24V / 200 Ah600 W2,400 Wh1,800 Wh1.3 days
48V / 400 Ah2,000 W9,600 Wh6,000 Wh1.6 days

For UK winter, divide daily output by 2.5 — a setup that recovers in one summer day will take 2–3 December days, and longer in Scotland.

Common UK scenarios

Caravan and motorhome

A 100–200 W roof panel and a single 100 Ah leisure battery is the standard configuration sold by every UK touring dealer. With a 50% nightly draw from a fridge and lighting, a 100 W panel just keeps pace in summer; 200 W gives genuine headroom for cloudy weeks. For year-round touring (including winter pitches in Cornwall or the Lake District), upsize to 300 W and consider lithium.

Static caravan or off-grid cabin (weekend use)

Battery sized for two days of autonomy, panel array sized to recharge in two days of average sun. A 200 Ah / 12 V bank with 400 W of panels suits a Welsh weekend cabin running lights, a 12 V fridge and a small TV. See the solar wire size calculator when planning the run from rooftop array to interior battery — UK cabin layouts often mean 8–12 m of cable, and the losses add up fast at 12 V.

Whole-property off-grid (rural Wales, Scotland, Cornwall)

48 V system voltage, 600+ Ah lithium battery, 4–8 kW of panels and MPPT charge controllers. At this scale you’re sizing the array for one-day-of-autonomy recharge under worst-case (December) sun hours, which means substantial summer overproduction. Pair with a wind turbine or LPG generator for January–February, when southern UK averages just 1.0–1.5 peak sun hours per day.

Backup battery for grid-tied (FIT or SEG export plus self-consumption)

You usually don’t size the panel array for battery recharge — you size it for daily home consumption and the battery cycles in the background. Charge time only matters during outages, which is when panel orientation and tilt make the most difference. Winter storms cause most UK power cuts and are also when sun hours are lowest — so a battery sized for 24 hours of essential loads is the realistic target.

What the calculator deliberately ignores

  • Solar irradiance variation across the day. Real UK production curves are a bell shape, sharply asymmetric in winter when the sun rises late and sets early. 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 and float on lead-acid systems, which is why most UK MPPT controllers default to a 14.4 V absorption stage.
  • 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.
  • UK winter cold derating. Below 0°C, lithium charging should be disabled (most BMS units do this automatically); below -10°C, lead-acid acceptance drops by half. Relevant for Scottish and Pennine off-grid sites.

Sizing rule of thumb (UK)

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

  • Panel watts ≈ Battery Wh × 0.7 for summer UK use (4 peak sun hours)
  • Panel watts ≈ Battery Wh × 1.5 for year-round UK use (averaging 2.5 sun hours)
  • Panel watts ≈ Battery Wh × 3.0 for winter UK use (1.0–1.5 sun hours)

This sizes for a clear day. For dependable off-grid power across British weather, multiply by a further 1.5–2× to handle 3–5 day overcast stretches without running the bank flat — a particular issue in November and February when frontal systems can park over the country for a week at a time.

Cost context (UK 2026)

A typical leisure-battery upgrade from a UK touring caravan dealer in 2026:

  • 100W mono panel + PWM controller + cabling + leisure battery: £180–£260 from Halfords or Tayna
  • 200W mono panel + 20A MPPT + cabling + 110Ah AGM battery: £350–£480
  • 400W array + 40A MPPT + 100Ah LiFePO₄: £900–£1,300

For full off-grid systems, MCS-certified installers quote £8,000–£14,000 for a 4 kW array plus a 5 kWh lithium bank installed (Solar Energy UK 2026 average). Compare quotes from multiple installers via Checkatrade or MyBuilder before committing — the spread between the cheapest and most expensive MCS quote is typically 40%.

Sources

Frequently asked questions

How long does a 100W solar panel take to charge a 100Ah battery in the UK?
From a 50% depth of discharge in summer: roughly 2 days assuming 4 peak sun hours and 75% system efficiency. The maths: a 12V/100Ah battery at 50% DoD needs 600 Wh; a 100W panel produces around 300 Wh of usable energy per UK summer day (100W × 4h × 0.75). 600 ÷ 300 = 2 days. In winter, with peak sun hours dropping to 1.5 in the south of England and as low as 0.8 in Scotland, the same recovery takes 5–8 days. This is why UK off-grid systems are usually oversized 3–4× for winter operation — Energy Saving Trust guidance reflects this in its caravan and motorhome figures.
What is 'peak sun hours' for the UK and what value should I use?
Peak sun hours is the equivalent number of hours per day your location receives 1,000 W/m² of solar irradiance. The Met Office and Energy Saving Trust publish annual averages: London and the south of England average roughly 2.7 kWh/m²/day; the Midlands sit around 2.5; Manchester and northern England get about 2.3; Edinburgh and central Scotland average 2.2; the Highlands drop to 2.0. For seasonal extremes, summer (June–August) doubles those values and winter (December–February) cuts them in half. For year-round leisure battery sizing use 2.5; for off-grid winter design use 1.5 in the south, 1.0 in Scotland.
Why does the calculator use 75% system efficiency instead of 100%?
Real UK solar charging loses energy at four points: charge controller (PWM around 70%, MPPT around 92%), wiring resistance (a 2–4% drop on a properly sized cable run), panel temperature derating (panels rated at 25°C — UK summer roof temperatures still reach 50°C, costing about 10% output), and battery round-trip efficiency (lead-acid 80–85%, LiFePO₄ 92–96%). Compounded, total system efficiency lands at 70–80% in typical UK conditions. The 75% default matches MCS Standard 3005 design guidance for stand-alone PV systems.
Can I charge a 12V leisure battery faster with 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 cable runs (drop is proportional to current squared). The MPPT controller then converts the high-voltage DC down to battery voltage at 92–96% efficiency. PWM controllers can't do this conversion — they pin panel voltage to battery voltage and waste the surplus. For caravans and motorhomes the cable runs are short enough that PWM still works; for campervan-to-roof rack runs of 5 m or more, MPPT pays back inside the first season.
How long to charge a 100Ah leisure battery from a 200W panel in the UK?
About 1 day in summer (4 peak sun hours × 200W × 0.75 = 600 Wh, exactly matching the 50% DoD recovery). In a UK April or September shoulder season, expect 1.5–2 days. In December, recovery from a single night's discharge can take 4 days or longer in the north — at which point the battery effectively sits flat through winter and you depend on EHU (electrical hook-up) at a campsite or a generator top-up.

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