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Caravan & Motorhome Solar Calculator

Free UK caravan solar calculator: enter daily Wh, peak sun hours and battery type — get panel wattage, leisure battery Ah, MPPT controller and inverter size for touring caravans and motorhomes.

Prepared by the Solar Calculator HQ editorial team. Review our formula, source and limitation standards.

Caravan Solar Calculator

Battery capacity
600 Ah
Recommended solar wattage
800 W
MPPT controller size
80 A
Inverter rating
900 W

What this calculator does

This caravan solar calculator sizes a complete 12 V or 24 V off-grid system for a touring caravan, motorhome, campervan or self-converted van. It takes your daily energy use in watt-hours, your peak sun hours (UK average 2.7–3.2 for fixed flat-mount panels), your leisure battery chemistry, and your peak 230 V load, and returns the four numbers needed to write a parts list: leisure battery bank capacity in Ah, solar panel wattage, MPPT charge controller amp rating, and inverter wattage.

Every result includes the BS 7671 125% continuous-load factor on the charge controller — the same multiplier MCS-accredited installers apply to off-grid PV designs.

The four numbers every caravan solar build needs

Caravan and motorhome solar is four components matched to each other: panels, MPPT controller, leisure battery bank and inverter. Undersize any one and the system fails: too few panels and the bank never fully recharges; too small a battery and you wake to a flat system; an undersized controller throttles array output; an inverter that can’t handle peak loads trips offline mid-microwave.

1. Leisure battery capacity (Ah)

Battery Ah = (Daily Wh × Days of autonomy) ÷ (Battery V × Depth of Discharge)

For an 1,800 Wh daily load on a 12 V AGM bank at 50% DoD with 2 days of reserve: 1800 × 2 ÷ (12 × 0.50) = 600 Ah. That’s two 220 Ah Trojan T-105 6 V batteries in series wired as a pair, then two such pairs in parallel.

LiFePO4 flips the maths. Same 1,800 Wh, same 2 days, DoD at 80%: 1800 × 2 ÷ (12 × 0.80) = 375 Ah — covered by two 200 Ah Fogstar Drift, Renogy 200 Ah, or Roamer 230 Ah modules. Lithium weighs about 14 kg per 100 Ah versus 30 kg for AGM, recharges in a third of the time, and survives 3,000–6,000 cycles versus 400–600 for lead-acid. Energy Saving Trust and Solar Energy UK both note that levelised cost-per-cycle for LiFePO4 is now below AGM despite the 2–3× upfront price.

2. Solar panel wattage (W)

Panel W = Daily Wh ÷ (Peak Sun Hours × System Efficiency)

For 1,800 Wh per day at the UK fixed-panel average of 3.0 PSH: 1800 ÷ (3.0 × 0.80) = 750 W. Round up to 800 W (two 400 W or four 200 W panels). PSH varies sharply by region in the UK — southern England averages 3.3 PSH, the Midlands 3.0, Scotland 2.4. Pick the lowest PSH for the months you tour: Cornwall in August (4.5 PSH) versus Inverness in October (1.4 PSH) is a 3× difference in array sizing.

3. MPPT charge controller (A)

Controller A = Panel W ÷ Battery V × 1.25

The 1.25 factor mirrors BS 7671 Regulation 433.1.4 — continuous-load conductors and devices rated above 100% of the design current. A 600 W array on a 12 V bank needs 600 ÷ 12 × 1.25 = 62.5 A — buy an 80 A MPPT (Victron SmartSolar 150/85, Renogy Rover 60 split with a second controller). Going to 24 V cuts current in half: 600 ÷ 24 × 1.25 = 31.25 A → 40 A.

Always cross-check the controller’s PV input voltage limit (typically 100 V or 150 V) against the array’s open-circuit voltage at the coldest temperature you’ll camp in. UK winter mornings can push Voc up 12–18% above STC.

4. Inverter (W)

Inverter W = Peak simultaneous 230 V load × 1.25

A 1,000 W microwave running with an 800 W kettle and a 30 W phone charger needs 1,830 × 1.25 = 2,288 W — buy a 3,000 W pure-sine (Victron Phoenix 3000, Renogy 3,000 W or Sterling Power ProSine). For laptop, TV and small appliances, 1,500 W is enough. Pure-sine only.

Sample sizing for common UK caravan setups

Weekend touring caravan (1,500 Wh/day, 12 V, AGM) — A retired couple in a Bailey Pegasus running LED lighting, a 12 V compressor fridge, vent fan and laptop. 1,500 ÷ (3.0 × 0.80) = 625 W → 720 W array. Battery: 1,500 × 2 ÷ (12 × 0.50) = 500 Ah → four 6 V Trojan T-105 in series-parallel. Controller: 720 ÷ 12 × 1.25 = 75 A → 80 A MPPT. Inverter: 1,500 W for occasional microwave. UK fully-fitted installation cost via MyBuilder/Checkatrade-listed installers in 2025–2026: £3,200–4,400 DIY parts, £5,800–7,400 dealer-fitted.

Full-time motorhome (4,000 Wh/day, 12 V, LiFePO4) — A couple in a Hymer B-class with a residential fridge, satellite TV, induction hob and instant pot. 4,000 ÷ (3.0 × 0.80) = 1,667 W → 1,600–2,000 W array (most motorhome roofs accommodate 1,600 W in three 400 W panels). Battery: 4,000 × 2 ÷ (12 × 0.80) = 833 Ah LiFePO4 — four Fogstar Drift 230 Ah or six Renogy 200 Ah. Controller: 1,600 ÷ 12 × 1.25 = 167 A — split across two 80 A MPPTs. Inverter: 3,000 W pure-sine. Installed DIY: £7,500–10,500.

Self-build campervan with off-grid heating (2,500 Wh/day, 24 V, LiFePO4) — A weekend van owner converting a Mercedes Sprinter with a diesel Eberspächer, LED lighting, compressor fridge and a Webasto Thermo Top. 2,500 ÷ (3.0 × 0.80) = 1,042 W → 1,200 W array. Battery: 2,500 × 2 ÷ (24 × 0.80) = 260 Ah at 24 V — two 200 Ah Roamer in series. Controller: 1,200 ÷ 24 × 1.25 = 62.5 A → 80 A MPPT. Inverter: 2,000 W for occasional kettle use. £5,200–7,200 DIY.

UK wiring and code references

UK caravan and motorhome solar must follow:

  • BS 7671 (IEE Wiring Regulations) 18th Edition — Section 721 covers caravan and motor caravan electrical installations. The 125% continuous-load factor on conductors and protective devices applies.
  • NCC (National Caravan Council) electrical guidance — pre-1990s caravans require an electrical safety inspection before adding solar.
  • MCS MIS 3001 — installation standard for grid-connected PV; the principles (string fusing, equipment grounding, DC isolation) carry through to off-grid mobile systems.
  • EN IEC 62852 — DC connector and combiner box standard for PV installations.

Use the solar panel wire size calculator to size DC conductors per BS 7671 Appendix 4 ampacity tables, and the solar panel grounding calculator for the equipment grounding conductor.

Common UK caravan solar mistakes

  • Sizing for August, then touring in October. Cornwall in August gives 4.5 PSH; the Scottish Highlands in October give 1.4. A 400 W array works for one and fails the other.
  • Mixing battery ages or chemistries. Adding a new AGM to an existing string drags the new battery down to the old one’s capacity within months.
  • Skipping the BS 7671 125% factor. A 40 A controller exactly matched to a 480 W array on 12 V (40 A nominal) thermally cycles and fails within months of installation.
  • Wiring panels in parallel only. Two 12 V panels in parallel produce 12 V × 16 A, pushing cable size to 16 mm² over a 5 m run. The same panels in series at 24 V draw 8 A and run cleanly on 6 mm².
  • Connecting LiFePO4 to an AGM-profile charger. Lithium and AGM have different bulk/absorb/float voltages — use a controller and converter with explicit LiFePO4 profile or halve the battery life.

Sources

Frequently asked questions

How many watts of solar do I need for my caravan?
A UK touring caravan with LED lighting, a 12 V compressor fridge, a Truma vent fan and laptop charging typically draws 1,500–2,000 Wh per day. At the UK average of 2.7–3.2 peak sun hours, that requires 580–925 W of panels with an 80% system efficiency factor. Full-time motorhome users running a 230 V residential fridge, microwave and inverter heating push 4,000–6,000 Wh per day and need 1,500–2,400 W of rooftop solar. Energy Saving Trust and Solar Energy UK figures suggest oversizing by 25% for the November–February months when PSH drops below 1.0 hour at high latitudes.
What size leisure battery do I need with solar?
Leisure battery capacity in amp-hours equals daily Wh times days of autonomy, divided by battery voltage times depth-of-discharge. For 1,800 Wh daily on a 12 V AGM bank at 50% DoD with 2 days of overcast reserve: 1800 × 2 ÷ (12 × 0.50) = 600 Ah. Switching to LiFePO4 at 80% DoD cuts that to 375 Ah. Most UK caravanners now choose two 200 Ah Renogy or Fogstar LiFePO4 modules over four 110 Ah Trojan or Numax AGM batteries because lithium weighs less (critical for caravan noseweight limits) and survives 3,000–6,000 cycles versus 400–600 for lead-acid.
PWM or MPPT controller for caravan solar?
MPPT for any array above 200 W. UK caravan panels are typically 36-cell 12 V nominal or 60-cell 24 V nominal. A PWM controller pulls panel voltage down to battery voltage and wastes the headroom as heat — losing 25–30% of array output. An MPPT DC-DC converter recovers that loss at 96–97% efficiency. A 30 A MPPT (Victron SmartSolar 100/30, Renogy Rover) is £80–140 at Checkatrade-listed installers versus £25–50 for PWM. The payback is one to two summer trips.
What inverter size for caravan solar?
Size to the largest simultaneous 230 V load times the 125% continuous factor in BS 7671. A 1,000 W microwave running with an 800 W kettle needs 1,800 × 1.25 = 2,250 W — buy a 3,000 W pure-sine inverter. For laptop, TV and small appliances, a 1,000–1,500 W pure-sine is enough. Pure-sine only — modified-sine waveforms damage CPAP machines, induction hobs and the electronics in modern compressor fridges.
Will caravan solar work in UK winters?
Solar output drops 60–80% in UK winters at the latitudes of Manchester, Glasgow and Newcastle. December PSH falls to 0.6–1.0 hour from a summer peak of 4.0–5.5. If you tour year-round, size the array for the worst month — that often means a 600 W array even though 300 W would handle June. Roof-tilt brackets steepen the panel angle and help shed snow; a folding 100–200 W suitcase panel deployed in open sky covers shaded touring sites.
Do I need EHU when I have solar?
Solar replaces electric hook-up (EHU) for most touring scenarios — three to seven days of off-grid on a CL site or aire is realistic with a properly sized system. EHU still matters for two cases: running a Truma electric heater overnight in winter (2,000 W draw exhausts any reasonable battery bank in 2–3 hours) and for fast LiFePO4 bulk charging via a Victron or NOCO mains charger after a long string of cloudy days. The typical UK caravanner uses EHU on 30–50% of nights pre-solar and 0–15% post-solar.

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