Van Cooling & Air Conditioning: Power Draw, Roof Fans & What Actually Works

Heating a van is a solved problem — a diesel heater does it for pennies. Cooling one is not, because moving heat out of an insulated metal box against a 30°C ambient takes real energy, and there is no cheap trick that changes the physics. This guide covers what each cooling option actually draws, what it actually achieves, and why for most UK vans the correct answer is a £200 roof fan rather than a £2,000 air conditioner.

Why Cooling Costs So Much More Than Heating

A diesel heater burns fuel and uses electricity only to run a glow plug and a small fan — around 30W once running. It produces 2,000W of heat from about 0.2 litres of diesel an hour.

Cooling has no equivalent. An air conditioner has to move heat from inside to outside against the temperature gradient, and that work has to come from your battery. A compressor air conditioner producing 2,000W of cooling draws 500–900W of electrical power to do it. Run that for six hours and you have consumed 3,000–5,400Wh — more than most van battery banks hold in total.

That single ratio governs everything below. Heating is cheap because you are burning fuel; cooling is expensive because you are moving heat with electricity.

Option 1: Roof Fan — The Answer For Most Vans

A powered roof vent (Maxxair, Fiamma, Dometic) extracts hot air from the top of the van and draws cooler air in through an open window or door. It does not lower the temperature below ambient, but it stops the van sitting 10–15°C above ambient, which is where the real misery lives.

Power draw is trivial: 0.5–3W on low, 15–30W on maximum. Overnight on a low setting costs 5–25Wh — nothing. Cost is £180–320 fitted, plus cutting a 400mm hole in the roof.

What it achieves is substantial. A sealed van parked in sun reaches 45–55°C inside on a 25°C day. With a roof fan extracting and a window cracked, that drops to roughly 27–30°C — close to ambient. You are not getting cold air, but you are removing the greenhouse effect, and in the UK that is almost always enough.

Fit a rain-proof model so it can run through a shower, and fit it toward the rear over the sleeping area, drawing air from the front. Reversible fans that can blow inward as well as extract are worth the small premium for winter condensation control.

For UK van life, if you fit one cooling device, this is it. A reversible powered roof vent solves 90% of the summer problem for under 3% of the power an air conditioner needs.

Option 2: 12V Compressor Air Conditioning

Purpose-built 12V/24V DC air conditioners for vehicles (Dometic RTX, Nomadic, Kingtec) run a proper refrigeration cycle and genuinely cool air below ambient — typically 8–15°C below intake.

Power draw is the problem. A 2,000W-cooling unit draws 400–700W electrical on startup and 250–450W continuous once the space is cool. Overnight — say eight hours at an average 300W — is 2,400Wh.

That means a 2,000Wh battery bank cannot run one for a full night, and a 5,000Wh bank runs one night and needs a full day of good solar to recover. In practice, DC air conditioning is only viable with 400Ah+ of lithium and 600W+ of solar, which is a £4,000–6,000 electrical system before you have bought the air conditioner at £2,000–3,500.

Where it is justified: full-time living in genuinely hot climates, or a van used for touring southern Europe in summer. Where it is not: a UK van that sees 28°C on perhaps ten nights a year.

Option 3: 230V Portable Air Conditioner

A domestic portable unit costs £250–500 and works well — on hookup. Draws 700–1,200W, which is 20–35A on a UK campsite’s 16A supply once you account for other loads, so it is close to the limit of what a hookup will give you before it trips.

Off-grid it is not viable at all: 1,000W through a 2,000W inverter is roughly 92A from a 12V battery, and eight hours of that is 8,000Wh. No van carries it.

The other issue is the exhaust hose. Single-hose portables pull cooled air from inside the van and blow it out of the window, creating negative pressure that draws hot air in through every gap. In a small space this halves their effectiveness. Dual-hose units avoid it but are bulkier and rarer.

Worth having only if you spend most summer nights on hookup and have somewhere to store a 30kg box the rest of the year.

Option 4: Evaporative Coolers — Skip These

Evaporative or “swamp” coolers blow air over a wet pad. They draw only 30–80W, which makes them look extremely attractive on paper.

They work by adding moisture to the air, and their effectiveness collapses as humidity rises. UK summer humidity typically runs 60–80%, at which an evaporative cooler delivers a 1–3°C drop while raising interior humidity toward saturation. In a small insulated space with condensation problems already, adding water vapour to achieve almost no cooling is a bad trade.

They are genuinely effective in dry heat — Spain in August at 20% humidity, they work. In Britain they do not.

Option 5: The Free Ones That Work Better Than Expected

Insulation cuts cooling load exactly as it cuts heating load. An uninsulated metal roof in direct sun reaches 60–70°C and radiates into the van all evening. 25mm of PIR board on the roof drops interior peak temperature by 6–10°C for zero running cost, and it is the single most effective cooling upgrade available.

Reflective window covers on the windscreen and cab windows cut solar gain dramatically — the cab glass is usually the largest uninsulated area in the vehicle. External covers work far better than internal because they stop the heat before it enters. £30–60 and worth every penny.

Parking orientation and shade. Parking with the windscreen away from the afternoon sun, or under a tree, is worth more than any 300W of air conditioning. Obvious, and consistently under-used.

Cross-ventilation. A roof fan extracting at the back with a window open at the front creates airflow through the whole van. Opening two windows on the same side achieves almost nothing.

Where the Heat Actually Comes From

Before spending anything, understand what you are fighting. Heat enters a parked van through four routes, and they are not equal.

Solar gain through the roof is the biggest by a wide margin. A dark van roof in July sun reaches 60–70°C. That surface then radiates downward into the living space for hours after the sun has moved, which is why a van that felt tolerable at 6pm is still uncomfortable at eleven. Roof insulation and a light-coloured roof both attack this directly.

Cab glass is second and often larger than people expect. A windscreen and two door windows total 2–3m² of uninsulated glazing pointed at the sky. Untreated, that alone can add 5–8°C to the interior on a sunny afternoon.

Body panels in direct sun come third — significant on the sunny side, negligible on the shaded side, which is why parking orientation matters so much.

Internal sources come last but are not nothing: a compressor fridge rejects its heat into the van, a laptop adds 40–60W, and two sleeping people produce roughly 200W between them. In a well-insulated small space overnight, body heat alone raises interior temperature measurably.

The order matters because it tells you where money goes furthest: roof, then glass, then shade, then active cooling. Most people start at the wrong end of that list.

Real Power Comparison

Watt-hours consumed across an eight-hour summer night:

• Roof fan, low: 8–24Wh
• Roof fan, high: 120–240Wh
• USB desk fan: 20–40Wh
• Evaporative cooler: 240–640Wh, for 1–3°C in UK humidity
• 12V compressor air con: 2,000–3,600Wh
• 230V portable via inverter: 6,400–9,600Wh

The roof fan on high uses less than one tenth of what the smallest real air conditioner needs. That ratio is why the recommendation is what it is.

Three Cooling Setups With Costs

UK realistic — £250–400

Reversible powered roof vent, external windscreen cover, one USB fan for directed air at the bed. Draws under 30W at worst. Handles every UK summer night short of a genuine heatwave, and the roof fan earns its place year-round for condensation control.

Southern Europe touring — £2,500–4,000

Roof fan plus a 12V compressor air conditioner, backed by 400Ah lithium and 600W of solar. The air conditioner runs on solar surplus through the afternoon and for two or three hours at bedtime rather than all night. A Bluetti Elite 400 (3,840Wh) is one route to the storage this needs without wiring a 400Ah bank yourself.

Hookup-based — £400–700

Roof fan plus a dual-hose 230V portable used only on hookup. Cheapest path to genuine cold air, provided you accept it does nothing off-grid and you have storage space for it.

Running Air Conditioning Off-Grid: The Honest Maths

If you are determined to run DC air conditioning without hookup, here is what it takes.

Assume 300W average draw over six hours of night use: 1,800Wh. Add the fridge at 500Wh and everything else at 300Wh, and you need 2,600Wh of overnight discharge — which means a 5,000Wh bank to stay above 50% state of charge.

Replacing 2,600Wh daily needs roughly 600W of solar in high summer, when a 600W array makes 2,300–2,700Wh on a clear day. That works in July and fails completely from September onward — which is fine, because you do not need cooling then.

So the specification is: 5,000Wh lithium, 600W solar, a 50A DC-to-DC charger for cloudy days, and a 12V compressor unit. Around £5,000–7,000 all in. The Bluetti Elite 400 at 5,120Wh covers the storage side of that in a single unit.

Whether that is worth it for a UK van is a straightforward question: how many nights a year are actually too hot to sleep? For most people the honest answer is five to fifteen, and £5,000 is a lot for fifteen nights.

Four Mistakes That Waste Money

Buying an evaporative cooler for British summers. The humidity defeats it. It will disappoint you and add condensation.

Fitting air conditioning before insulating the roof. You are paying electricity to fight heat you could have stopped for £80 of PIR board.

Using a single-hose portable in a small van. The negative pressure it creates pulls hot air in through every gap and roughly halves its own effect.

Sizing the battery for the air conditioner alone. The fridge, lights and devices are still running. Budget the full night’s load, not just the cooling.

Verdict: Fan First, Insulate Second, Air Con Almost Never

For a UK van, fit a reversible powered roof vent and an external windscreen cover, and insulate the roof properly. That is £250–400, draws under 30W, and handles almost every night the British summer produces. It also pays back in winter, because the same fan controls the condensation that ruins van interiors.

Consider 12V air conditioning only if you tour southern Europe in summer or live in the van full-time somewhere genuinely hot — and understand before you order that the air conditioner is the cheap part. The battery and solar system to feed it costs two to three times as much again.