Dometic FreshJet FJX

Comfort depends on humidity as well as temperature. We can tolerate up to about 26°C (78°F) only when the air is dry; in humid air, around 20°C (68°F) is the comfortable limit. Most people feel best at roughly 18-22°C (64-72°F) with 40-70% relative humidity. Achieving that in a motorhome means cooling the air and drying it at the same time, which needs a compressor air conditioner - systems relying on direct or indirect evaporative cooling are less effective.

How warm the air feels can differ from what the thermometer says, and humidity makes a big difference. In dry heat, sweat evaporates quickly from the skin and cools us, so we cope with high temperatures; in humid heat, sweat evaporates less, so we feel hotter, and the damp, sticky air can affect circulation and sleep. That's why a good air conditioner needs to lower the humidity as well as the temperature. Dometic suggests around 26°C (78°F) is usually comfortable.

If you mostly tour in cooler climates, the air conditioner won't be pushed hard and a lower-rated unit will be enough. If you regularly travel somewhere hot - especially with children or pets on board - don't skimp on output, as a motorhome or caravan can quickly get uncomfortable on a hot day, and when nights stay warm too, you'll only rest properly with an adequately sized unit.

Much like a household fridge, but far more powerful: a refrigerant draws heat out of the space being cooled and releases it outside - and in a motorhome or caravan it should also reduce the humidity. Evaporative systems are a common alternative: a roof-mounted unit evaporates water into a filter, and a fan pulls outside air through it so heat is exchanged.

They use little power and are light, but their effectiveness depends on the outside humidity, so performance varies - it falls away above 85% humidity, exactly when cooling is most needed - and they always add humidity inside the vehicle, whatever the conditions. Caravans and motorhomes don't have thick walls like a house, so even well-insulated ones heat up in the sun, easily reaching 30°C or more inside - in temperate climates as well as hot ones.

How hot it gets also depends on window size and number (small windows heat up more slowly than large glazed areas), on whether you're parked in sun or shade, and on whether you air the vehicle out or keep everything shut.

Yes - a generator gives you a reliable power supply wherever there's no mains hook-up, so you can keep your home comforts when touring off the beaten track. Dometic makes generators for exactly this.

Usually, yes. An air conditioner draws more current at start-up than any other appliance you're likely to have on board, but only for a fraction of a second - it uses far less once it's running. Most campsite hook-ups are protected by a time-delay fuse, so those brief peaks don't normally cause a problem.