What is the highest BTU for a patio heater? A misguided question. British thermal units are a standard measurement of heat energy and outdoor heaters can produce anything from 10,000 to a scorching 80,000 BTUs. Most are somewhere around 40,000. But, according to Stephen Levy of specialists Heat Outdoors, all that is completely irrelevant if the heat is blown away by the wind – see above. How many square feet does a patio heater cover? “About eight square metres, assuming a high quality 2kW shortwave heater,” Levy says. That’s roughly equivalent to a garden table and the garden chairs around it. 2kW refers to the electricity a heater uses, by the way, rather than the heat it emits: more kiloWatts do not necessarily mean more warmth, especially if the heater has a poorly-made reflector or cheap bulbs. To heat a bigger space, it’s usually better to get more than one heater. For low-glow and gas heaters, shelter from the wind is by far the most important factor. Talking of which… How effective are gas …