Compared to using age-old technology such as an open-coal fire, most modern heating appliances are actually pretty efficient; look at the red bars in the chart below and you'll see that, for every joule (the standard modern unit of measuring energy) of fuel you feed into them, you ...
That's how many joules (the standard measurement of energy) you'd get from a kilogram of mass. In theory, if you could turn about seven billion hydrogen atoms completely to energy, you'd get about one joule (that's about as much energy as a 10-watt lightbulb consumes in a tenth of...