Generally natural gas is purchased per unit. 1 unit equals 1 therm, 1 therm of gas produces 29.3kw. A gas fire that has an input of 4.3kw will therefore run for 6.81hours (6 hours 45 minutes approx). Similarly if your gas appliance is running on LPG, 1kg of propane is equal to 196 litres, which productes 13.72kw. Therefore a LPG gas fire or stove that has an input of 4.3kw will run for 3.19hours (3 hours 10 minutes approx). Input of gas varies by stove and fire model, so you can calculate this running cost if you wish. Natural gas equation: 29.3kw divided by input of gas fire = running hours per unit of gas. LPG equation: 13.72kw divided by input of gas fire = running hours per 1kg. We haven’t gone into gas costs as obviously this varies on your tariff.