A direct fired unit converts nearly all of its fuel into heat. As a rule of thumb it burns about 0.1 litre of diesel or 0.08 kg of propane per kW per hour. A 30 kW unit therefore uses some 3 litres of diesel or 2.4 kg of gas per running hour.
Electric units consume exactly their rated output: 9 kW is 9 kWh per hour. Per kilowatt delivered, electricity is usually the most expensive option in Belgium, but there is no fuel supply to organise, no combustion and no ventilation requirement.
With an indirect fired unit part of the heat leaves through the chimney. Allow 10 to 15 per cent more fuel for the same room temperature — the price of clean, dry heat.
What drives the cost most is not the fuel but the number of running hours. A thermostat, closed doors and a sheet across openings save more than a few cents difference per kWh.
For longer periods in one place diesel or gas is usually cheapest; for short jobs or indoor work without a flue, electric is the most practical.