How much heating power will I need to heat a garage?

The output of a heater is measured in Watts (W) and kiloWatts (kW) for electric heaters and British Thermal Units (Btu) for all the others. 1 Watt is approximately 3.4 Btu. There are 3 things to calculate what power of heater you need, you should consider:

The volume of the space to be heated,
The rise in temperature required.
How well insulated the space is.

The volume of the space is calculated by the following formula: width x length x height (all in meters) to get cubic meters.

This figure should be multiplied by the required rise in temperature, in °Celsius.

Then multiply this number again, depending on the level of insulation in the building:

No Insulation x 3.5
Light Insulation x 2.5
Medium Insulation x 1.5
Heavy Insulation x 0.5

Then to return a result in Btu, multiply this by 4

An example of a single garage, with a temperature rise of 10°C needed with no insulation, the calculation would be about:

40 cubic meters x 10° x 3.5 x 4 = 5600 this gives an answer in Btu.

As you can see from the calculation, if you put a small amount of insulation and close up some of the holes where air can get through, you will drastically reduce the amount of heat that you will need.