When you start an engine when it’s cold it will use more fuel, as compared to when it’s already warm. When starting a preheater follow these guidelines regarding how much in advance you switch it on; 90 minutes when it’s below -15 °C, 60 minutes when it’s between -15 and 0 °C, 20 minutes when it’s 0 to +10 °C. Having the engine heater connected for a longer period than is actually recommended only results in more energy being used.