By law all portable electric space heaters are limited to 1500 watts power usage. Electric heaters of all varieties are efficient at converting electrical energy to heat, but because of the power limitation the most heat you will get from one will be about 5000 BTU's. You will not replace the heat from a central heating system with on portable heater, but by implementing some strategies that utilize the strengths of the different types of space heaters you can save. Here's a good link for more about portable room heaters .