No space heater is an energy-efficient replacement for a central heating system, but the right heater can be an effective way to warm a small, chilly space for a short period of time. Different types of heaters excel at different types of heating tasks, though, so the most efficient heater for your situation depends on how and where you're going to use it.
Space heaters are typically powered by electricity, kerosene, propane or natural gas. Electric heaters are extremely efficient at converting electricity to heat, but because the production of electricity is a relatively inefficient process, electric heaters are less efficient overall -- and more costly to operate -- than heaters that run on other fuels. Only electric heaters are safe to use in unventilated indoor spaces, however.
The most efficient space heater is one that's sized appropriately for the space it's being asked to heat. The heat output of space heaters varies widely, typically between 10,000 and 40,000 British thermal units per hour, and higher heat output comes at the cost of higher energy usage. If you have only a small space to heat, choose a heater that produces enough heat to adequately warm that square footage and nothing more.
Radiant and Convection Heat
Convection heaters use fans to create air circulation that spreads their heat throughout the space they're heating. Because they move heated air effectively, convection heaters heat rooms quickly, which makes them efficient when they're used in relatively large spaces. Radiant heaters don't use fans, but rather radiate their heat passively to directly warm people and objects near them. If you need to stay warm in a small space for a short period of time, a radiant heater can be more efficient than a convection heater because the heater doesn't waste energy trying to heat a large space or running a fan.
Thermostats and Controls
A space heater quickly becomes inefficient if it continues to try to heat a space when the space is already warm. A heater equipped with a thermostat saves energy by shutting off the heater when the air around the unit reaches the desired temperature, and adjustable temperature controls allow you to decrease the unit's heat output when you don't need it to operate at its maximum capacity.