One thumb of the rule to remember is that a heater takes 10 watts to heat up one square foot of space. Then there are heaters that are energy efficient and use fewer watts to produce more energy. For example, EconoHome 500SS Heater Panel only consumes 400W to heat up 200 square feet of area. That’s only 30 percent of what an average heater uses. You can install two of these on opposite walls to heat a room of 300 square feet. Energy efficient heaters bring down the electricity bill by up to 50 percent.