What You Need to Know About Heaters and Thermostats: Wattage

By mark_watola on 11/13/2020

We already discussed heater and thermostat voltage and circuit capacity.

In this segment we will discuss heater and thermostat wattage. Remember, safety is paramount, so make sure to know your thermostat and space’s voltage, and your current heater’s voltage if you plan to replace it. Familiarize yourself with your thermostat, home, and heater voltages if you plan on replacing parts.

Heater Wattage

After finding a heater matching the voltage of your space and thermostat, determine the heater wattage you need.

Watts measure power output, to describe the rate of energy generated or used. Heater wattage expresses the amount of power used in one hour.

If the heater is a primary source of heat, approximate 10 watts per square foot to heat a space. If the heater is a secondary source of heat, account for 6-8 watts per square foot.

200 sq.ft. x 10 (watts per square foot) = 2,000W

A heater’s output is measured in BTUs, or British Thermal Units.

Heater wattage can be used to calculate approximate BTUs. To determine BTU using the listed wattage, multiply by 3.412. Conversely, BTUs can be used to calculate wattage by multiplying by 0.293.

500W x 3.412 = 1,706 BTUs

2000 BTUs x 0.293 = 586W

Not all heaters are 100% efficient, so there may be some discrepancy between wattage (power consumed) and BTUs (heat output).

Heater and Thermostat Wattage Chart

Thermostat Wattage

Thermostats can control multiple heaters, but how many depends on the wattage of the heaters. Be careful not to exceed the maximum thermostat wattage, by only using up to 80% of the maximum wattage. Relays and transformers may be required.

If the total maximum amperage is not listed, calculate the thermostat’s wattage capacity by multiplying the voltage by its amps.

120V x 20A = 2400 Watts

In this case the thermostat (120V, 20A) can handle one or multiple heaters not to exceed the total 2400 watts.

If you need help finding the right heater for your thermostat, contact us!

For more information check out our blog:

The Best Thermostat Breakdown

Can I Use a Smart Thermostat in my Home?

How to Replace an Analog Thermostat

Diagnosing a Dead Thermostat






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