What You Need to Know About Heaters and Thermostats: Wattage
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).
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!
Recommended Reading
Read our thermostat buyer's guide to find the best thermostat for your HVAC system needs. An analog thermostat must be changed by hand and may not always show an accurate temperature reading while digital thermostats have an easy-to-read display and an automatic feature which will shut off the AC once the home reaches the desired temperature.
Depending on the type of Smart thermostat you choose, they may operate using line or low voltage. Make sure to check what type of voltage a Smart thermostat operates with before purchasing or installing.
When replacing your thermostat, make sure to shut off all power to the heating and cooling system before doing any electrical work. Additionally, we recommend contacting an electrician to replace your thermostat.
Symptoms of a bad thermostat can mean your furnace or air conditioner refuses to start, the air conditioner runs constantly and won’t turn off, the thermostat doesn’t respond to temperature changes, or the HVAC system short cycles. You can make some simple fixes, like checking the batteries, the circuit breaker, and the wiring, or clean your mechanical thermostat.
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