Electric Heater Size Calculator: Watts and BTU by Room Size
Electric heater size should match the room’s design heat loss. Estimate heat loss in BTU/h, then convert useful resistance-heat output at 3,412 BTU/h per kilowatt. The result is a thermal requirement, not approval to connect that load to an existing receptacle or branch circuit.
Electric Heater Size Calculator
Enter project-specific values, calculate, then change an input to compare scenarios. The result stays in this page and sends no visitor data anywhere.
Thermal output is not electrical design approval. A qualified electrician must verify voltage, circuit and conductor requirements.
From heat loss to watts
Electric resistance heat converts electrical energy to heat at the point of use. EIA’s standard energy conversion is 1 kWh = 3,412 BTU. For a rate, 1 kW of resistance heat is approximately 3,412 BTU/h. Divide the calculated BTU/h by 3.412 to obtain watts.
A 6,824 BTU/h room load therefore corresponds to about 2,000 watts of delivered resistance heat. That calculation says nothing by itself about allowable circuit load, voltage, receptacle, plug configuration, conductor size, overcurrent protection, controls, or installation method.
Why room size is not enough
A 200-square-foot interior room above conditioned space can need much less heat than a 200-square-foot corner room over a garage. Window U-factor, exposed perimeter, ceiling and floor boundaries, leakage, ceiling height, and outdoor design temperature all matter. The calculator uses those inputs rather than one watts-per-square-foot rule.
Portable heater versus permanently installed heat
Portable heaters are listed cord-and-plug appliances intended for use exactly as their instructions specify. Keep required clearances, place them on stable surfaces, and connect them only as permitted by the listing. Do not use extension cords, power strips, improvised adapters, or damaged receptacles.
Permanently installed heaters include baseboard, wall, ceiling, unit, and duct heaters. Their branch circuit, disconnecting means, controls, mounting, clearances, and conductor temperature requirements must be designed from the exact nameplate and instructions under applicable code.
Electrical circuit limitations
Do not add heater wattages until a circuit “almost reaches” a breaker rating. Continuous-load rules, other loads on the circuit, terminal ratings, conductor material and ampacity, voltage, and local requirements all affect the design. A breaker that trips is a warning, not a request for a larger breaker.
Never replace a breaker with a higher rating to run a heater unless a qualified electrician has designed and installed a compliant circuit. The existing conductors and devices may be protected only at the current rating. Do not open energized panels or perform live measurements.
Example interpretation
If a bedroom loses 5,100 BTU/h, the equivalent resistance output is about 1,495 watts. A nominal portable heater near 1,500 watts may appear close, but verify the product’s actual rating, circuit availability, instructions, and required clearances. If the room load is 9,000 BTU/h, buying multiple portable heaters is not automatically a safe solution; a permanent system and dedicated electrical design may be appropriate.
Thermostats and distribution
Heater location and control influence comfort. Baseboard heaters depend on air circulation and clear space. Wall heaters have defined mounting and clearance requirements. Line-voltage thermostats and relays must be rated and installed for the exact load. A large heater in one location may not correct cold surfaces or poor air mixing.
What the calculator omits
The estimate does not include warm-up recovery, slab edges, thermal bridging, mechanical ventilation, bathroom exhaust, open doors to other rooms, or intermittent occupancy. It assumes the entered room is the served zone and uses steady-state design heat loss.
Request a professional room-by-room load calculation for permanent equipment, additions, finished basements, high ceilings, bathrooms, garages, or unusual construction. Have a qualified electrician verify every permanent circuit and any situation where portable heat would substantially load an existing circuit.
Heat pumps are electrically powered but different
This calculator’s watts output applies to electric resistance heat. A heat pump moves heat and can deliver more than 3,412 BTU/h per input kilowatt under many conditions, but capacity and efficiency change with outdoor temperature. Do not use the resistance conversion to size a heat-pump circuit or predict heat-pump power.
Electric-heater FAQ
How many BTUs is a 1,500-watt heater?
About 5,118 BTU/h of resistance-heat output using 3.412 BTU/h per watt.
Can I use two portable heaters on one circuit?
Do not assume so. A qualified person must evaluate the circuit, other loads, devices, and product instructions. Overloading can overheat wiring and connections.
Should I buy the next larger wattage?
Match the verified load and available listed equipment while respecting the electrical design. Do not use arbitrary oversizing as a safety factor.