A space heater can cost less than central heat when it lets you lower heating elsewhere enough to offset its electricity use. Simply adding a heater while leaving the central thermostat unchanged usually adds cost. The answer also changes with your central system: a gas furnace, heat pump and electric furnace have different operating costs.
For example, a 1,500W heater running at full power for four hours costs $1.20 at $0.20/kWh. To save money, your central heating must use more than $1.20 less energy over that period. There is no universal four-hour cutoff that makes a heater cheaper.
Space heater vs central heat: start with your system
| Central system | What changes the decision? |
|---|---|
| Natural-gas furnace | Compare electricity with the variable gas rate and furnace efficiency. Heating the whole home and heating one room are different jobs. |
| Heat pump | It can supply several units of heat per unit of electricity. Replacing its heat with resistance heat can increase cost. |
| Electric furnace or baseboard | Both use resistance heat. Savings come mainly from heating less space or reducing temperature and runtime. |
If you need the heater’s hourly and monthly bill alone, use our 1500W space heater cost tables. This comparison asks whether that added cost is smaller than the central heating it replaces.
Compare the cost of the same amount of heat
The following examples supply 10 kWh of heat at the equipment, about 34,120 Btu. All prices and efficiencies are illustrative inputs, not national averages or a forecast for your home.
| Heating method | Example input | Cost for 10 kWh of heat |
|---|---|---|
| Portable resistance heater | 10 kWh electricity × $0.20 | $2.00 |
| Heat pump, COP 3 | 10 ÷ 3 kWh electricity × $0.20 | $0.67 |
| Electric resistance furnace | 10 kWh electricity × $0.20 | $2.00 |
| Gas furnace, 90% efficiency | 34,120 ÷ (100,000 × 0.90) therm × $1.50 | $0.57 |
COP means heat output divided by electrical input. COP 3 is an assumed operating condition, not a promise throughout winter. Actual performance changes with outdoor temperature, defrost, equipment and controls. Resistance backup changes the mix; see our guide to AUX heat and high electricity bills.
The gas example uses 90% as a simplified efficiency assumption. Rated AFUE describes seasonal fuel conversion, not exact efficiency during every cycle. The table excludes duct losses, separate gas-furnace blower electricity and installation costs; the heat-pump example assumes the stated COP covers its electrical input. It does not establish equal room comfort or whole-house bills.
The break-even test for heating one room
Assume one heater uses 1.5 kW for four equivalent full-power hours per day: 1.5 × 4 × $0.20 = $1.20/day, or $36 over 30 identical days. Thermostat cycling can reduce full-power hours; elapsed hours alone do not tell you consumption.
Your central heating reduction must exceed the following daily amounts just to cover that heater:
| Central system being reduced | Daily reduction needed in this example |
|---|---|
| Gas at $1.50/therm | More than 0.80 therm, ignoring any blower savings |
| Heat pump or electric furnace at $0.20/kWh | More than 6 kWh of heating electricity |
For a heat pump operating at COP 3, 6 kWh of avoided electricity represents 18 kWh of avoided equipment heat output. That is why warming one occupied room can work, but replacing efficient heat with resistance heat without substantially reducing the home’s heating requirement may not.
Suppose lowering the central thermostat saves 0.6 therm/day at $1.50/therm: $0.90 saved minus $1.20 added means a $0.30 daily increase. If it saves 1.0 therm/day, the net saving is $0.30/day. These are assumed reductions to show the threshold, not measured savings from a particular thermostat setting.
How to check whether your own bill falls
Use the variable electricity price, including usage-based delivery charges. For gas, use the variable price per therm. Exclude fixed customer fees that remain payable either way. If your bill uses Ccf, apply its listed conversion factor; do not treat one Ccf as exactly one therm.
Record utility interval usage, outdoor temperatures, occupied hours and thermostat settings before and after a modest change. Compare days with similar weather and household activity. Separate gas water heating and cooking where possible. One warmer day is not evidence that the heater saved money.
For electric homes, compare total heating electricity with and without the portable heater. Do not count the central-system reduction while forgetting the new plug-in load. Our monthly electric heating guide explains baseline usage; it cannot isolate heating perfectly from unrelated loads.
When a portable heater is a poor trade
Several occupied rooms, long heating hours or a central thermostat that remains unchanged weaken the case. Three 1,500W heaters at full power consume 4.5 kWh each hour, costing $0.90/hour at the example rate. Their capacity still does not guarantee whole-home comfort.
Keep adequate heat for occupants and pipes. Do not switch off central heating during freezing conditions just to pursue this calculation, and do not close supply or return vents indiscriminately. Follow your system’s operating guidance.
Plug a portable heater directly into a suitable wall outlet, never an extension cord or power strip. Keep it at least three feet from combustible materials and turn it off when leaving the room or sleeping. If the breaker trips, stop and investigate using our space heater breaker-trip guide rather than repeatedly resetting it.
Written by an electrical construction and project-coordination practitioner based in South Korea, using U.S. public technical references and explicitly stated calculations.
A fireplace insert adds decorative effects to the comparison. Our electric fireplace electricity cost guide separates heat from flame-only operation and shows how to account for thermostat cycling.
Sources & Official References
ENERGY STAR: Air-source heat pumps
ENERGY STAR: Heat pump performance and COP definitions
ENERGY STAR: Furnace efficiency and AFUE
EIA: Btu and energy conversions
ESFI: Space heater safety
References checked October 28, 2026. All dollar examples are calculations using hypothetical rates.
Is there something you’d like to see?
Suggest an electrical topic, practical guide, or calculator you would like to see on JHOTAS-LAB. Your suggestion may help shape future content.
