Space Heater vs Central Heating

Introduction: what this calculator compares

This calculator estimates and compares the operating cost of two common heating approaches: (1) a plug-in electric space heater used in a single room, and (2) a central gas furnace that heats the whole home through ducts or radiators. Many households use a space heater to warm only the room they occupy, while keeping the thermostat lower for the rest of the house. Whether that saves money depends on your electricity price, natural gas price, furnace efficiency, and how many hours each system runs.

The model here is intentionally simple and focuses on energy cost. It does not attempt to predict indoor temperature, heat loss through walls/windows, or how long a furnace must run to maintain a setpoint. Instead, it answers a practical question: “If I run each option for these hours, what will it cost per day and per month?”

How to use the calculator

1. Enter your space heater wattage (commonly 750 W, 1,500 W, or similar) and the hours per day you plan to run it.
2. Enter your electricity rate in $/kWh. You can usually find this on your utility bill.
3. For central heating, enter the furnace heat output in BTU/hr, the efficiency (AFUE as a percent), your natural gas price in $/therm, and the hours per day the furnace runs.
4. Select Calculate Costs to see daily and approximate monthly totals (30-day month).

Tip: If you are trying to compare “heating the same comfort level,” you may need to adjust the furnace run hours and/or the space heater hours. A furnace might cycle on and off; use your best estimate of total run time per day.

Formulas and assumptions (units matter)

The calculator uses standard unit conversions. Electricity is billed in kilowatt-hours (kWh). Natural gas is commonly billed in therms, where 1 therm = 100,000 BTU.

Space heater (electric)

• Daily electricity use: Space heater daily energy equals wattage times hours divided by 1000. $E=\frac{W\times h}{1000}$ kWh/day
• Daily cost: Space heater daily cost equals daily kilowatt-hours times electricity rate. $C=E\times r_e$

Central furnace (gas)

• Efficiency is entered as a percent and converted to a fraction (e.g., 85% → 0.85).
• Therms consumed per hour (based on delivered BTU/hr and efficiency): Therms per hour equals BTU per hour divided by efficiency times 100000. $Q=\frac{\mathrm{BTU/hr}}{\eta \times 100000}$
• Daily cost: Furnace daily cost equals therms per hour times gas price times hours per day. $C_f=Q\times r_g\times h_f$

Monthly costs are estimated as daily cost × 30. If you want a more precise month, multiply by the number of days in your billing cycle.

Worked example (using the default inputs)

Suppose you run a 1,500 W space heater for 6 hours/day and pay $0.15/kWh. The heater uses (1500/1000) × 6 = 9 kWh/day, costing 9 × 0.15 = $1.35/day (about $40.50/month).

Now compare a furnace delivering 60,000 BTU/hr at 85% efficiency, with gas at $1.20/therm, running 8 hours/day. Therms per hour are 60000 / (0.85 × 100000) ≈ 0.7059 therm/hr. Daily cost is 0.7059 × 1.20 × 8 ≈ $6.78/day (about $203.40/month).

This does not mean a space heater can replace central heat for an entire home. It means that for a limited number of hours in a single room, the energy cost can be lower—especially when the alternative is heating the whole house.

Limitations, safety notes, and interpretation

This calculator estimates cost from rated power and run time. Real-world results can differ due to thermostat cycling, duct losses, varying furnace stages, and how much heat your home loses to the outdoors. A furnace may run fewer hours on mild days and many more hours during cold snaps. Likewise, a space heater may not maintain comfort in a large room, a drafty space, or an area with high ceilings.

• Comfort and coverage: Space heaters are best for localized heating; central systems provide more even whole-home temperatures.
• Electrical limits: A 1,500 W heater can draw ~12.5 A on a 120 V circuit; avoid overloading outlets and power strips.
• Safety: Keep heaters away from curtains and bedding, use tip-over protection, and never leave them unattended.
• Fuel types: This page models a gas furnace priced in therms. If your central heat is oil, propane, or electric, the inputs and conversion differ.
• Efficiency: The efficiency input is treated as a simple multiplier. Actual seasonal efficiency can vary with maintenance and operating conditions.

Sample scenarios (cost scaling)

The table below illustrates how costs can scale with run time using the default rates and equipment sizes. Your results will change with your local utility prices, furnace efficiency, and the hours you enter.

Practical guidance for better comparisons

If your goal is to reduce heating bills, consider using this calculator alongside a few practical observations:

• Measure actual run time: Many thermostats show furnace runtime history. Using real runtime makes the comparison more meaningful.
• Check your bill units: Electricity is usually in kWh; gas may be in therms, CCF, or m³. Convert to therms if needed.
• Account for multiple heaters: Two 1,500 W heaters running together are effectively 3,000 W.
• Insulation often wins: Air sealing, weatherstripping, and attic insulation can reduce both electric and gas heating costs.
• Consider time-of-use pricing: If your electricity rate changes by time of day, use the rate that matches when you run the heater.

Use the results as a planning tool: try different hours, rates, and efficiencies to see which variables drive your costs the most.