Space heater vs furnace cost calculator

Compare a portable electric heater with a gas furnace over your chosen operating days. You can compare the same amount of useful heat or enter a separate assessed furnace heat demand. Useful heat means heat delivered to the space, rather than energy bought from your supplier.

For the portable heater, enter its electrical watts, hours available each day and heating-on share. That share is the percentage of those hours when it draws the stated power. Use 100% if your power figure already represents an average; applying a second reduction would undercount use.

For a room-versus-house comparison, choose assessed furnace heat and enter the daily heat from an installer assessment. The calculator does not assume that one small heater can replace heating throughout a house. Enter the same operating season for both options so the annual costs have a clear basis.

Portable heater input energy = watts ÷ 1000 × hours × heating-on share. In same-heat mode this equals useful resistance heat. Furnace therms = entered useful heat ÷ seasonal efficiency ÷ EIA kWh per therm. Multiply each input energy by its matching price and operating days.

Convert the heater's watts to kilowatts by dividing by 1000. Convert the heating-on share and furnace efficiency to fractions once. The furnace efficiency is the share of fuel energy that becomes useful heat. A lower efficiency means more gas is needed for the same delivered heat.

Electricity is priced in cents per kWh, so divide that price by 100 to obtain dollars per kWh. Gas uses dollars per therm directly. A therm is an amount of gas energy, not a volume. The result shows the quantities, prices and operating days used in each cost.

These illustrative inputs are separate from your saved answers. Prices, use and performance are held constant for this worked scenario; they are not personal defaults.

Same-heat mode matches heat amounts, not room temperatures or comfort. Heat location, insulation, air movement and controls still matter. A room-only schedule and a whole-house schedule can have different heat needs even when their running hours match.

Fixed gas service charges are outside this running-cost comparison. Turning down the furnace does not remove them while the gas account stays open. Installation, repairs and buying a portable heater are also outside the estimate.

Keep fuel use, efficiency and price on the same basis. Do not enter cubic feet of gas in a therm field. Ask your supplier for the bill's conversion if needed. The estimate holds prices and schedules constant; it does not model weather changes. Missing or incomplete personal inputs never become zero automatically.

Read the daily and annual energy quantities as well as the money difference. If one option supplies more heat, that difference helps explain the cost. Adjust the assessed heat only when you have evidence for the area and schedule being compared.

The published electricity option uses EIA state residential averages for July 2026, released on 24 September 2026 and checked on 14 October 2026. These describe past average prices. They are not your supplier's offer or a promise of future prices. Choose your bill price for a personal comparison. Include the taxes and usage charges that change with consumption; keep fixed account charges separate.

Ask a qualified heating contractor to check any room-versus-house heat assessment and the furnace's seasonal efficiency. Take your bill and equipment details so the contractor can identify costs that remain whichever option you use.

Follow the heater maker's placement, clearance and electrical instructions. A lower calculated cost does not confirm that a heater is suitable for a room or circuit. The calculator does not assess wiring, combustion safety, ventilation or protection against frozen pipes.