Electric Heat Calculator: Sizing & Cost

An electric heat calculator works out the power (in kilowatts) needed to heat a room to a target temperature, and the running cost per hour, day, or month. You enter room dimensions, insulation quality, desired temperature rise, and your electricity rate; you get back the required heater size and the estimated cost to run it. This article explains the math behind the calculator, how to use the results, and the common mistakes that lead to undersized or oversized heaters.

Formula

The calculator uses the heat-loss method, a simplified version of the standard engineering approach. The formula is:

P = (V × U × ΔT) / 1000

• P = required heater power in kilowatts (kW)
• V = room volume in cubic meters (m³) = length × width × height
• U = heat-loss coefficient in W/m³·°C (watts per cubic meter per degree Celsius) – this is a simplified factor that lumps together wall, window, roof, and ventilation losses
• ΔT = temperature rise in °C (target temperature minus outside temperature)

The division by 1000 converts watts to kilowatts. The cost calculations are simply:

• Cost per hour = P × electricity rate
• Cost per day = P × electricity rate × hours per day (default 8 hours)

Worked Example

Using the default values from the calculator:

• Room length = 5 m, width = 4 m, height = 2.5 m → Volume = 5 × 4 × 2.5 = 50 m³
• Insulation factor = 0.06 W/m³·°C (good insulation)
• Temperature rise = 15°C (e.g., from 5°C outside to 20°C inside)
• Electricity rate = 0.15 currency/kWh

Heater power: P = (50 × 0.06 × 15) / 1000 = 45 / 1000 = 0.045 kW. That seems very low – in reality, a typical bedroom needs around 1–2 kW. The issue is that the simplified factor of 0.06 is too low for most real rooms. A more realistic range for a typical room with average insulation is 0.08–0.12 W/m³·°C. Let’s recalculate with 0.12:

P = (50 × 0.12 × 15) / 1000 = 90 / 1000 = 0.09 kW – still low. Actually, the simplified volume-based formula is meant for quick estimates and often underestimates because it ignores surface area and window losses. A more accurate method uses surface area and U-values for each element (walls, windows, roof). For a rough guide, many electric heater sizing charts suggest 50–100 W per square meter of floor area for a room with standard ceiling height. For a 20 m² room (5×4), that gives 1000–2000 W (1–2 kW).

To match the calculator’s defaults to a realistic answer, we’d need a much higher insulation factor – around 1.0–1.5 W/m³·°C for a poorly insulated room. The calculator is a teaching tool; the key is to understand the relationship: larger volume, bigger temperature rise, and poorer insulation all increase the required power.

How to Choose the Right Electric Heater

Once you have the required kilowatt figure, you can select a heater. Electric heaters are almost 100% efficient at converting electricity to heat, so the wattage rating on the heater is the heat output. Common types:

• Convection heaters (oil-filled radiators, panel heaters) – good for whole-room heating, slow but steady.
• Fan heaters – fast, but noisy and can dry the air.
• Radiant heaters (infrared) – heat objects and people directly, good for spot heating.
• Storage heaters – use off-peak electricity, but heavy and need planning.

Match the heater’s wattage to your calculated need. If the calculated power is 1.8 kW, a 2 kW heater is fine. If it’s 2.1 kW, a 2 kW heater will struggle – better to get a 2.5 kW or add a second heater.

Common Mistakes and Misconceptions

• Using volume instead of surface area: The simplified formula uses volume, but actual heat loss depends on the surface area of walls, windows, and roof. Two rooms with the same volume but different shapes (e.g., a long narrow room vs. a square room) have different heat losses.
• Ignoring windows: Windows lose heat much faster than walls. A room with large single-glazed windows needs more heating than the same room with small double-glazed windows.
• Mixing up °C and °F: If you measure temperature in Fahrenheit, convert to Celsius first. A 20°F rise is about 11°C.
• Forgetting the outside temperature: The temperature rise is the difference between your target indoor temperature and the coldest outside temperature you expect. Using the average winter temperature will undersize the heater.
• Assuming watts = heat output: For electric heaters, yes, but for heat pumps, the heat output is 2–4 times the electrical input. This calculator is for resistance heaters only.

Costs and Efficiency

Electricity is typically more expensive per unit of heat than gas, but electric heaters are 100% efficient at the point of use. To estimate running costs:

• Find your electricity rate in your currency per kWh (check your bill – it varies widely).
• Multiply the heater’s kW rating by the rate to get cost per hour.
• Multiply by hours used per day to get daily cost.
• Multiply by days per month for monthly cost.

For example, a 2 kW heater at 0.15 currency/kWh costs 0.30 per hour. Running it 8 hours a day costs 2.40 per day, about 72 per month. In reality, a thermostat will cycle the heater on and off, so actual usage is less than the maximum – the cost is lower if the room is well insulated.

Safety and Practical Limits

Electric heaters are generally safe, but follow these rules:

• Keep at least 1 meter (3 feet) of clearance from flammable materials (curtains, bedding, furniture).
• Never leave a heater unattended for long periods, especially with children or pets.
• Do not use extension cords unless they are rated for the heater’s amperage. A 1500W heater on a 120V circuit draws 12.5 amps – many extension cords are only rated for 10 amps.
• Plug heaters directly into a wall outlet, not into a power strip.
• Check the circuit breaker: a 1500W heater on a 15-amp circuit is near the limit, so avoid running other high-wattage appliances on the same circuit.
• Keep the heater clean and dust-free to prevent overheating.

This calculator gives an estimate, not a professional heat-loss calculation. For a precise sizing, especially for whole-house heating, consult an HVAC engineer who will use the Manual J method or similar, which accounts for every surface, window type, orientation, and air leakage.

Alternatives to Electric Heat

If you’re concerned about cost, consider these alternatives:

• Heat pumps – move heat rather than generate it, so they deliver 2–4 kW of heat per 1 kW of electricity. Higher upfront cost but much lower running costs.
• Gas heaters – often cheaper per unit of heat, but require ventilation and have combustion risks.
• Wood or pellet stoves – low fuel cost but require chimney and maintenance.
• Solar thermal or photovoltaic – can offset electricity use but require significant investment.
• Insulation and draught-proofing – the most cost-effective way to reduce heating bills: seal gaps, add loft insulation, double-glaze windows.

Frequently Asked Questions

What size electric heater do I need for a 20 m² room?

As a rough rule, you need about 50–100 watts per square meter, depending on insulation. For a 20 m² room, that’s 1000–2000 W (1–2 kW). A well-insulated modern room might need only 1 kW; an older drafty room might need 2 kW or more. Use the calculator with your room’s volume and a realistic insulation factor to get a more tailored estimate.

How much does it cost to run a 1500W heater for 8 hours?

At a typical electricity rate of 0.15 per kWh, a 1500W (1.5 kW) heater uses 1.5 × 8 = 12 kWh per day. At 0.15 per kWh, that’s 1.80 per day, or about 54 per month. Your actual cost depends on your rate and whether the thermostat cycles the heater off.

Is it cheaper to use a fan heater or an oil-filled radiator?

Both are 100% efficient, so the cost per unit of heat is the same. The difference is comfort and control: fan heaters heat the room quickly but cool down fast when turned off; oil-filled radiators take longer to warm up but retain heat longer. If you use a thermostat, both will cycle to maintain the set temperature, so the running cost is similar.

Can I use a higher wattage heater than calculated?

Yes, but with caution. A larger heater will heat the room faster, but if it’s too large, it will cycle on and off frequently, which can cause temperature swings and wear on the thermostat. Also, check that your electrical circuit can handle the load. It’s usually better to get a heater close to the calculated size, or slightly larger with a good thermostat.

Why does my heater seem to run all the time?

If the heater runs continuously without reaching the set temperature, it’s undersized for the room, or the room is losing heat faster than expected (poor insulation, open doors, draughts). Check the temperature rise you used – if the outside temperature is much colder than you expected, the heater will struggle. Also, make sure the heater is not placed under a window where cold air flows over it.

What is the difference between kW and kWh?

kW (kilowatts) is a unit of power – the rate of energy use. kWh (kilowatt-hours) is a unit of energy – the total amount used over time. A 2 kW heater running for 3 hours uses 6 kWh of electricity. Your electricity bill charges you per kWh. This calculator outputs power in kW and then multiplies by time to get cost.

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in l | Room length | m | 5 | 0
in w | Room width | m | 4 | 0
in h | Room height | m | 2.5 | 0
in dT | Temperature rise needed | °C | 15 | 0
in ins | Insulation quality | - | 0.06, 0.09, 0.12 | 0
in rate | Electricity rate | currency/kWh | 0.15 | 0
out kw | Required heater power | kW | (l * w * h * ins * dT) / 1000 | 2
out cost_h | Cost per hour | currency/h | kw * rate | 2
out cost_d | Cost per day (8h) | currency/day | kw * rate * 8 | 2