Calculator Inputs
Formula Used
The calculator estimates heat demand from room volume, temperature rise, insulation, ventilation, window loss, safety margin, and efficiency.
| Volume | Volume = Length × Width × Height |
|---|---|
| Temperature Rise | ΔT = Target Indoor Temperature − Outdoor Design Temperature |
| Base Heat Loss | Base Loss = Volume × Insulation Factor × ΔT |
| Ventilation Loss | Ventilation Loss = 0.335 × Air Changes × Volume × ΔT |
| Window Loss | Window Loss = Window U-Value × Window Area × ΔT |
| Required Watts | Required Watts = Total Heat Loss × Margin Factor ÷ Efficiency |
| BTU Per Hour | BTU/h = Watts × 3.412142 |
| Single Phase Current | Current = Watts ÷ Voltage ÷ Power Factor |
| Three Phase Current | Current = Watts ÷ √3 ÷ Voltage ÷ Power Factor |
| Breaker Planning | Planning Current = Current × 1.25, then choose the next standard breaker size. |
How to Use This Calculator
- Enter the room length, width, and height.
- Select meters or feet for your dimensions.
- Enter the target indoor temperature and outdoor design temperature.
- Choose the insulation level that best matches the room.
- Add air changes, window area, and window U-value.
- Enter margin, efficiency, voltage, phase, and power factor.
- Add daily operating hours and energy price.
- Press the calculate button to view watts, BTU, amps, breaker guidance, heater count, and running cost.
- Use the CSV or PDF button to save the result.
Example Data Table
| Space | Length | Width | Height | Target °C | Outdoor °C | Insulation | ACH | Voltage |
|---|---|---|---|---|---|---|---|---|
| Bedroom | 4 m | 3 m | 2.7 m | 21 | 0 | Good | 0.5 | 240 |
| Workshop | 6 m | 4 m | 3 m | 18 | -5 | Average | 1.2 | 240 |
| Garage | 7 m | 5 m | 3 m | 16 | -2 | Poor | 1.5 | 240 |
Sizing Overview
An electric heater should match the room load, not guesswork. A small heater runs without comfort. A large heater cycles too often and may stress circuits. Good sizing starts with room volume, temperature rise, insulation, air leakage, and window loss. Electrical checks are also needed. The calculator combines those items into a practical heating recommendation.
Why Correct Capacity Matters
Heat demand changes with building shape and weather. A tall room needs more energy than a low room. Poor insulation lets heat escape faster. Drafts add fresh cold air that must be warmed. Windows lose heat quicker than most walls. The tool estimates these losses and then adds a safety margin. That margin helps during colder evenings, door openings, and thermostat recovery.
Electrical Planning
The result includes watts, kilowatts, and BTU per hour. It also estimates current from voltage, phase, and power factor. Continuous electric heating often needs extra circuit allowance. The breaker suggestion applies a planning factor before selecting the next common size. This is guidance only. Final wiring should follow local electrical rules and qualified review.
Running Cost View
A heater may be safe yet expensive. The cost panel estimates daily, monthly, and yearly energy expense from hours of operation and tariff. Use it to compare insulation upgrades, lower thermostat settings, or fewer operating hours. Small changes can reduce winter bills.
Better Input Tips
Measure room dimensions carefully. Use the same unit system for room and window area. Choose insulation honestly. Select higher air changes for drafty rooms, workshops, garages, or frequently opened spaces. Enter realistic outdoor design temperature, not an average afternoon value. Pick target temperature for comfort, equipment protection, or process needs.
Use in Design
This calculator is useful for bedrooms, offices, stores, sheds, workshops, and small plant rooms. It can compare portable heaters, baseboard units, fan heaters, and panel heaters. It does not replace engineered heat loss software for complex buildings. It gives a clear first estimate and highlights electrical demand before purchase. Always leave clearance around heaters, follow manufacturer instructions, and protect circuits correctly.
Interpreting the Result
Read the suggested capacity as minimum output. Choose the nearest available heater above it. When several rooms connect openly, calculate each space separately and add results.
FAQs
1. What does this heater sizing calculator estimate?
It estimates required heater watts, kilowatts, BTU per hour, electrical current, breaker planning current, heater count, and running cost from room and electrical inputs.
2. Should I choose the exact calculated wattage?
Choose the nearest available heater size above the calculated value. Do not select a much smaller heater, because it may run continuously without reaching comfort temperature.
3. Why is insulation important?
Insulation affects heat loss. Poor insulation lets heat escape faster, so the calculator applies a higher heat loss factor and recommends more heater capacity.
4. What is air changes per hour?
Air changes per hour estimate how often room air is replaced by outside air. Drafty rooms, garages, and workshops usually need higher values.
5. Does the breaker result replace an electrician?
No. The breaker result is a planning guide. Final circuit design must follow local codes, cable ratings, heater instructions, and qualified electrical review.
6. Why include heater efficiency?
Most resistance heaters are near 100 percent efficient at converting electrical input into heat. The field is included for advanced derating or special equipment comparisons.
7. Can I use feet instead of meters?
Yes. Select feet as the dimension unit. The calculator converts room dimensions and window area internally before applying metric heat loss formulas.
8. How is running cost calculated?
Running cost equals heater kilowatts multiplied by operating hours and energy price. Monthly and yearly estimates use 30 and 365 day periods.