Heater Power Calculator (kW, Heat-up Time, Amps)
Heater kW from mass, specific heat, temperature rise and heat-up time, kW for flowing water or oil, single- and three-phase current, output at other voltages.
Formulas
- Heat kWh = mass (kg) × specific heat (kJ/kg·K) × ΔT (K) ÷ 3600
- Heater kW = heat kWh ÷ heat-up time (h) × safety factor
- Flowing fluid kW = flow (kg/s) × specific heat × ΔT × safety factor (L/min × density ÷ 60 = kg/s)
- US units (Tempco): W = lb × Btu/lb·°F × °F ÷ (3.412 × h), kW = gal/min × 60 × lb/gal × sp. heat × °F ÷ 3412
- Current: single-phase A = W ÷ V, three-phase A = W ÷ (√3 × V)
- Output at another voltage = rated W × (applied V ÷ rated V)²
Safety factor (Tempco)
Tempco says to add a safety factor: 10 % for small, closely calculated systems, 20 % as the common figure, and 25–35 % for larger systems with many unknowns. For flowing fluids it recommends 20 % for heat losses to jacket and piping, voltage variation and wattage tolerance. Surface losses during heat-up and operation (open liquid surfaces, uninsulated walls) are not in this calculation — add them for systems with large losses.
kW to heat water in 1 hour (no safety factor)
| Water | ΔT 10 °C | 20 °C | 40 °C | 60 °C |
|---|---|---|---|---|
| 50 L | 0.58 | 1.16 | 2.32 | 3.49 |
| 100 L | 1.16 | 2.32 | 4.65 | 6.97 |
| 200 L | 2.32 | 4.65 | 9.30 | 13.9 |
| 500 L | 5.81 | 11.6 | 23.2 | 34.9 |
| 1000 L | 11.6 | 23.2 | 46.5 | 69.7 |
Water specific heat 1.0 Btu/lb·°F (4.187 kJ/kg·K) and density 8.34 lb/gal (0.999 kg/L) are Tempco's figures. Two hours halves the kW; safety factor 1.2 multiplies it by 1.2.
Heater current (power factor 1, balanced three-phase)
| Heater | 1-phase 220 V | 3-phase 380 V | 3-phase 440 V |
|---|---|---|---|
| 1 kW | 4.5 A | 1.5 A | 1.3 A |
| 3 kW | 13.6 A | 4.6 A | 3.9 A |
| 6 kW | 27.3 A | 9.1 A | 7.9 A |
| 10 kW | 45.5 A | 15.2 A | 13.1 A |
| 20 kW | 90.9 A | 30.4 A | 26.2 A |
| 30 kW | 136.4 A | 45.6 A | 39.4 A |
Heaters are resistive, so W = V × A (single-phase). Three-phase values are line currents with equal resistance on each phase (balanced).
How to read it
- The shorter the heat-up time, the bigger the heater. Heating in 30 minutes takes twice the kW of 1 hour.
- The tank, container and parts inside heat up too. Tempco calculates the material, the container and the hardware separately and adds them (same formula, each with its own weight and specific heat).
- Melting or boiling (latent heat) is not in this calculation. Turning water into steam takes a lot of heat on top of raising its temperature.
- A 240 V heater on 220 V gives (220/240)² ≈ 84 % of its rating. On a higher voltage the output rises and heater life can suffer (Tempco 16-9).
FAQ
What is the heater kW formula?
kW = mass (kg) × specific heat (kJ/kg·K) × temperature rise (°C) ÷ (3600 × heat-up hours), times a safety factor. Raising 200 kg of water 40 °C in 1 hour: 200 × 4.187 × 40 ÷ 3600 ≈ 9.3 kW, about 11.2 kW with a 1.2 safety factor.
How many kW to heat 100 gallons of water?
Tempco's water table (10 % safety factor included) gives 26.9 kW to raise 100 gallons by 100 °F in one hour. Its equation 2 is kW = gallons × °F ÷ (372 × hours).
What is the current of a three-phase heater?
A = W ÷ (√3 × line voltage). 10 kW on 480 V three-phase is about 12.0 A; on 240 V three-phase about 24.1 A, and on 240 V single-phase 41.7 A (Tempco's table).
How do I size a heater for flowing water?
kW = flow (kg/s) × specific heat × temperature rise. Tempco's example: 5 gal/min of water from 50 to 100 °F with a 20 % safety factor needs 44 kW.
For heater cable size and voltage drop, use the Voltage Drop & Wire Size Calculator. For three-phase power and current, use the Three-Phase Power Calculator. For temperature sensor values, use the Thermocouple & Pt100 Converter.
Basis
Formulas, safety factors and examples are from heater maker Tempco's «Engineering Data» (tempco.com catalogue pages 16-2 heat requirements, 16-5 water heating table, 16-6 liquids and gases with the 5 gal/min water example and circulation heater flow chart, 16-9 wattage at other voltages, 16-10 single- and three-phase amperage table). Tests check that this calculator reproduces them. 1 Btu/lb·°F = 4.1868 kJ/kg·K and 1 kWh = 3412.14 Btu (NIST SP 811). Use your material's own specific heat and density, and electrical codes and the heater's specification for current, breakers and wiring.