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KNOWLEDGE · K6

Electric heater duty calculation: formula and key design loads

This guide covers the heat-duty formula, common property values, additional loads and the process data needed for a preliminary quotation.

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  • One formula for all media
  • Properties at actual temperature
  • Latent heat & losses included
  • Margin 10–20%

Sizing BasicsPublished 2026-09-11中文版 ↗

01One formula

For sensible heating: P = ṁ × c × ΔT / 3600 (kW), with ṁ in kg/h, c in kJ/(kg·°C), ΔT in °C. With phase change (boiling water to steam, for example), add the latent term: P_latent = ṁ × r / 3600, where r is the latent heat of vaporization in kJ/kg.

For static (batch) heating of a tank, use P = m × c × ΔT / (3600 × t), with t the required heat-up time in hours. Add the heat absorbed by the tank steel and the losses through insulation — on large tanks that alone can be 20–30% of the total.

02Property quick table

Water

c ≈ 4.18 kJ/(kg·°C), ρ ≈ 1000 kg/m³. Rule of thumb: 1 tonne of water per hour per °C ≈ 1.16 kW.

Thermal oil

c ≈ 2.0–2.3 kJ/(kg·°C) (varies strongly with temperature), ρ ≈ 850–950 kg/m³. Half the heat capacity of water per kilogram — which is why oil systems circulate harder.

Air

c ≈ 1.01 kJ/(kg·°C), standard ρ ≈ 1.29 kg/m³. Every 1,000 Nm³/h per °C ≈ 0.36 kW.

Steam (latent)

r ≈ 2,257 kJ/kg at atmospheric pressure. One tonne per hour of steam ≈ 630 kW of pure latent duty.

03Three loads people forget

Surface losses

Radiation and convection from uninsulated or outdoor equipment: typically 5–15% of duty; take the high end for poor insulation.

Start-up load

Warming the equipment and pipework metal from cold. For batch or intermittent production this can dominate the selection.

Fluctuation margin

Inlet temperature swings and output ramp-ups — 10–20% margin keeps the heater out of its ceiling in real operation.

04After the kW: what a real specification needs

Power is only the first layer. Then settle: heater architecture (direct or circulation type), design pressure (working pressure plus margin), materials against the medium's corrosivity, orientation and flange standards, control (staged switching or SSR modulation) and protection (over-temperature, low-flow, level).

When you request a quotation, send it complete in one pass: medium name and phase, flow, inlet/outlet temperatures, working pressure, hazardous-area requirements, orientation, control expectations. A complete RFQ skips two rounds of back-and-forth and prices more accurately.

↗Related reading

Industrial Electric Heater Running Cost: A Worked kWh Example

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Medium, flow, temperatures, pressure. With those four, duty and architecture are a same-day answer.

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FAQ

Frequently asked

01Why take properties at the actual temperature?+

Oil and gas specific heats shift noticeably with temperature; across a 200°C rise, using inlet-temperature properties skews the result by more than 10%. Use values at the mean process temperature for sizing.

02Is a 30% margin 'safer'?+

No. Oversized heaters run at low turndown, control poorly and waste capital. 10–20% is standard; handle start-up load as its own calculation.

03No flow meter — how do I estimate flow?+

Discount the pump nameplate flow for system resistance, or cross-check with bucket-and-stopwatch or tank level change. Flow is the first input of the duty formula — it is worth verifying properly.

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