Heater Wattage Calculator: Heat-Up Time, Mass, ΔT
Heater wattage is the energy needed to raise a mass by a temperature difference, divided by the time you have, plus an allowance for heat lost on the way: P = m × cp × ΔT ÷ t. Enter what you are heating, the start and target temperatures and the heat-up time below to get the kilowatts, the current, and the element size that keeps watt density inside the limit. No signup; the formula and a worked example are underneath.
Calculate heater wattage
Runs in your browser. The link updates as you type, so you can copy it to share this exact calculation.
What you are heating
Filled in from the material; overwrite it with your own value.
Losses from the tank walls, lid and surface. 20% is a starting assumption, not a rule; insulated vessels need less, open tanks more.
Supply and element (optional)
Result
- Temperature rise
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- Mass heated
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- Energy needed
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- Power, no losses
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- Line current
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- Heated area at the watt density limit
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- Total heated length at that diameter
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- Fewest elements within demo limits
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- Each element
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- Required power
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The heat-up formula
Heating a batch from one temperature to another takes a fixed amount of energy. Spreading that energy over the heat-up time gives the power:
- Energy Q = m × cp × ΔT, in kJ when m is in kg, cp in kJ/kg·K and ΔT in K (or °C)
- Power P = Q ÷ t, in kW when t is in seconds
- With losses Prequired = P × (1 + allowance)
- Current I = P ÷ V on single phase, or P ÷ (√3 × V) on three phase, for a balanced resistive load
- Element size heated area A = P ÷ watt density limit, and heated length = A ÷ (π × sheath diameter)
A temperature difference in °F converts to K by multiplying by 5/9. Mass comes from volume times density for liquids. The specific heat values filled in are typical room-temperature figures: water 4.18, light mineral oil about 1.9, carbon steel about 0.49 and aluminum about 0.90 kJ/kg·K. They change with temperature and grade, so use your own data where it matters.
Worked example
Heat 200 liters of water from 15 °C to 65 °C in one hour, on a 480 V three-phase supply, with a 20% allowance for losses.
- m = 200 L × 1.00 kg/L = 200 kg, and ΔT = 50 K
- Q = 200 × 4.18 × 50 = 41,800 kJ, or 11.6 kWh
- P = 41,800 kJ ÷ 3,600 s = 11.6 kW, and with 20% losses 13.9 kW
- I = 13,933 W ÷ (1.732 × 480 V) = 16.8 A per line
- At a 12 W/cm² immersion limit the heated area must be at least 1,161 cm², which is about 3.7 m of heated length on a 10 mm sheath
That fits one long element inside the demo's limits, but three elements of about 1.25 m heated and 4.6 kW each is usually easier to fit in a tank and balances the three phases.
What the calculator does and does not cover
It sizes for heat-up: getting a batch to temperature in the time you have. It does not model boiling or any other phase change, heat needed to hold temperature against continuous losses or incoming cold product, or flow heating, where power follows from flow rate rather than batch mass. Size for whichever is larger. The element check uses the Ignitionary demo configurator's defaults: watt density limits of 12.0 W/cm² immersed, 6.0 clamped to a surface and 4.5 in forced air, up to 15 kW and 4,000 mm developed length per element, with at least 40 mm cold zone at each end. Your own limits may differ.
To check an element you already have, use the watt density calculator. The same limits run live in the demo configurator and its CAD API, and a heating element manufacturer encoded 200+ formulas like these in its configurator, as described in the customer story.
Frequently asked questions
Multiply the mass of water in kg by its specific heat, 4.18 kJ/kg·K, and by the temperature rise in K to get the energy in kJ. Divide by the heat-up time in seconds to get kW, then add an allowance for heat losses. For example, 200 L from 15 to 65 °C in one hour needs 11.6 kW before losses.
It depends on the vessel. Insulated, covered tanks lose little; open tanks lose a lot from the surface. This calculator defaults to 20% as a starting assumption. For a firm number, measure or calculate the losses at operating temperature and add them to the heat-up power.
Rearrange the same formula: time equals mass times specific heat times temperature rise, divided by the heater power after losses. In the calculator, change the heat-up time until the required power matches your heater.
Divide the power by the watt density limit for the application to get the minimum heated area, then divide by π times the sheath diameter to get heated length. The calculator does this using the demo configurator's limits and splits the load into elements that stay within 15 kW and 4 m each.
This is one rule. Ignitionary encodes all of yours.
Watt density, bend radius, developed length, sheath temperature, pricing: the live demo checks every one as you configure, and the heating element manufacturer in our customer story encoded 200+ formulas the same way.