Sizing the Electric Replacement for a Fired Steam Boiler
When a gas- or oil-fired boiler lands on the decarbonisation list, the first question on the feasibility sheet is blunt: how many kilowatts of electricity does it take to make the same steam? The existing plant is rated in boiler horsepower, the electric boiler, electrode boiler or high-temperature heat pump you are pricing is rated in kilowatts, and nothing in the two catalogues lines up until both sit on the same scale.
What Changes When the Flame Goes Away
Output stays, input collapses
Pressure decides the technology
The incoming supply becomes the project
Peak rating is not annual energy
Turning the Fired Rating into an Electrical Demand Figure
Most retrofit studies begin with a walk round the boiler house and a list of plate ratings. Each one goes through the same short loop.
Enter the boiler horsepower from the plate
Type 15, 60, 250 or whatever the existing unit carries. The kilowatt figure appears as you type, and a comma works as well as a dot if your survey spreadsheet exports that way.
Take the kilowatts on to amps and a transformer size
Electric boilers run at close to unity power factor, so at 480 V three-phase the current is roughly 1.2 A per kilowatt. Around 1 MW the low-voltage route runs out and the study switches to a medium-voltage electrode unit.
Reverse it when the quotation comes back in kilowatts
Suppliers answer in kW. Press the swap button (↔) to run kW → bhp and confirm that the 1 500 kW unit on the offer genuinely covers the 150 bhp duty you asked about, rather than sitting just under it.
Drop the clean figure into the load study
The copy button hands over the bare number with no unit and no spaces, which is exactly what a demand spreadsheet or a load-flow model expects in a cell. Ctrl + C inside a field does the same.
Fired Boiler Sizes and the Electrical Supply They Demand
Standard packaged steps, the electric duty that replaces each one, and the connection it implies at 480 V three-phase. Currents assume unity power factor, which is close enough for resistance and electrode plant.
| Fired boiler rating | Electric duty | Current at 480 V, 3-phase | Supply that implies |
|---|---|---|---|
| 15 bhp | 147.1 kW | 177 A | 225 kVA transformer; an existing LV board may cope |
| 20 bhp | 196.2 kW | 236 A | 300 kVA transformer, dedicated feeder |
| 30 bhp | 294.3 kW | 354 A | 500 kVA transformer |
| 50 bhp | 490.5 kW | 590 A | 750 kVA transformer, new switchboard section |
| 100 bhp | 981.0 kW | 1 180 A | 1 500 kVA transformer, busbar rather than cable |
| 150 bhp | 1 471 kW | 1 770 A | 2 000 kVA transformer; low voltage near its practical ceiling |
| 300 bhp | 2 943 kW | 3 540 A | Electrode boiler fed at medium voltage |
| 500 bhp | 4 905 kW | 5 900 A | Medium-voltage electrode plant, network reinforcement likely |
Read down the current column and the shape of the problem appears. A gas main carries several megawatts of energy through a 100 mm pipe without anyone noticing; the same duty on wires is thousands of amps. That is why anything past roughly 150 bhp stops being a boiler project and becomes a substation project — and why splitting the duty, with an electric unit on the base load and the fired one held for peaks, so often wins on cost.
What the Converter Does During a Boiler Survey
Both ends live while you sweep the boiler house list
Type in either box and the other keeps up, so you can run through every unit on site — 15, 30, 60, 125 — without clearing the field between plates.
Straight back to bhp when the vendor answers in kW
One press of the swap arrows checks a quoted electric rating against the fired duty it is meant to replace, which is the direction most of the correspondence runs in.
Every power unit on hand for the fuel-side check
The searchable dropdowns cover BTU/h, kcal/h, MW and the rest, so one page handles the burner input figure and the steam output figure without opening a second tool.
Figures that drop straight into the demand sheet
Results carry up to eight decimals with thousands spaced for reading, and the copy button strips all that formatting back to a plain number.
Boiler Electrification Questions
Should the replacement be an electric boiler or a heat pump?
It turns almost entirely on the temperature the process really needs. A heat pump moves several kilowatts of heat for every kilowatt of electricity, so a 100 bhp duty could land near 300 kW of connection instead of 981 kW — an enormous saving where a waste-heat source exists and the delivery temperature suits. Once genuine saturated steam at normal plant pressure is required, the practical options narrow to resistance or electrode plant, or to mechanical vapour recompression where there is flash or exhaust steam worth lifting.
Why does the electrical connection come out so much larger than anyone expected?
Because the fuel side was never treated as a capacity problem. Energy arrived down a pipe at a rate nobody on site had reason to express in kilowatts. Convert it and a middling boiler turns out to be the largest single load on the premises: 50 bhp is 490 kW, comparable to the whole existing demand of a small factory. The heat duty has not grown at all — only the medium carrying it, and wires carry energy far less densely than gas does.
Does the efficiency gain mean I need fewer kilowatts than the converted figure?
No — you need the converted figure exactly, and the gain shows up somewhere else. The fired unit had to burn about 1 225 kW of fuel to deliver 981 kW of steam at 80 % efficiency; the electric unit delivers that same 981 kW from 981 kW at its terminals. Either way the connection is sized on output. What disappears is the stack loss, the standing loss and the combustion-air ventilation, not the size of the switchgear.
Is it worth dropping from steam to hot water while the plant is being changed?
Very often yes, and it is the question that unlocks heat pumps. Plenty of sites distribute steam only because a boiler was the obvious purchase decades ago; the actual users are washing, space heating or a jacketed vessel that would work happily on pressurised water at 90–120 °C. Survey the users before the boiler. If one true steam user remains — sterilisation, direct injection, a press — keep a small steam plant for it and move the rest onto water instead of electrifying the entire rating.
Our gas meter reads more than the converted kilowatts. Which figure do I size on?
Size on the converted output and use the meter to sanity-check the load factor. Metered fuel is input; the boiler rating is output, and the gap between them is combustion efficiency plus everything the plant lost while idling. Take the annual gas figure, apply the efficiency and you have the annual steam energy — divide by running hours and you learn the average duty, which is usually a fraction of the plate. That average tells you what the electricity will cost; the converted peak tells you what the connection has to carry.
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