Size direct electric combination and storage combi boilers within residential electrical grid limits.
Homeowners often wonder why their shower turns freezing or drops to a trickle when someone runs the kitchen tap. Unlike system boilers with hot water cylinders, a combi boiler must heat water on-the-fly. Test how simultaneous taps impact water temperature and flow rate below:
🔥 45°C (@ 35°C Rise)
The boiler comfortably supplies the shower at full mains pressure and temperature.
Direct electric heating has a fixed energy limit: at 230V single phase (100A fuse max 14.4 kW), maximum instantaneous hot water delivery is approximately 5.9 L/min.
Instantaneous shower flow delivery across electric combi sizes.
| Electric Combi Size | Amperage Draw (230V) | DHW Flow (@ 35°C Rise) | Internal Storage Tank | Shower Performance |
|---|---|---|---|---|
| 9.0 kW Direct Combi | 39.1 Amps | 3.7 L/min | None (Instant) | Hand Basin / Low Flow |
| 12.0 kW Direct Combi | 52.2 Amps | 4.9 L/min | None (Instant) | Eco Aerated Shower Head Only |
| 14.4 kW Direct Combi | 62.6 Amps | 5.9 L/min | None (Instant) | Moderate Shower |
| 12.0 kW Storage Combi (e.g. Elektra C19) | 52.2 Amps | 12.0 L/min | 50L Internal Thermal Store | Full High-Pressure Shower |
Storage electric combis bridge the flow gap using an internal insulated thermal store.
The 2 variables that move the electric combi boiler result most, and how BS 7671 (18th Edition) & CIBSE Domestic Guide expects each one to be established.
Electric storage combis heat an internal 40L–50L reservoir slowly, discharging full 12 L/min shower flow on demand.
Aerated shower heads mix air into the stream, providing a strong sensation with lower water consumption.
The mistake below costs more than any other on a electric combi boiler job — circuit and tariff oversights that make an otherwise correct kW rating unusable.
Consequence: A 12 kW direct electric boiler delivers only 4.9 L/min, resulting in a cold, weak shower if high flow is drawn.
Put these 2 questions to your heating engineer before you approve a quoted output — each one checks the specification against BS 7671 (18th Edition) & CIBSE Domestic Guide.
Direct electric combis are constrained by domestic 100A fuse limits (maximum 14.4 kW / 5.9 L/min). Storage electric combis solve this by incorporating a compact internal thermal heat store.
Formulations benchmarked against BS 7671 (18th Edition) & CIBSE Domestic Guide (IET & CIBSE) for Electric Combi Boiler Size Calculator.
2 questions we are asked most about electric combi boiler sizing, answered against IET & CIBSE guidance.
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