Electric Combi • Engineering Sizer

Electric Combi Boiler Size Calculator: kW & Flow Rate

Q: Can an electric combi boiler run a powerful shower?

Direct electric combis produce around 5 L/min (adequate for eco showers). For a powerful high-pressure shower, select an electric storage combi with an internal hot water tank.

Q: What electrical supply is needed for an electric combi boiler?

A 12kW electric combi requires a 100A main fuse, a dedicated 16mm² cable, and a 63A breaker on the consumer unit.

Size direct electric combination and storage combi boilers within residential electrical grid limits.

BS 7671 (18th Edition) & CIBSE Domestic Guide • BS EN 12831 Verified • Dual Unit (kW & BTU)

BS 7671 (IET Wiring Regs) Electrical Engine

Electric Boiler Size, Amps & Supply Limit Calculator

230V Single Phase / 400V 3-Phase

Electric Boiler Capacity (kW): 12 kW (52 Amps)

4 kW (Studio) 12 kW (Max Single-Phase 60A/80A) 24 kW (3-Phase Only)

Main Incoming Supply Fuse:

Other High-Load Appliances:

Electrical Current Demand

52.2 Amps

✓ Compatible with 100A Main Fuse

Required Dedicated Cable Size: 16.0 mm² Twin & Earth

Dedicated MCB / RCBO Rating: 63A Type B Breaker

Combined Total Peak House Draw: 97.2 Amps

⚡ Electrical Constraint: A standard single-phase 100A supply can support up to a 14.4 kW electric boiler. For larger 18 kW–24 kW electric heating, a 3-phase (400V) electrical supply upgrade from your DNO is mandatory.

⚠️ High Amperage Safety Notice (BS 7671 & Part P) Electric boilers draw continuous heavy currents that can overload main household cut-out fuses (60A/80A/100A) and create fire hazards if undersized cables are used. All electrical heating connections must be carried out by a qualified Part P registered electrician and notified to your local Distribution Network Operator (DNO).

Hydraulic Flow Simulator

Can a Combi Boiler Run Two Showers at Once?

Simultaneous Outlet Physics

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:

1. Select Installed Combi Boiler Size

Incoming Mains Cold Water Flow Limit 16.0 L/min

10 L/min 16 L/min 26 L/min

Toggle Active Simultaneous Outlets in Use

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Master Ensuite Shower 9.0 L/min

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Family Bathroom Shower 9.0 L/min

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Kitchen Hot Tap 5.5 L/min

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Washing Machine Fill 4.5 L/min

Hydraulic Delivery State Optimal Flow

Delivered Tap Water Temperature

45.0°C

🔥 45°C (@ 35°C Rise)

Total Demanded Flow Rate: 9.0 L/min

Max Boiler Thermal Delivery (@45°C): 13.1 L/min

Verdict:

The boiler comfortably supplies the shower at full mains pressure and temperature.

💡 If you frequently run 2 showers simultaneously, choose an 18–24 kW System Boiler + 210L Cylinder instead of an oversized combi.

Thermodynamic Methodology

Electric Instantaneous DHW Physics Formula & Variables

BS 7671 / CIBSE Guide G

Mathematical Formulation

DHW_Flow [L/min] = ( Power [kW] × 60 ) / ( 4.186 × ΔT [35°C] )

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.

Power

Maximum allowable single-phase boiler wattage (typically 12 kW or 14.4 kW) Unit: kW

DHW_Flow

Maximum possible instantaneous shower delivery rate Unit: Litres per minute

Lookup Standards

Electric Combi Boiler Output & DHW Flow Limitations

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.

Key Principles

Critical Sizing Factors & Engineering Influences

The 2 primary variables that determine actual heat demand, and how BS 7671 (18th Edition) & CIBSE Domestic Guide calculates each.

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Internal Thermal Storage Heat Buffering

Electric storage combis heat an internal 40L–50L reservoir slowly, discharging full 12 L/min shower flow on demand.

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Eco Aerated Shower Heads (6 L/min)

Aerated shower heads mix air into the stream, providing a strong sensation with lower water consumption.

Prevent Costly Errors

Common Sizing Mistakes & How to Avoid Them

Fuel delivery and burner miscalculations to prevent on electric combi boiler size calculator: kw & flow rate installations — circuit and tariff oversights that make an otherwise correct kW rating unusable.

Common Pitfall

Expecting 14 L/min from a Direct Electric Combi

Consequence: A 12 kW direct electric boiler delivers only 4.9 L/min, resulting in a cold, weak shower if high flow is drawn.

Correct Approach: Select a storage electric combi or pair with an unvented cylinder.

Quality Assurance Audit

Electric Combi Boiler Size Calculator: kW & Flow Rate: Installer Consultation Audit

Put these 2 essential questions to your heating engineer before approving an installation quote — benchmarked against BS 7671 (18th Edition) & CIBSE Domestic Guide.

Readiness Score: 0 / 2

Main Incoming Electrical Fuse Inspected Confirmed 100A cutout fuse installed by DNO.

Low-Flow Aerated Shower Handsets Fitted Calibrated shower head to ≤ 6.0 L/min for direct electric systems.

Technical Deep Dive

Direct Electric vs Thermal Storage Electric Combis

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.

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CIBSE & BS EN Standards-Compliant Framework BENCHMARKED

Formulations benchmarked directly against published British & European standards: BS 7671 (18th Edition) & CIBSE Domestic Guide (IET & CIBSE) for Electric Combi Boiler Size Calculator: kW & Flow Rate. Maintained by the Boiler Size Calculator Technical Editorial Desk.

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Standards Revision: 2026

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