Consumer Unit Calculator

Consumer Unit Calculator — estimate consumer unit for your project. Fast, accurate, free. No signup required.

Service sizing should be checked against both demand and the building envelope. Use the Power Consumption calculator for electrical demand, the Home Energy calculator for improvement savings, and the Water Sizing calculator for connected plumbing and storage capacity.

The building services calculator covers mechanical, electrical, and plumbing (MEP) sizing calculations for construction — from boiler output and radiator sizing to EV charger power requirements, battery storage capacity, and mechanical ventilation airflow rates. Building services are the hidden infrastructure that makes buildings habitable: heating, hot water, ventilation, and electrical systems account for 60–80% of a building's energy consumption and must be correctly sized to function efficiently, comply with Building Regulations, and avoid costly over- or under-specification. A boiler that is too small cannot maintain design temperatures in cold weather; one that is too large short-cycles continuously, reducing efficiency and component lifespan.

UK Building Regulations Part L (Conservation of Fuel and Power) and Part F (Ventilation) set minimum standards for heating system efficiency, insulation levels, and ventilation rates. The SAP (Standard Assessment Procedure) calculation that produces an EPC rating depends directly on the building services specification, including boiler efficiency, hot water cylinder size, and mechanical ventilation type.

The mode is selected automatically based on your topic: boiler/radiator (heat output sizing), hot water (cylinder capacity), EV charger (charge time), battery (backup capacity), or air change rate/MVHR (ventilation flow).
For boiler sizing: enter the total heated floor area, the inside-to-outside temperature difference (typically 24°C for UK design conditions: 21°C inside, −3°C outside), and the property's overall U-value or heat loss rate.
For hot water: enter the number of occupants, daily hot water demand per person (typically 35–50 litres), and the target cylinder temperature (typically 60°C to prevent legionella).
For EV charging: enter battery capacity (kWh), charger power (kW), and current state of charge (%) to calculate charging time.
For ventilation: enter room dimensions and required air change rate to calculate the necessary mechanical ventilation airflow.

Building services sizing formulas

Heat loss (simplified): Q = A × U × ΔT where A = floor area (m²), U = overall heat loss coefficient (W/m²K), ΔT = design temperature difference (°C); Q in watts.

Boiler output: P_boiler = Q × 1.25 (add 25% margin for hot water, startup, and system losses).

Hot water cylinder: V = Occupants × Daily_demand × 1.5 (litres; 1.5× daily demand for adequate reserve).

EV charge time: t = Battery_kWh × (1 − SOC%) ÷ Charger_kW (hours).

Ventilation flow: Q_vent = Room_volume × ACH (m³/hr), where ACH = air changes per hour required.

Example (boiler): 120m² house, U = 0.25 W/m²K, ΔT = 24°C. Q = 120 × 0.25 × 24 = 720W per degree (simplified); design heat loss ≈ 7.2 kW. Boiler: 7.2 × 1.25 = 9 kW minimum — a 12kW combi is appropriate.

Reading building services results

Standard sizes and outputs

Typical boiler sizes: 24–28kW combi for 1–3 bedroom properties; 30–35kW combi or system boiler for 4+ bedrooms. Hot water cylinders: 120L for 1–2 persons; 150L for 3 persons; 180–210L for 4–5 persons (Building Regulations require cylinders to be capable of reaching 60°C in legionella prevention mode). EV charger types: 3.6kW (16A single-phase socket) = 12–14 hours to charge from empty; 7kW (32A home wallbox) = 6–8 hours; 22kW (3-phase commercial) = 2–3 hours; 50kW rapid DC = 30–60 minutes. Ventilation (Building Regulations Part F): extract rates — kitchen 30 l/s (intermittent) or 13 l/s (continuous); bathroom 15 l/s (intermittent) or 8 l/s (continuous); whole-house MVHR targets 0.5 ACH background ventilation.

Construction tips and best practices

Oversizing a boiler reduces efficiency — a combi boiler should run for at least 10 minutes per cycle; oversized boilers short-cycle, reducing efficiency from a rated 90% to 70–75% in practice.
For EV home charging, ensure your consumer unit has capacity for a 7kW (32A) circuit before installing a home wallbox — older installations may need an upgrade to 100A service fuse.
MVHR systems require annual filter cleaning and 5–10 yearly heat exchanger maintenance — factor this into the whole-life cost comparison with simpler ventilation strategies.
Legionella prevention: domestic hot water cylinders must be capable of reaching 60°C (and should be programmed to do so at least weekly) to prevent legionella growth in stored water.
Battery storage sizing: total useful storage = capacity × depth of discharge (typically 80–90% for lithium batteries). A 10kWh battery with 90% DoD provides 9kWh usable.

Space heating accounts for approximately 62% of UK household energy consumption — more than all other uses combined.
A 7kW home EV charger adds approximately 30–40 miles of range per hour of charging, enough to charge most EVs overnight from a typical daily commute deficit.
MVHR (Mechanical Ventilation with Heat Recovery) can recover 85–95% of heat from exhaust air — in a well-insulated Passivhaus, MVHR can supply most of the space heating requirement using recovered heat alone.
A 210-litre hot water cylinder takes approximately 2–2.5 kWh to heat from cold using a heat pump (COP 3.0) — versus 5–6 kWh for a direct electric immersion heater.

Common mistakes to avoid

Sizing a boiler without accounting for hot water demand — a combi boiler must also deliver domestic hot water on demand; a boiler sized only for space heating may struggle to meet simultaneous hot water and heating demand.
Installing an EV charger without checking the meter tails capacity — a 7kW charger draws 32A continuously; combined with other household loads this can exceed the 60A or 80A service fuse on many older UK properties.
Assuming all battery storage is usable — most lithium batteries protect themselves by limiting discharge to 80–90% DoD; a nominal 10kWh battery delivers 8–9kWh usable.
Sizing ventilation extract only without considering supply — in a well-sealed new build, mechanical extract without mechanical supply creates a pressure imbalance that can cause draughts, door slamming, and increased heat loss through adventitious infiltration.

Heating system installation in the UK is subject to Building Regulations Part L (Conservation of Fuel and Power) and must be notified to Building Control or carried out by a Gas Safe registered engineer (for gas) or NICEIC/ELECSA registered electrician (for electrical systems). EV charger installation requires compliance with BS 7671 (18th Edition IET Wiring Regulations) and must be notified under Part P of the Building Regulations; chargers must also comply with the Electric Vehicles (Smart Charge Points) Regulations 2021. MVHR and mechanical ventilation systems must comply with Building Regulations Part F and be commissioned and tested to CIBSE Commissioning Code A. Hot water systems must comply with the Water Supply (Water Fittings) Regulations 1999 and, where unvented, must be installed by a registered G3 competent person. This calculator provides preliminary sizing estimates only.