Electricity Supply Capacity Calculator (kVA)

How the calculation works

What supply capacity means and why it matters

Your electricity supply can only deliver a limited amount of power at any moment. In a home, that limit is set by the main fuse and the service cable, and it is expressed in kilovolt-amperes (kVA). This calculator estimates the lowest standard supply band that covers the total demand of your home, so you can judge whether the current supply is likely to be enough.

The choice matters in both directions. If the capacity is too low, the main fuse can trip when the oven, kettle, washing machine and an EV charger run together, and someone has to reset the supply or reduce the load. If the capacity is well above what the home needs, the connection is larger than it needs to be. The aim is to find the band that covers realistic peak use with a reasonable margin.

How the band model works

The calculator adds five components together:

• P_base: the base demand of the home, set by its number of bedrooms (studio or 1-bed, 2-bed, 3-bed, or 4-bed and larger).
• P_cooker: the extra demand from the cooking type.
• P_water: the extra demand from water heating.
• P_climate: the extra demand from electric heating or cooling.
• P_EV: the extra demand from charging an electric vehicle.

Total demand is the sum of these five values. The calculator then selects the lowest band whose value is equal to or above the total. If the total equals a band value exactly, that band is enough, because the rule is 'at or above'.

The bands are 1.15, 2.30, 3.45, 4.60, 5.75, 6.90, 10.35, 13.80, 17.25 and 20.70 kVA. On a 230 V single-phase supply, current in amperes equals kVA multiplied by 1,000 and divided by 230. The bands therefore correspond to 5 A, 10 A, 15 A, 20 A, 25 A, 30 A, 45 A, 60 A, 75 A and 90 A. These bands are reference steps for the planning model. The supply a home actually has is set by its network operator and existing connection, so use the result as a guide when you discuss capacity, not as a tariff choice.

Base demand by home type

The base figure covers the everyday load of a home: lighting, a fridge-freezer, a washing machine, a television, heating controls and general-purpose sockets. It assumes gas or solar water heating, no dedicated electric heating and no EV charger. The values are 3.45 kVA for a studio or 1-bed, 4.60 kVA for a 2-bed, 5.75 kVA for a 3-bed and 6.90 kVA for a 4-bed or larger home.

The base follows the number of bedrooms, so a large open-plan 2-bed flat and a small 2-bed flat receive the same base value. If the home has heavier everyday use than typical, such as a home office with several computers or a large fridge-freezer running alongside other loads, round the home size up one step as a margin.

Cooking

A gas hob adds nothing to the electrical demand, because its burners do not draw power from the electricity supply. An electric ceramic hob or an induction hob adds 2.5 kVA, which the model uses as the planning allowance for the hob and oven working together at a typical level. A four-zone hob at full power can draw several kilowatts on its own, so the allowance reflects typical cooking, not the maximum rating. If you regularly cook on all the rings and the oven at full power for long periods, ask your installer to check the design against the hob's rated power.

Water heating

An electric storage water heater adds 2.0 kVA. It heats for long periods, often overlaps with cooking and bathing, and is therefore counted in full. A gas combination boiler, a gas storage system and a solar system add nothing in this model, since their electrical draw is small compared with the other loads. The calculator has no option for a heat pump water heater. If you have one, ask the installer for its rated power and allow for it by choosing the next home size up as a margin.

Space heating and cooling

Electric heating and cooling are the largest source of uncertainty in this model. One air conditioning unit adds 1.5 kVA and two or more units add 3.0 kVA. A heat pump providing central heating adds 3.5 kVA, and electric convector heaters spread through the home add 3.0 kVA. The model treats the heating as running alongside the other loads, which approximates a cold winter evening. In a mild climate, or with a smaller system, the real peak is lower.

A heat pump can move a home up a band. If you are planning one, check the heat pump installer's design load alongside this result, because the installer will work from the actual heat loss of the building.

EV charging

EV charging is the factor most likely to change the band. A Mode 2 portable cable adds 2.3 kVA, a 3.7 kW wallbox adds 3.7 kVA and a 7.4 kW wallbox adds 7.4 kVA. A 7.4 kW charger draws 32 A from a 230 V single-phase supply, which is about 7.4 kW, so the allowance matches its rated power. Smart chargers that reduce output when the household is using a lot of power may lower the real peak, but the calculator uses the rated power and does not model that behaviour.

The effect can be large. A 2-bed flat with a gas hob starts at 4.60 kVA. With a 3.7 kW wallbox it reaches 8.30 kVA, which needs the 10.35 kVA band. With a 7.4 kW wallbox it reaches 12.00 kVA, which needs the 13.80 kVA band. Wallboxes of 11 kW or 22 kW are normally installed on a three-phase supply and fall outside this calculator.

Worked examples

The following examples follow the same steps as the calculator:

• A 1-bed flat with a gas hob, gas water heating, no electric heating and no EV charger: 3.45 kVA, which is exactly the 3.45 kVA band (15 A).
• A 2-bed flat with an induction hob, an electric storage water heater and no EV charger: 4.60 + 2.5 + 2.0 = 9.10 kVA, which needs the 10.35 kVA band (45 A).
• A 2-bed flat with a gas hob and a 3.7 kW wallbox: 4.60 + 3.7 = 8.30 kVA, which needs the 10.35 kVA band (45 A).
• A 3-bed house with a gas hob, a heat pump and a 7.4 kW wallbox: 5.75 + 3.5 + 7.4 = 16.65 kVA, which needs the 17.25 kVA band (75 A).

Limits of the estimate

• Demand is summed as if every load runs at once. That makes the result a cautious estimate, and it can recommend a higher band than a household with staggered use would need.
• The calculator does not model solar generation, battery storage or heat pump water heaters.
• Large or specialist loads, such as a hot tub, a swimming pool pump, a workshop or a commercial kitchen, are not in the model and need a separate assessment.
• The model assumes a single-phase 230 V supply. Homes with a three-phase supply, or those planning one, need a different assessment.
• The result depends on the home size and the appliances you select. A home with an unusual layout or heavy everyday use may need a higher step.

Common mistakes

• Treating a gas hob as electric, or forgetting an electric storage water heater.
• Forgetting future plans. A heat pump or EV charger added in a year's time can push the home over the band you chose today.
• Relying on the current fuse rating alone. The fuse shows what the supply allows now, not what the home needs.
• Assuming a bigger band is always better. A higher supply does not fix wiring that is undersized for the loads, and an upgrade may involve work on the service cable.

Regulations and who to contact

The electrical design of the home is governed by BS 7671, the wiring regulations, which set the rules for circuit design, protection and diversity. Miniature circuit breakers in the consumer unit are typically to BS EN 60898. In England and Wales, Part P of the Building Regulations covers electrical work in dwellings; in Scotland, the Building Standards apply. Notifiable work can be carried out by a registered competent person who can self-certify it, or it can be notified to building control.

Changes to the supply itself are handled by the network operator, which owns the service cable and the cut-out. Ofgem regulates energy networks and suppliers in Great Britain, so it is a useful reference for how connections and charges are regulated.

When to call a professional

The calculator gives an orientation estimate. Confirm the final band with the network operator and a qualified electrician, who should check the service cable, the meter tails and the consumer unit. Circuits for an EV charger or an electric water heater should be designed to BS 7671 and certified on completion with an Electrical Installation Certificate. Never replace or upsize a fuse yourself.