Do You Really Need a Supply Upgrade? Amps, Heat Pumps and EV Chargers, Explained

Every few weeks someone asks us some version of the same question: I am getting a charger and a heat pump fitted, do I need a new supply? The honest answer is that it depends entirely on three numbers, and most homeowners have never been told any of them.

The three are: the rating of your cut-out fuse, the maximum demand of the installation after diversity has been applied, and how much of that spare capacity the new kit will actually eat. Get those straight and the decision tends to make itself.

What the 100-amp question actually means

Your supply arrives as a single-phase service, usually at 230 volts, through a sealed fuse known as the cut-out. That fuse might be rated at 60, 80 or 100 amps. A 100A supply gives you roughly 23kW to play with in total. A 60A supply gives you around 14kW. That is the whole budget for the house, not per circuit.

Older terraces and 1930s semis are frequently on 60A or 80A. Many also have a looped supply, meaning the service cable is shared with a neighbour, which adds a layer of complexity when you want to add a big load. Newer builds are usually 100A, but not always.

You are not supposed to break the seal and look. A decent electrician will read the rating off the cut-out or check with the distribution network operator.

What the new kit genuinely draws

These are the figures that surprise people, mostly because they are lower than expected in some places and much higher in others.

• A 7kW home charger pulls about 30A, continuously, for hours at a time. It is the longest sustained load most homes will ever run.
• An 8 to 12kW air source heat pump typically draws somewhere between 2 and 4kW while running, so roughly 9 to 17A. That is a big improvement on a boiler replacement in carbon terms, but it is not a small load. Cold snaps and hot water reheats push it to the top of that range.
• An induction hob rated at 7.2kW sounds terrifying until diversity is applied. Very few households run every zone flat out at once, so the realistic figure is often less than half the nameplate number.
• An electric shower is the one people forget. An 8.5kW unit is about 37A; a 10.5kW unit is closer to 45A. Showers are short, but they are brutal while they last.

Diversity: why you cannot just add everything up

This is where a proper load calculation earns its keep. BS 7671 sets out how to estimate maximum demand, applying diversity factors so you are not assuming every appliance in the house switches on simultaneously.

A worked example helps. Take a three-bed semi on a 100A supply. After diversity, the existing installation might work out at around 50 to 60A. Add a 30A charge point and you are at 80 to 90A. Tight, but workable, especially with load management in place. Now put the same house on a 60A supply and it falls apart: a 30A charger plus an electric shower plus an oven is not a comfortable mix.

That is the whole decision in a paragraph. Two houses that look identical from the street can land on opposite sides of the line, which is why nobody can give you a responsible answer over the phone without the numbers.

Signs your supply is already close to the edge

There are a few practical tells. Lights that dip noticeably when the kettle boils or the shower starts. Fuses or breakers that trip more than once a season. A cut-out marked 60A on an all-electric house. Persistent voltage sag that makes LED lamps flicker. Warm or discoloured accessories, though that is usually a wiring or connection problem rather than a supply capacity issue, and needs investigating either way.

If any of that sounds familiar, sort out the underlying fault before you consider adding 30A of continuous load to it.

What a DNO upgrade actually involves

Your distribution network operator is the company that owns the cables in the street: UK Power Networks, SSEN, NGED, Northern Powergrid or SP Energy Networks, depending on postcode. Not your supplier, and not your electrician.

An upgrade usually starts with a load application and a questionnaire. Depending on the answer you might get a site visit, a new service cable, a new cut-out, possibly a meter reposition, and occasionally excavation plus a road permit. Timescales run from a few weeks to several months, and costs vary enormously: a fuse upgrade on an existing cable is one thing, a completely new service run to a property is quite another. Looped supplies often need unlooping first, which can involve the neighbour and their permission.

Three-phase is the other option. It solves capacity beautifully and costs considerably more, and some network operators now offer it more readily than they did a few years ago.

The cheaper path: manage the load instead

This is where most homes end up, and rightly so. Modern charge points can be fitted with dynamic load management: a current clamp on the meter tails watches the whole house and tells the charger to back off, or pause entirely, when demand climbs. A heat pump with a soft start plus a priority switch between the charger and the heat pump does a similar job more crudely but very effectively.

Then there is timing. Most charging happens overnight anyway, and off-peak tariffs reward you for it. A heat pump's hot water reheat can usually be nudged away from the evening peak. Induction hobs, being diversified and short-lived, are rarely the problem people fear.

Add those together and a lot of homes that look marginal on paper turn out to be perfectly fine on a 100A supply, or even an 80A one, without a spade going anywhere near the pavement.

How to get a straight answer for your own house

Ask for a maximum demand calculation in writing before anyone quotes you for the work. Get the DNO enquiry started early, because their timelines, not the installer's, tend to set the schedule. And be deeply suspicious of anyone who promises a 7kW charger on a looped 60A supply with a shrug and no plan.

The kit itself is well understood now. The variable is your supply, and that is worth an hour of arithmetic before you spend four figures on an upgrade you might not need. If you would like yours looked at properly, JEGS Electrical can run the numbers and tell you plainly where you stand.