How Is Maximum Demand Calculated for a Heat Pump?

If you're planning a heat pump installation, or you're an installer trying to work out whether a property's electrics can cope with one, you've probably come across the term "maximum demand." It sounds like it should have one clean, agreed formula behind it. It doesn't. There isn't a single calculation that every electrician in the UK is legally required to use, and understanding why is the first step to understanding how maximum demand actually gets assessed in practice.

Maximum demand is the estimated peak amount of electrical current a property is likely to draw at any one time, once every circuit, including the proposed heat pump, is taken into account. It's a judgement call based on how a property is actually used, not a mechanical sum of every breaker rating in the consumer unit. Getting this estimate right matters because it determines whether an installation can simply go ahead (Connect and Notify) or whether it needs formal approval from the local electricity network first (Apply to Connect), which can add time to a project.

Why You Can't Just Add Up Every Breaker

This is probably the single biggest misconception homeowners and even some installers have about electrical capacity.

Picture a fairly ordinary consumer unit:

• Cooker – 40A

• Ring main – 32A

• Upstairs sockets – 20A

• Downstairs lights – 6A

• Upstairs lights – 6A

• Proposed heat pump – 16A

Add those together and you get 120 amps. Taken at face value, that would mean almost every house in the country needs an electrical supply upgrade before a heat pump can go in. That's obviously not the case, and the reason comes down to what a breaker is actually protecting.

A circuit breaker is sized to protect the cable behind it from overheating, not to describe how much current that circuit constantly draws. A 40A cooker breaker doesn't mean the cooker pulls 40 amps around the clock; it means the cable is rated to carry up to 40 amps safely if it ever needed to. In reality:

• Your oven cycles its elements on and off rather than running at full power continuously.

• Lighting circuits are almost never anywhere near their rated maximum.

• Socket circuits might just be powering a TV, a router, and a few chargers at any given moment.

• An immersion heater backup, common on many heat pump installations, often isn't running at all while the heat pump is actively heating the house.

This is the concept of diversity, and it's the reason electrical designers apply judgement rather than arithmetic when estimating maximum demand.

Understanding Diversity in Practice

Diversity accounts for the fact that not every appliance in a home runs at full rated power simultaneously. In a typical household, demand is spread out across the day: the kettle might be on in the morning, the cooker in the evening, sockets running low-draw electronics most of the time, and the heat pump modulating its output depending on outdoor temperature and heat loss. Diversity factors, like the 40% figure used in the method below, are a way of reflecting that real-world behaviour in a single, workable estimate rather than designing for a theoretical worst case that almost never happens.

The Heat Geek Method for Calculating Maximum Demand

One widely used method, taught by Heat Geek and one that holds up consistently well in practice, breaks the calculation into two simple steps:

Step 1: Take 100% of the largest breaker in the consumer unit.

Step 2: Add 40% of every other breaker, including the proposed heat pump.

This gives a realistic, defensible starting point for estimating a property's maximum demand before any more detailed network assessment takes place.

Worked Example

Using the consumer unit above:

• Largest breaker: 40A cooker circuit → 40A × 100% = 40 amps

• Remaining breakers: 32 + 20 + 6 + 6 + 16 = 80 amps → 40% of 80 = 32 amps

• Estimated maximum demand: 40 + 32 = 72 amps

What Happens When Maximum Demand Exceeds 60 Amps?

This 72-amp figure isn't just an academic exercise. It directly decides which route the installation takes with the local Distribution Network Operator, or DNO. Anything estimated above 60 amps typically pushes the installation from Connect and Notify into Apply to Connect, meaning the installer has to submit a formal application and wait for the DNO to confirm the local network has spare capacity before work can begin. Below that threshold, the installer can usually connect the system and simply notify the DNO afterwards.

Falling into Apply to Connect isn't a rejection of the installation, and it doesn't mean anything has gone wrong with the design. It simply means an extra administrative step is needed so the DNO can check that the transformer, cabling, and local substation serving that street can safely absorb the extra load. We cover this distinction in much more depth, including what the DNO actually checks and how long each route typically takes, in our article on whether you need to inform the electricity grid before installing a heat pump, which is worth reading alongside this one if you want the full picture of how electrical assessment fits into the wider installation process.

Is the Heat Geek Method the Only Way to Calculate Maximum Demand?

No, and it's important to be upfront about that. There is no legally mandated formula every electrician must use, and the Heat Geek method is best treated as an excellent starting point rather than a final answer. A thorough assessment should also factor in:

• Electric showers, which can be substantial standalone loads

• EV chargers, particularly if one is already installed or planned

• Immersion heaters used as heat pump backup during defrost cycles or servicing

• Battery storage systems, which can affect how demand is spread through the day

• Solar PV, which changes the picture but doesn't remove the need for a demand assessment

• Backup or secondary heaters that might run alongside the heat pump

• How the property is genuinely used day to day, since two identical consumer units can represent very different real-world demand depending on the household

This is exactly why correct system sizing and electrical assessment go hand in hand rather than being treated as separate boxes to tick. An undersized or incorrectly assessed supply can lead to problems down the line, and an oversized or poorly matched heat pump creates its own issues too, something we go into in what happens if a heat pump is oversized and what size heat pump does my house need.

Why This Step Gets Missed (And Why It Matters)

Maximum demand assessment is one of those early-stage planning steps that's easy for a rushed or less experienced installer to skip or underestimate, alongside things like an accurate room-by-room heat loss calculation and correct flow temperature targeting. It's rarely the flashy part of a quote, but getting it wrong can mean discovering partway through a project that a DNO application is needed after all, adding delay right when you expected the system to be going in. We've written more broadly about the pattern of what tends to go wrong when this stage is rushed in our guide to common commissioning mistakes with air source heat pumps and in 7 signs your heat pump may not be installed correctly, both worth a read if you're currently comparing quotes.

Frequently Asked Questions

Do I need to calculate maximum demand myself?
No. A competent MCS-certified installer will carry out this assessment as a standard part of the design process and handle any resulting DNO notification or application on your behalf.

What happens if my estimated maximum demand is over 60 amps?
Your installer submits an Apply to Connect application to your local DNO before work begins, so the network operator can confirm there's sufficient spare capacity, or arrange upgrade works if there isn't.

Does having an EV charger or electric shower change the calculation?
Yes. Both should be included as part of the wider assessment alongside the heat pump, since they add meaningfully to a property's overall electrical demand.

Is a higher maximum demand a sign the heat pump has been badly sized?
Not necessarily. Maximum demand is a separate question from whether the heat pump itself has been correctly matched to the property's heat loss. A well-sized heat pump can still push a property over the 60 amp threshold simply because of what else is already installed.

Thinking About Installing a Heat Pump?

Getting the electrical design right from the start, alongside heat loss calculations, system sizing, and flow temperature, is one of the most important parts of a successful installation. Our Pre-Installation Design Review service gives you an independent assessment before you commit, reviewing the proposed system, discussing the electrical requirements including maximum demand, and flagging any issues before installation begins. It's also worth browsing our case studies to see the kind of design details that get missed when this stage is rushed.

Already Have a Heat Pump?

If your heat pump isn't performing as expected, whether that's fault codes, poor heating performance, or a system that just doesn't feel right, our Fix My Heat Pump service provides independent remote technical support, often diagnosing the issue over a video call before an engineer needs to visit.

Thinking About Adding Cooling?

Many modern heat pumps can provide cooling as well as heating, though not every system or electrical setup is suitable. Our Heat Pump Cooling Advice service explains what's required, whether your existing system could support it, and what changes would be needed to do it safely.