An electrical load assessment works out the largest current your home is likely to draw at any one moment, then checks that your supply, main cable, consumer unit and circuits can carry it. It is not a sum of every appliance label. It is a calculation of realistic peak demand, and it decides whether you can add a solar and battery system, an EV charger or a heat pump as things stand, or whether something needs upgrading first.
This guide explains how the calculation works, what an installer checks on site, where the 60A line sits for the network operator, and how to do a rough check yourself before you ask for a quote.
TL;DR
- A load assessment finds your maximum demand, the realistic peak, not the total of all appliance ratings. Diversity allowances make the difference.
- Under BS 7671 Regulation 311.1, maximum demand has to be assessed before the supply, main cables and protective devices are sized.
- The size marked on your fuse holder is not proof of your supply capacity. Only your network operator (DNO) can confirm it.
- If maximum demand reaches 60A per phase, or the adequacy checks fail, you need DNO approval before an EV charger or heat pump goes in.
- Solar, batteries and EV chargers are best assessed together, because they share the same supply.
Table of Contents
- What an electrical load assessment actually measures
- How maximum demand is calculated: diversity explained
- What an installer checks on site
- The 60A line and the DNO
- Why EV chargers change the picture
- Adding solar and a battery: what changes
- A rough check you can do yourself
- Our perspective: assess first, then choose the kit
- Getting your home assessed
- Common mistakes in load assessments
- FAQ
- Sources
What an electrical load assessment actually measures
Two figures matter, and people often confuse them.
Connected load is the arithmetic total of everything that could draw power: every socket circuit, the cooker, shower, immersion heater, lighting and anything fixed that you add.
Maximum demand is the largest current the installation draws at one time. It is always lower than the connected load, because kettles, ovens, showers and chargers are rarely all at full power together.
The Electrical Installation Guide makes the same point: designing from the plain sum of all loads would be extravagantly uneconomical. Without allowing for this, an ordinary ring circuit would theoretically need huge cabling and a domestic supply fuse would be constantly at risk of tripping.
The assessment exists for two reasons:
- Protecting your wiring. Cables, breakers and the consumer unit must be sized for the current they will really carry.
- Protecting the network. Practitioners point out that the exercise is as much about not overloading the local transformer and service cable as about your own circuits. That is why the DNO gets involved once demand is high.
How maximum demand is calculated: diversity explained
The word “diversity” is not defined in BS 7671 itself. The working allowances come from Appendix A of the IET On-Site Guide, which gives figures for an individual household installation. They reflect the fact that not every device runs at once.
As set out in this maximum demand guide, the typical domestic allowances are:
| Load type | Diversity allowance |
|---|---|
| Lighting | 66% of connected load |
| General socket outlets | 100% of the largest circuit plus 40% of the others |
| Cooking appliances | First 10A plus 30% of the remainder |
The on-site guide also has allowances for other fixed loads, and an installer will use the current edition and apply judgement. Treat the table as an illustration of the method, not a design tool.
A worked illustration
Take a simplified house with two 32A ring final circuits, lighting at 10A connected and a cooker rated 40A with no socket on its control unit.
- Lighting: 66% of 10A = 6.6A
- Sockets: 32A + 40% of 32A = 32A + 12.8A = 44.8A
- Cooker: 10A + 30% of the remaining 30A = 10A + 9A = 19A
That is about 70.4A before the shower, immersion heater or any new equipment, and before you apply the allowances the guide gives for those. It shows why an assessment is a proper calculation: add a 7 kW charger to a house already near the line and the answer changes quickly.
Here is the catch. Some loads get little or no diversity. Anything that runs for hours at near full power, such as an EV charger, is treated as a continuous load, and designers rarely allow it to shrink on paper.
What an installer checks on site
The desk calculation is only half the job. A proper survey looks at the following.
The incoming supply. The cut-out holds the DNO’s fuse. Many homeowners assume a fuse holder marked 100A means 100A is available. Qualified electricians warn that this is a risky assumption, because the marking is on the holder and not necessarily on the fuse inside. Only the DNO can confirm the real rating and the capacity behind it. Northern Powergrid has explained that it now standardises on 80A fuses for new whole-current metered connections, and that an 80A fuse can still pass up to 100A for short periods, since household loads cycle through the day. Your own network operator will have its own practice.
The type of supply cable. The service cable must not be a “looped” supply, which is not acceptable for a new high-load addition such as an EV charger, according to this technical bulletin on EV charging installation.
Spare capacity in the consumer unit. This means both electrical headroom and physical space: is there a free way for a new protective device, and is the board suitable? The same bulletin cautions against feeding EV circuits from plastic consumer units.
Fixed equipment ratings. The survey records every fixed load at its real rating. Typical figures quoted in the same bulletin are:
| Equipment | Typical current |
|---|---|
| EV charger | 32A |
| Heat pump | 10 to 20A |
| Electric shower | around 41A |
| Cooker | 30 to 45A |
Existing earthing and cable condition. Higher loads make any weakness in earthing or old wiring more important, so this is checked alongside the numbers. If an inspection is due anyway, our electrical testing pairs well with the survey.
The 60A line and the DNO
The assessment result decides how the network operator is told about new equipment. In broad terms:
- If maximum demand, including the new load, stays under 60A per phase and the supply passes the adequacy checks, the installer can usually notify the DNO after the work.
- At 60A or more, or where adequacy checks fail, the DNO must approve before work begins.
This is set out in the Northern Powergrid installer communication, and the same 60A limit appears on the MCS EV charge point and heat pump connections form, which asks for details of the devices to be installed. Installers on forums describe the same experience: the fuse holder size is not the fuse, and the 60A figure is the trigger for permission.
Who your DNO is depends on your address. As a rough guide, National Grid Electricity Distribution covers Devon and Dorset, and SSEN covers much of Hampshire. Your installer will confirm the exact operator from your postcode and submit the application. Approval is not a reason to panic: it is a paperwork step, and it can add lead time, so it is worth knowing early.
Why EV chargers change the picture
Most household loads are intermittent. A kettle runs for three minutes, an oven for an hour. An EV charger can run at full output for several hours overnight. The Electrical Contractors’ Association’s guidance on electric vehicle charging frames this as the real shift: the challenge is not only capacity, but the move to high-current, long-hour loads.
Two practical consequences follow:
- Installers often uprate the protective device and cable size one step for EV circuits, because of that continuous duty.
- Where the existing supply is tight, load management can keep the house within its limit instead of paying for a supply upgrade. The ECA describes load curtailment as a modern, cost-effective way to fit high-load equipment into older domestic systems.
For how this works in practice, see our notes on EV charger load balancing and dynamic load management.
The size of the car battery also shapes how much charging you need. If you are choosing between a full EV and a hybrid, a plug-in hybrid battery and range guide helps you judge how much home charging you will actually do, and therefore how hard the charger will lean on your supply.
Adding solar and a battery: what changes
Solar and battery systems behave differently from a charger. Panels generate; they do not add to your import demand. But the inverter and battery connect to your installation, and the export side has its own rules. Systems up to 3.68 kW per phase sit under a simpler notification route than larger ones, as we explain in our guide to G98 vs G99.
A good assessment treats the whole home as one system:
- Import peaks: when will the house, charger and battery all pull from the grid together?
- Battery charging: a battery charging from the grid on a cheap tariff adds a load at exactly the time your EV may also be charging. Our piece on off-peak battery charging covers how to schedule this sensibly.
- Sizing: your real usage matters more than rules of thumb. We explain how 12 months of meter data avoids paying for a battery bigger than you need.
- Backup circuits: if you want power during an outage, the backup circuit has its own loads and earthing arrangements. See EPS backup power.
If you want a picture of how these parts fit together, our examples of whole home energy systems show typical combinations.
A rough check you can do yourself
You cannot replace a survey, but you can arrive at a quote conversation prepared.
- List your fixed, high-power appliances: cooker, shower, immersion heater, tumble dryer, any heat pump, any existing charger. Note each rating from the label or manual.
- Look at your cut-out. Photograph the fuse holder and meter. Do not open anything. Note that the marking on the holder does not prove the fuse rating.
- Find your consumer unit: take a photo of the front, with the cover closed, showing how many ways it has and whether it is plastic or metal.
- Gather a year of bills or smart meter data. Past usage shows your real pattern better than any estimate.
- Say what you plan to add. A 7 kW charger, a battery, a heat pump in two years: the assessment should allow for what is coming, not only what is there.
For oil-heated homes, working out how much energy the house uses over winter helps frame the heat pump question later. A guide to estimating winter heating oil needs shows one way to think about seasonal consumption before you replace a fuel-based system with electric heating.
What you should not do is open the cut-out, test the supply yourself or swap protective devices. The supply side belongs to the DNO, and the consumer unit is work for a qualified electrician.
Our perspective: assess first, then choose the kit
We are electrical contractors who install solar, batteries and EV chargers across Dorset, Hampshire and Devon, and the order of work matters to us. Most problems we see come from buying the equipment first and discovering the supply is the constraint afterwards.
Our view is simple. Assess the whole home before you pick products. A load assessment is cheap compared with a charger that cannot run at full speed, a battery that is too large, or a delayed install because nobody asked the DNO early enough. We prefer to look at the consumer unit, the cut-out and your usage together, and to explain what we find in plain terms.
We would also rather solve a tight supply with load management than with an expensive upgrade, where that is safe and suitable. And we would rather tell you that your current supply is fine than sell you work you do not need. Where a job needs MCS-registered work, we say so at the start, and our installs are carried out to the standards that come with our MCS and NICEIC accreditations.
— Simon
Getting your home assessed
If you are planning solar panels, a battery or an EV charger in Dorset, Hampshire or Devon, we will assess your supply and loads as part of the quote, so the system we propose fits the house you have. Start with our solar installations or EV chargers pages, or get in touch through our main site. Once a system is running, our Solar & Battery Service & Maintenance Plan is £175 per year.
Common mistakes in load assessments
- Summing nameplates. This overstates demand and can lead to needless upgrades.
- Applying diversity to everything. Continuous loads like EV charging do not shrink the way sockets and lighting do.
- Trusting the fuse holder. The marking is not the rating inside it.
- Ignoring the neighbours’ reality. A strong supply on paper may still sit on a stressed local network. That is the DNO’s call, not yours.
- Assessing one product at a time. A charger, then a battery, then a heat pump, each checked in isolation, can add up to a problem nobody saw.
- Leaving it too late. DNO approval needs time. Raise it at the quote stage.
FAQ
What is the difference between connected load and maximum demand?
Connected load is the total of all the equipment that could draw current. Maximum demand is the realistic peak at any one moment, found by applying diversity allowances. Supply and cable sizing uses maximum demand.
Do I need to tell my network operator before installing an EV charger?
It depends on your assessed maximum demand. If it is under 60A per phase and the adequacy checks pass, notification can usually follow the installation. At 60A or above, or if the checks fail, you need DNO approval before work starts, and your installer normally handles the application.
Does a 100A marking on my fuse holder mean I have a 100A supply?
Not necessarily. The holder’s marking and the fuse inside can differ, and some network operators now fit 80A fuses on new connections. Only your DNO can confirm the actual rating and available capacity.
Can I do my own electrical load calculation?
You can do a rough estimate of your appliances and usage to help plan, but it is not a substitute for a survey. The real assessment needs the supply, consumer unit and cabling inspected by a qualified electrician, and the figures checked against current regulations.
Will solar panels and a battery affect my load assessment?
Yes. The inverter, battery charging schedule and export limits all form part of the picture, particularly if you also have or plan an EV charger. Assessing them together avoids surprises at the DNO stage.
Sources
- How to Calculate Maximum Demand: Diversity Table, Elec-Mate
- Electrical Diversity Calculator: IET On-Site Guide, TradeCalculator
- Power loading of an installation, Electrical Installation Guide
- Northern Powergrid: LCT installer stakeholder communication, July 2023
- Technical bulletin: installation of EV charging points
- MCS EVCP & HP Connections Form v3.4
- ECA: Electric Vehicle Charging
