What an electrician is working out when they look at the consumer unit

The supply has a rating, and it is not negotiable on the day

Every house is fed through a service fuse at the intake, sized by the network operator and sealed by them, and the cable running in from the street is sized to match it. That fuse is the hard ceiling on everything the property can draw simultaneously. No amount of internal rewiring moves it, and nobody but the operator may change it.

Households rarely think in these terms because ordinary domestic loads never approach the limit. A kettle, an oven and a shower running together is a large draw by historical standards and still leaves headroom. The arithmetic changes when a heat pump running for hours meets a car charger running for hours and a battery charging from the grid at the same time. So the first thing an assessment establishes is what the supply is rated at, what is already connected, and what the proposed additions would draw. It’s an unglamorous exercise in adding up, and it is the reason a quotation sometimes comes back with an answer nobody wanted.

Diversity is the assumption that everything does not happen at once

Add up the nameplate ratings of everything in an average house and the total comfortably exceeds the service fuse, often by a wide margin. Houses do not blow their fuses constantly because the appliances are not all used together, and the wiring rules formalise that observation as diversity: a set of factors that reduce assumed simultaneous demand for each class of load.

Those factors are statistical rather than physical. They describe how people have historically behaved, which is a reasonable basis for a design and a fragile one when behaviour changes. Cooking loads are brief and staggered, showers are short, and lighting is now negligible, so the traditional assumptions hold up well for the traditional house.

Electric vehicles and heat pumps break the pattern in a specific way. Both run for hours rather than minutes, both draw close to their rated power for most of that time, and both tend to run at the same hours across a whole street. Coincidence is exactly what diversity assumes away, which is why these two loads are assessed differently from everything else.

Generation is a source, and the board has to be able to take it

A solar inverter does not consume from the consumer unit; it feeds into it. Current arrives at a breaker and flows backwards along the busbar towards the main switch and out to the street, passing whatever is being consumed on the way. The board was designed for power flowing one way, and a competent assessment checks that it copes with the other.

The practical issues are the busbar rating, the position of the generation breaker relative to the main switch, and whether the existing device types are suitable for a circuit that carries current in both directions. Some wiring codes are prescriptive about where the incoming generation connects; others leave it to the designer. It is not a detail an owner can reasonably adjudicate.

Batteries complicate this further because they are both a load and a source, sometimes within the same minute. And there is a plainly physical constraint that no amount of calculation gets round: a board with no spare ways has no spare ways. Occasionally the honest answer is a second distribution board rather than a heroic reorganisation of the first.

Load management can sometimes substitute for capacity

Where the supply is genuinely tight, the alternative to a bigger supply is a smaller peak. Current-sensing devices monitor the incoming tails and reduce or pause a controllable load — almost always the car charger — whenever total demand approaches the limit. The house never exceeds its fuse because something else quietly gives way.

The trade is real but usually invisible. A car charging overnight has many hours to absorb a few interruptions, and the vehicle arrives full regardless. A car being charged in a hurry between two journeys is a different matter, and that is the case where load management is genuinely felt as a limitation.

These schemes are not universally accepted. Some network operators recognise them explicitly and will permit a charger that would otherwise be refused; others do not, and require the supply to be capable of the full load whether or not it will ever be drawn. This is squarely a matter of local rules and the operator’s policy.

What an owner can usefully gather before anyone visits

A surveyor arriving with good information writes a better quotation than one arriving with none. Photographs of the meter position and the intake, of the consumer unit with its cover firmly in place, and of any labelling on the service fuse carrier are all genuinely useful. So is the certificate from the last electrical installation work, if it can be found.

What an owner should not do is open anything. The equipment between the street and the meter is live at all times, cannot be isolated by anyone in the household, and is not yours in any case. There isn’t a piece of information worth obtaining that way, and this is one of the few places in a house where curiosity has serious consequences.

The larger point is that the assessment is not a formality bolted onto the solar job. It is the thing that determines whether the design proposed is the design that can actually be built, and getting it done early — properly, by someone qualified, against the wiring rules in force where you live — is what prevents an expensive discovery halfway through the installation.