Before You Add Load: A Supply-Capacity Checklist for EV Chargers, Solar and Batteries

Adding an EV charger, a solar inverter or a home battery is often described as a simple upgrade. In practice, each of these technologies adds a significant load or a new source of energy to the installation, and each has its own earthing, protection and notification requirements. The most expensive surprises usually come from what was not checked before the quote was accepted. The checklist below is designed to help UK property owners ask the right questions before work begins.

1. Establish the existing supply capacity

Start with the supply itself. Most UK homes have a single-phase supply with a distribution network operator fuse rated at 60A, 80A or 100A, although older properties can differ. Some larger homes and commercial premises have a three-phase supply. The DNO fuse rating, the meter tails, the consumer unit busbar rating and the existing maximum demand all matter.

A proper assessment should include a load calculation using diversity, not just a count of the breakers. Electric showers, cookers, immersion heaters, electric heating and existing outbuildings can all consume capacity. An EV charger rated at 7.4kW draws around 32A on a single-phase supply. A three-phase 22kW charger draws proportionally across the phases. Solar inverters and batteries add their own charge and discharge currents, and the worst-case scenario is not always obvious.

Ask for the load calculation in writing. If capacity is tight, the options may include dynamic load management, a supply upgrade, a three-phase connection or an interlocked priority system. Those decisions should be made before the equipment is ordered, not after the first trip.

2. Confirm the earthing arrangement

The earthing system affects how the new equipment must be protected. UK installations are commonly TN-S, TN-C-S or TT. Each has different implications for fault current and protective device operation. EV charging is a particular case: where the supply uses PME earthing, the charge point may need open-PEN fault detection or an alternative earthing arrangement, depending on the equipment and the installation conditions. A TT system will normally require an earth electrode and appropriate RCD protection.

Solar inverters and batteries also have manufacturer-specific earthing requirements. The electrician should record the external earth loop impedance, confirm the main protective bonding and check that the new equipment can be safely integrated. A quote that does not mention earthing at all is not a complete quote.

3. Check RCD and circuit protection

Protective devices must match the equipment and the circuit. For EV charging, the requirement is not simply a generic RCD. Depending on the charge point, the installation may need a Type A RCD with 6mA DC detection or a Type B RCD. Some charge points include this protection internally, but the installer must confirm that the overall arrangement complies with BS 7671 and the manufacturer's instructions.

Solar systems need DC isolation and appropriate AC protection. Batteries need isolation arrangements that allow safe maintenance and emergency disconnection. Where the building falls within the scope of the AFDD requirements, arc fault protection may also need to be considered. Surge protection may be advisable where the new equipment is sensitive to transient overvoltages. The consumer unit should have enough capacity, and the devices should be compatible with the existing board.

4. Plan cable routes and derating

Cable size is not chosen by current rating alone. Volt drop, grouping, ambient temperature, installation method and thermal insulation all affect the safe current-carrying capacity. A cable clipped to a cool wall is not the same as a cable buried in insulation or run through a hot roof space. For outdoor runs, armoured cable or suitable ducting may be needed. For solar, the DC cable route from the roof to the inverter deserves particular attention.

Ask how the cable will be routed, how it will be protected and whether it will cross fire compartments or affect the building's structure. Long runs to a driveway, garage or outbuilding should be calculated, not guessed. The quote should identify the cable type and size, the route and any making good required.

5. Confirm DNO notification and building regulations

Grid-connected solar and battery systems normally require a G98 or G99 application to the distribution network operator before connection. EV charge points may also require DNO notification, depending on the load and the DNO's own rules. These are not paperwork formalities; they are part of connecting the equipment lawfully and safely.

In England and Wales, notifiable electrical work in dwellings is covered by Part P of the Building Regulations. Consumer unit replacements and new circuits are common examples. The installer should be able to notify the work through a competent person scheme or explain how building control will be involved. At the end, you should receive the appropriate electrical certificate and, where relevant, a building regulations compliance certificate.

6. Spotting a quote that skips compliance

A low price can be genuine, but it can also mean something has been left out. Be cautious if a quote contains no site survey, no load calculation, no reference to earthing, no detail on RCD type, no DNO application and no mention of testing or certification. Those omissions tend to become expensive variation orders later.

A better quote will include the survey findings, the proposed equipment, the protective device schedule, the cable route and size, the notification arrangements, the testing regime and the aftercare terms. It should also confirm who is responsible for the DNO application and what happens if the supply needs upgrading. S&DG Electric works to that standard because compliant work is not an optional extra. It is the basis of a safe installation.

The practical checklist

• Confirm the DNO fuse rating and whether the supply is single-phase or three-phase.
• Obtain a written maximum demand and diversity calculation.
• Identify the earthing arrangement and any EV-specific PME requirements.
• Confirm the RCD type and whether the charge point includes DC fault detection.
• Check consumer unit capacity, compatibility and AFDD or SPD requirements.
• Review cable routes, derating, volt drop and outdoor protection.
• Confirm G98 or G99 notification for generation and storage.
• Confirm Part P notification and the certificates you will receive.

Adding EV charging, solar or battery storage can make a property more efficient and more resilient, but it should never be treated as a plug-and-play exercise. A little preparation at the quotation stage protects both the budget and the building. If a quote feels vague, ask for the missing details. A competent electrician will welcome the question.