A 7 kW EV Charger on a 9 kW Supply: Load Balancing

A 7 kW wallbox draws about 30 A on its own. In a home with a small supply, the car plus an oven, a heat pump or an electric shower can exceed the main fuse rating. The usual fixes are a supply upgrade or a charger turned down to a low fixed current. Dynamic load balancing is a third option: a meter at the grid connection measures the whole home, and the charger uses only the capacity that is left. The car charges at full power when demand in the home is low and slows down when large appliances run, so the total stays under the limit.

Why a 7 kW charger can trip the supply

A single-phase 7 kW charger runs at about 30–32 A at 230 V. A supply limited to 9 kW is about 39 A, and a 40 A single-phase supply is about 9.2 kW. Charging on its own fits. But an ordinary evening adds an oven and hob of around 3 kW, a heat pump, a kettle and a dishwasher, and the total goes well over the limit.

A main fuse or breaker does not trip the instant the limit is exceeded. It can carry a small overload for a long time, but a large one, such as the car plus the oven, trips it within minutes. The whole home is then without power until someone resets the breaker or, if the fuse has blown, the network operator replaces it.

Both traditional fixes have a cost:

• A supply upgrade means an application to the network operator, possibly a new service cable or equipment, and a wait.
• A fixed low charging current, for example 9 A (about 2.1 kW), leaves room for the evening peak but wastes the free capacity for the rest of the night, and the car may not be full in the morning.

How dynamic load balancing works

1. A meter with a current sensor sits at the grid connection, on the incoming supply, and measures what the whole home draws.
2. The controller compares this with the household limit, minus a small safety margin, and works out the free capacity.
3. It tells the charger how much current it may use. The charger passes this on to the car through the standard control signal, and the car adjusts its charging current.
4. When an appliance starts, the charging current drops. When it stops, the charging current rises again.
5. If the free capacity falls below the minimum charging current of 6 A (about 1.4 kW single-phase), charging pauses and resumes as soon as there is room.

Because the controller measures the whole home, it does not need to know which appliance is running. It simply reacts to the total.

Worked example: one evening, three ways

The chart on our ChargeFlow page shows an illustrative evening with a single-phase 7 kW charger and a household limit of 9 kW. The car is plugged in at 17:15 and needs 25 kWh. From about 17:50 the oven and hob run, then the heat pump, a kettle and later the dishwasher.

At full power, the oven starting at 17:58 takes the home over the limit and the breaker trips. With the oven still on after the reset, it trips again at 18:18, and the owner stops charging. With a fixed 9 A, nothing trips, but the car gets less than half of what it needs. With load balancing, charging drops while the oven is on, rises again when the load falls, and the car is full before the dishwasher starts.

The values are illustrative, not measured. Real results depend on the home's loads, the car and the limit set.

When load balancing makes sense

• Small single-phase supplies, which are common in older homes and in several European countries.
• Homes with other large loads: heat pumps, electric showers, induction hobs, electric heating.
• Where a supply upgrade is slow or expensive, or not possible at short notice.
• To stay within the notification route in the UK, where the maximum demand of the premises has to stay within the criteria (see below).
• With solar: the same meter can let the charger follow the solar surplus during the day.

Load balancing does not create capacity. If the car needs more energy each night than the free capacity can deliver, or if the household limit is very low, a larger supply may still be the right answer.

UK: what the rules say

• Installation. EV charging installations fall under Section 722 of BS 7671 and the IET Code of Practice for Electric Vehicle Charging Equipment Installation. Section 722 allows load curtailment, including automatic load reduction, to be taken into account when determining maximum demand. Each charge point needs its own RCD of at least Type A.
• Network operator. Under the ENA EV charge point and heat pump connections form (v3.4), a single new charge point can be connected and then notified to the distribution network operator (DNO) within 28 days if the criteria are met. One criterion is that the maximum demand is below 13.8 kVA per phase (60 A single-phase), or that the premises load is limited to below the known cut-out fuse rating. A load-limiting device is taken into account in the maximum demand assessment. Above that threshold, the DNO must confirm supply capacity before connection. The cut-out must not be opened to check its rating.
• Smart charging. Since 30 June 2022, most charge points sold in Great Britain, other than public and rapid charge points, have had to meet the Electric Vehicles (Smart Charge Points) Regulations 2021, including smart functionality, default off-peak charging hours and a randomised delay. These requirements are separate from load balancing, and a charger for the UK home market needs both. Ask your supplier for the compliance statement of the exact model.

Germany: what the rules say

• Notifying the network operator. Under § 19 NAV, every charging device for electric vehicles must be notified to the network operator before it is put into operation. If the total rated power exceeds 12 kVA per installation, the network operator's prior approval is needed. So an 11 kW wallbox only needs to be notified, while a 22 kW wallbox needs approval.
• Controllable consumption devices (§ 14a EnWG). Private wallboxes that can draw more than 4.2 kW from the grid and were put into operation from 1 January 2024 count as controllable consumption devices. The network operator may not refuse or delay their connection by pointing to possible local overload. In return, it may reduce their power draw temporarily, but at least 4.2 kW must always remain available, and the customer gets reduced network charges.
• Load balancing still matters. § 14a concerns the public network. It does not protect the home's own main fuse. A home with a 9 kW limit still trips if the wallbox and the oven run at full power together.

Checklist for installers

1. Find the real limit. Check the cut-out or main fuse rating, any contracted power limit and the existing maximum demand. Do not open the cut-out.
2. Place the sensor correctly. The current sensor must measure the whole home at the grid connection, upstream of all large loads.
3. Set the limit with a margin. Set it below the fuse rating, with room for measurement tolerance and for loads that start suddenly.
4. Check the car. Load balancing relies on the car following the charging current. Some older vehicles do not accept current adjustment.
5. Know the minimum. Below 6 A, charging pauses. Make sure the customer understands that charging may stop during peaks.
6. Notify correctly. UK: the ENA form, with the load-limiting device included in the maximum demand assessment. Germany: § 19 NAV notification or approval, and § 14a where it applies.
7. One controller per charger, unless stated otherwise. Check how many chargers the controller can manage before quoting a two-car home.

How VaySunic can help

ChargeFlow is our 7, 11 and 22 kW AC wallbox, with charging current adjustable from 6 A to the maximum in 1 A steps. With the EnergyPilot local controller at the grid connection, it balances the load dynamically and keeps the home under the main fuse limit. One EnergyPilot controls one charger. The same setup lets the charger follow the solar surplus, and with PowerCube or SolarHub storage the battery can add power when the supply is small.

Certification status varies by market, including for the UK smart charge point rules and the German network operator processes. Contact us for the current certificates of a specific model, or see our distributor programme.

Sources

• ENA, EVCP & HP Connections Form v3.4 (PDF, via MCS)
• IET, Electric vehicle charging installations FAQs (BS 7671)
• Certsure, EV charging and BS 7671 Amendment 1:2020 (presentation, PDF)
• The Electric Vehicles (Smart Charge Points) Regulations 2021, SI 2021/1467
• Niederspannungsanschlussverordnung (NAV), § 19
• Bundesnetzagentur, Steuerbare Verbrauchseinrichtungen (§ 14a EnWG)