How many EV chargers fit on my grid connection?

With a simultaneity factor, an office on 160 kW gets 2 to 14 charge points. Calculated per quarter hour, 4 fit, or 39 with smart charging. Here is how.

What a simultaneity factor is

A charger often has two charge points; this article counts charge points, with one car per point. A simultaneity factor, also called a diversity factor, says what share of the charge points is in use at the same time. Add up the power of all charge points and multiply by the factor, and you have an estimate of the combined charging load. If that fits in the headroom that current consumption leaves under the transport capacity (the power the grid operator allows per quarter hour), the charge points fit.

Which factor you take depends on where you look:

• 1, according to the Dutch installation standard NEN 1010 for the circuits to the charge points: all charge points can be used at the same time, so you calculate as if they are, unless charging is controlled¹. For the connection, that is the most cautious choice.
• 0.8 in the busiest quarter hour of the worked example below, with 10 charge points: 8 of them charge at once.
• 0.17 on average in the same example: a car charges in 6 of the 35 quarter hours it is parked.

As long as the connection had headroom to spare, it made little difference which factor you chose: a charge point too many or too few was lost in the margin. Where the connection can no longer be upgraded, that margin is gone and every quarter hour counts.

Why one factor does not give the answer

A factor estimates the combined charging load. The question is whether that fits in the headroom left under the transport capacity at that moment. Both change every quarter hour.

• The headroom is not a fixed number. A business's consumption peaks for a few hours a day. The transport capacity minus the year's highest peak is the headroom in the tightest quarter hour. A calculation with that number treats the whole day as if it had no more headroom than that.
• Simultaneity depends on when cars arrive. Employees come in over a couple of hours and plug in straight away. Then most of them charge at once, even if over the whole day they only charge a small part of the time.

An average simultaneity is like the average number of cars on the motorway over a whole day. It is correct, and it says nothing about the traffic jam at half past eight.

Worked example: an office with a warehouse on 160 kW

A wholesaler has an office and a warehouse on one large connection with 160 kW of contracted transport capacity. The warehouse runs on working days from 6:00 to 16:00, the office from 8:00 to 17:30. In winter a heat pump heats the building; on warm summer days the air conditioning cools it. The business uses 459 MWh a year, 52 kW on average. The highest quarter hour is 132 kW, on Wednesday 26 February at 9:45.

The business is fictional. Its quarter-hour profile was built for this article and covers all of 2025, with a seasonal pattern but no cold spell.

There are 60 parking spaces, so from 2027 Dutch law asks for at least 6 charge points, or ducting for half of the spaces². The business wants 11 kW charge points for its employees' cars. Each charge point gets one car on working days, which arrives between 7:00 and 9:00 and stays 8 to 9.5 hours. A car needs 6 to 24 kWh, which matches a commute of about 30 to 125 km there and back³. On average a charge point stays empty on one working day in seven. Every week has the same pattern, holidays included. 95% of what the charge point delivers ends up in the car's battery.

What the factor says. The headroom above the highest peak is 160 − 132 = 28 kW. An 11 kW charge point counts as 11 kW times the factor.

So the same connection gives 2, 3 or 14 charge points, depending on which moment the factor describes.

Without smart charging, per quarter hour. Every car charges at 11 kW from arrival until it has its kWh. Add that to the business's consumption quarter hour by quarter hour, and you see on how many days consumption goes above the contracted transport capacity. In a congested grid area that is not allowed, not even for a few quarter hours a year (see sizing a battery for a business).

Without smart charging, 4 fit, fewer than the 6 the law asks for from 2027 if the business does not choose ducting. That is more than a factor of 1 or 0.8 says, because 79% of the cars are full by 10:00, before the business reaches its daily peak, usually between 10:00 and 12:00. And it is far fewer than the average factor says, because the cars all arrive in the same two hours. With ten charge points, seven cars charge at once at 9:00 on Monday 13 January, while the business itself is already at 119 kW.

You only know whether you picked the right factor once you have run the numbers per quarter hour. By then you no longer need the factor.

“Our clients want to grow. Extra warehouses, machines, production lines. But they can't get additional capacity from the grid operator. Wattix helps us show what's still possible within their existing connection.”

Jesse Moms, Kremer Installatietechniek

With smart charging. Smart charging divides the headroom under 160 kW among the cars plugged in, every quarter hour. Consumption then always stays within the transport capacity, and the question becomes whether every car is full when it leaves. For that, what counts is not the power of the charge point but the energy. A car that needs 14 kWh and stays 9 hours needs 1.6 kW on average, not 11 kW.

With smart charging, 39 fit. That is 8,664 charging sessions in 2025, and every car gets the energy it came for on every working day. On 13 January consumption sits at 160 kW for ten hours to do so, and not one quarter hour above it.

With 40 charge points, three cars leave without all the kWh they need on three days in January and December, 10 kWh short in all. With 43 charge points the shortfall over the whole year is 484 kWh, 0.4% of what the cars need. But that shortfall falls on 45 sessions spread over 22 days, and mostly in winter, when the heat pump uses part of the headroom.

Three things decide how much you can rely on that 39.

• The charging sessions. They are the most uncertain part of the calculation. If every car needs a quarter more energy, 29 fit instead of 39. And the example repeats the same pattern every week, while real occupancy varies more.
• The smart charging. The worked example plans charging with each car's departure time and kWh, and with a forecast of the building's consumption. Load balancing that always splits the power equally between the connected cars leaves more cars short on tight days, and then fewer charge points fit.
• The margin. The example calculates right up to 160 kW, with the heating of an ordinary year. If you want to stay a few kW under the transport capacity, or absorb a cold week, fewer charge points fit.

“You always have to make assumptions. But a platform like Wattix gives you the freedom to present results to your client in your own way, with the confidence that the underlying calculation model provides the same solid foundation every time.”

Fred Theunissen, De Installatiemakers

How to calculate it per quarter hour

1. Take a year of quarter-hour data from the connection. A large connection is metered per quarter hour, and that is what the grid operator checks the transport capacity against. A whole year is needed because the tightest days often fall in winter. On a smaller connection, up to and including 3x80A, the main fuse is the limit, and it responds to every moment, not to the quarter-hour average.
2. Write down the charging sessions. Per charge point: when the car arrives, when it leaves and how many kWh it needs. If you only know the commuting distance, use about 19 kWh per 100 km³. Also run a variant with more energy per car, because this is where most of the uncertainty is.
3. Calculate without smart charging. Let every car charge at full power from arrival and add that to consumption quarter hour by quarter hour. If no quarter hour goes above the transport capacity, it fits without smart charging too. You then size the installation to NEN 1010 with a factor of 1.
4. Calculate with smart charging. Divide the headroom under the transport capacity among the cars that are plugged in, quarter hour by quarter hour, giving priority to the car with the least time to spare, and check per session how many kWh the car has received when it leaves. Include the charging efficiency, 95% in this example.
5. Add charge points until a session falls short. The last number without a shortfall is the answer with smart charging. Then you decide whether a few tight days are acceptable, or whether solar panels or a battery add headroom.

What Wattix does here

In Wattix you enter, per charge point, the power and the weekly charging sessions: when the car arrives, when it leaves and how many kWh it needs. Wattix plans charging per quarter hour within the contracted transport capacity, alongside the consumption in the connection's quarter-hour data, and shows per session how much energy the car gets and where the shortfall is. The worked example was calculated with the same engine. How it works is covered in smart charging in Wattix.

From there you run variants: more or fewer charge points, with or without a battery, or charging on solar power. See charging infrastructure within the grid connection. Where the uncertainty in a calculation like this lies, and how accurate it is, is covered in what an energy simulation is.