Work out how many EV chargers a site can run at full power within its import capacity, interval by interval, from its measured load.
Chargers at full power in every interval
0chargers
Check the values to see a result.
Assumes chargers can be throttled to fit the headroom in every interval. Peaks inside an interval are not in interval averages.
Headroom = site capacity − reserve − site loadchargers at full power = headroom ÷ charger power, rounded down
The site load already exceeds its 450 kW after the reserve in interval 3, so no charger fits at full power then.
Tip: Use the busiest measured day, not an average one: the fewest chargers fit when the site load peaks.
How many EV chargers a site can run depends on how much of its connection the rest of the site is already using, interval by interval. Take the import capacity, keep a reserve, subtract the measured site load, and divide what is left by the power of one charger. The busiest interval sets how many can run at full power all the time; the quieter ones show how much more a load-managed system could deliver.
Headroom = import capacity − reserve − site load
Import capacity is the agreed maximum import of the connection in kW (a capacity agreed in kVA is multiplied by the power factor). The reserve is a margin kept free for the site’s own peaks, measurement error and switching. Site load is the measured demand without charging, in the same units, for each interval.
Chargers = ⌊headroom ÷ charger power⌋
Rounded down, in the interval with the least headroom. Use the charger’s input power from the supply, not the power it gives the car: a 22 kW AC charger draws 22 kW, and a DC charger draws a little more than its rated output.
Energy = Σ min(headroom, all chargers’ power) × interval hours
The most energy the chargers could take from the headroom across the period, if they were throttled to fill it. It is an upper bound: cars do not always need charge when the headroom is there.
A fixed allocation, sized for the busiest interval, needs no controls but leaves capacity unused most of the day. Dynamic load management measures the site load and shares the headroom among the chargers as it changes, so more chargers fit on the same connection. Either way, the calculation needs measured interval data from the busiest days, not an average day or the sum of nameplate ratings.
After a 50 kW reserve, 450 kW is available. The site peaks at 420 kW, leaving 30 kW, so only 1 of the twelve 22 kW chargers can run at full power all the time. Across the two hours the headroom would still allow 230 kWh of charging with load management.
Chargers at full power in every interval 1 charger Open in the calculator
Overnight the depot uses at most 180 kW of its 360 kW after the reserve. That leaves 180 kW, enough for 16 chargers at 11 kW at full power all night, and 1,950 kWh of charging over eight hours.
Chargers at full power in every interval 16 chargers Open in the calculator
In the third half-hour the site takes 460 kW, 10 kW above its 450 kW after the reserve. No charger fits at full power then, so the result is 0: without load management, the chargers need a larger connection or the site’s own peak cut first.
Chargers at full power in every interval 0 chargers Open in the calculator
For a 500 kW connection with a 50 kW reserve: how many chargers of each size fit at full power at each peak site load.
| Peak site load (kW) | 7.4 kW | 11 kW | 22 kW | 50 kW DC |
|---|---|---|---|---|
| 100 | 47 | 31 | 15 | 7 |
| 150 | 40 | 27 | 13 | 6 |
| 200 | 33 | 22 | 11 | 5 |
| 250 | 27 | 18 | 9 | 4 |
| 300 | 20 | 13 | 6 | 3 |
| 350 | 13 | 9 | 4 | 2 |
| 400 | 6 | 4 | 2 | 1 |
| 440 | 1 | 0 | 0 | 0 |
As many as fit in the headroom between your connection’s capacity, less a reserve, and your measured peak demand. Measure the site for at least a few busy weeks at 15 or 30-minute intervals before deciding; the interval demand calculator turns meter readings into demand.
A controller that measures the site’s load continuously and sets each charger’s current so that the total stays under the connection’s limit. It lets more chargers share the same connection, at the cost of slower charging during the site’s peaks.
Use kW throughout if the import limit is agreed in kW. If it is in kVA, convert it with the site’s power factor, or measure apparent power and work in kVA; the kW, kVA, kvar and power factor calculator does the conversion.
Because the chargers have to fit when the site peaks, not on a typical afternoon. An average day hides the peaks, and even 15-minute averages hide shorter ones, so keep the reserve for what the intervals cannot show.
A ZEM measures the site’s load at the incomer, interval by interval, which is the data this calculation needs; a ZDR adds local control of loads for demand response.
