How is a battery sized for a site?
The planner's spreadsheet from two years ago is pinned to the container's wall: sessions per hour, peak power, energy per session.
It ended in two numbers: 400 kW and 400 kWh.
The questionHow did the planner arrive at those two numbers?
two cars at 300 kW at once
600 kW minus 200 kW from the grid
about ten sessions of 30 kWh, minus 200 kWh from the grid
three busy hours, usable window and losses, rounded up
Computed for a composed site. The planner's assumptions, stated: up to three busy hours in a row, each with a deficit of 100 kWh, so 300 kWh from the battery without time to refill; the battery is used between 10% and 95%, so 85% of its capacity is usable; about 8% is lost in the inverter and the cells on the way out. 300 kWh ÷ 0.85 ÷ 0.92 is about 384 kWh, rounded up to 400 kWh.
Power and energy are separate choices
- Power (kW) decides how many cars the site can serve at full speed at once.
- Energy (kWh) decides how long the battery can keep up before it is empty: energy is power multiplied by time.
A battery with high power but little energy handles a short peak; a battery with much energy but little power can cover a long busy period at moderate speed. The units are explained in eMobility and EV basics, stop 04.
The refill limit
A battery can only give what the grid has put in. On a busy day, the grid's 200 kW have to cover both the cars and the refill. If demand stays above the connection for hours, the battery runs empty and the chargers fall back to what the grid alone can deliver. That is why the planner sized for the busiest hours of the year, not for the average.
Check your understanding
3 questions. Answer all of them to complete this stop. Each answer explains itself.
What decides how many cars can charge at full speed at once?
Power sets the speed at a given moment; energy sets how long that speed lasts.
In the busiest hour, cars need 300 kWh and the grid supplies 200 kWh. What does the battery supply?
300 kWh demand minus 200 kWh from the grid leaves 100 kWh from the battery.
What happens if demand stays above the grid connection for many hours?
Stored energy is finite; without enough refill time, the battery empties.
- LEIFIphysik (FWU Institut für Film und Bild): Grundwissen: Elektrische Energie, E = P · t. https://www.leifiphysik.de/elektrizitaetslehre/elektrische-energie/grundwissen/elektrische-energie. Checked 13 Oct 2026.