Home / Energy transition & the future
Energy transition & the future · Charging and the grid
Why does charging time matter?
How much electricity your car needs is one question. When it charges and at what power is another. A simple example shows how parking times, solar electricity and the shared electricity grid are connected.
Reading guide: the basics explain the connections. The numerical example is freely chosen. The outlook describes research questions and goals. Sources checked on 21 September 2026.
01 · The basics
The same amount of energy can be delivered in different time windows.
Kilowatt-hours (kWh) measure the amount of energy drawn. Kilowatts (kW) measure power at a given moment. At constant power: energy = power × time.
A car drawing 11 kW at the charging point for two hours receives 22 kWh there. Charging immediately on arrival or later initially changes the timing. Shifting the charging time alone does not reduce the energy required.
In practice, charging power varies. The vehicle, state of charge, temperature and installation affect the charging profile. Energy at the charging point also includes charging losses, so 22 kWh at the charging point does not automatically mean an extra 22 kWh in the battery.
02 · A freely chosen example
Four cars. The same energy. A different profile.
Suppose four cars are parked at four suitable charging points from 18:00 to 22:00. Each needs 22 kWh at the charging point and, in this example, can charge at a constant 11 kW. All must be ready by 22:00.
A · All at once
44 kW
highest combined charging power
- 18:00–20:00: four cars × 11 kW.
- 20:00–22:00: all four cars have stopped charging.
88 kWh in total
4 × 11 kW × 2 hours
B · In two groups
22 kW
highest combined charging power
- 18:00–20:00: two cars × 11 kW.
- 20:00–22:00: the other two cars × 11 kW.
88 kWh in total
2 × 11 kW × 2 hours + 2 × 11 kW × 2 hours
What the example shows: Coordination reduces the highest combined charging power from 44 to 22 kW. Each car still receives 22 kWh. The extra flexibility in timing makes the second arrangement possible.
Only the sum of the four charging powers is shown. Household consumption, PV, batteries, phase allocation, cables and transformers are not calculated. These figures do not establish available connection capacity or indicate whether a real grid would be overloaded.
What if one car needs to leave earlier?
Its charging target must then be met earlier. The two groups can only be arranged within the limits of parking times, vehicles and the installation. Note the energy requirement and departure time for each vehicle. When charging is shared, priorities and available power also need to be agreed.
Try it yourself · Four cars
When should the cars charge?
Choose each car’s charging start and departure freely. Each needs 22 kWh at the charge point and charges in this example at 11 kW - so it takes two hours. Change each car’s charging start and departure time and watch their combined demand.
Times can be set to the minute. All charging starts refer to the same day. A departure earlier than its charging start is on the next day. Equal times mean no charging window. The presets restore the evening example from 18:00–22:00.
Baseline: All at once.
The bars show charging in this example. Each vertical line marks a departure. Charging stops at departure at the latest.
Time intervals in a table
| Time | Cars charging | Total |
|---|
Assumptions and calculation
This is an illustrative learning example. Energy = power × charging time. Each car needs two hours at a constant 11 kW to receive 22 kWh at the charge point. If charging starts later or departure is earlier, only the energy charged before departure counts. Charging stops once 22 kWh has been reached. There is no automatic rescheduling or allocation of a shared power limit.
The charts adapt their time range to your inputs; “+1 day” marks the next day. The combined power scale stays at 0–44 kW. The charging power represents energy at the charge point; charging losses are not added a second time.
Household demand, solar PV, batteries, phase allocation, cables and transformers are not modelled. This example does not establish the connection capacity or overload of a real grid. Actual charging profiles may differ from the constant power used here.
Calculations run in your browser. Entries are not sent to the website. Only when you choose “Add to my plan” is a snapshot kept in this browser tab. You can export or clear it in the planning sheet.
03 · Put it in context
What should charging control achieve?
Use solar electricity at home
For direct solar charging, the car and a usable PV surplus must be available at the same time. Surplus charging adjusts charging power to the solar electricity left over. Annual generation alone does not show this overlap in time. How much you can use also depends on parking times and the charging target.
Explore further: HTW Berlin: Solarstromer tool and model assumptions
Take account of your electricity tariff
With time-dependent prices, charging time can affect electricity costs. Tariff terms, metering and control need to work together. A low-price signal does not automatically indicate spare capacity in your street: many users responding at once can create new power peaks.
Stay within shared power limits
At building level, load management can coordinate charging points with other electrical loads. For the public grid, what matters is the loading in the affected part of the network. Grid-oriented control takes that situation into account; electricity prices alone are not enough.
Explore further: Federal Network Agency: controllable electrical loads
In your discussion with the installer, agree which goal takes priority, what limits apply and how the required charge should be reached by departure.
04 · Plan today
Coordination starts with everyday routines and the technology already in place.
A timer can set a simple start time. An energy management system can also take account of live measurements, several charging points or PV generation. Check specifically which functions work together across the vehicle, home charger, meter and control system.
Electrical loads covered by Section 14a EnWG are also subject to grid-oriented control rules. These allow the network operator to limit their electricity draw when overload is imminent; ordinary household consumption is exempt. The Federal Network Agency explains the exact eligibility criteria and rules for existing installations.
Federal Network Agency: rules and scope
In Ulm, discuss grid connection, registration and controllability with your electrical contractor and the network operator responsible for your address. For connections in the Ulm Netze network area, its connection information is the local starting point.
Explore further: Ulm Netze: questions about charging installations
Plan a home charger for your house · Prepare charging as a tenant or owners’ association · Charging without your own parking space
05 · My next step
Four pieces of information for a practical discussion.
- My needs: What energy requirement or driving distance must be covered by the next departure?
- My time windows: When is the car actually at the charging point — on weekdays, at weekends and on exceptional days?
- My building: Which charging points, PV system and other major electrical loads are already present or planned?
- My priority: Is the main goal a reliable charging target, more use of my own solar electricity, tariff timing or sharing limited power?
My question for the installer: How can our installation meet the required charging target within these time windows, what metering and control does it need, and what happens with little sun or limited power?
Record answers and open points in the shared planning checklist. Where several people are involved, agree who will obtain the next piece of information and by when.
06 · Research and emerging developments
How can more flexible systems work together?
Another option is bidirectional charging: suitably equipped vehicles can also supply energy. Fraunhofer ISE’s BiFlex-Industrie research project describes how vehicle fleets could be integrated technically and economically into buildings and the electricity system. The project announcement linked here dates from 21 February 2024; it describes goals and planned demonstrators, rather than evidence of a solution available today for every private car.
Explore further: Fraunhofer ISE: BiFlex-Industrie research approach, 2024 announcement
For an individual household, practical questions therefore remain: do the vehicle and installation support the desired function? When is the car available? How are mobility needs, control, costs and operation managed?
Whether an additional home charger can connect at a particular point also requires a professional grid connection assessment. This considers network structure, equipment, voltages and simultaneous loads and generation. The charging example above explains one relationship; it does not calculate these quantities.
Sources and status
Checked on 21 September 2026. The four-car example is our own freely chosen calculation using idealised constant power. It uses neither measured data from Ulm nor data from a research study.
- Federal Network Agency — integrating controllable electrical loads: simultaneity, grid control and scope.
- HTW Berlin — Solarstromer tool: simultaneous solar use, parking times and simulation model limitations.
- Ulm Netze — connection information for charging installations: local starting point within the relevant network area.
- Fraunhofer ISE — BiFlex-Industrie, announcement of 21 February 2024: explicitly presented as a research approach.
Plan electricity, heating and mobility together · Back to home