Constrained grid supply
Limited grid site power
When the grid connection is too small for the site’s peaks, a battery can charge steadily from it and supply the demands above its limit. It works when the connection can still cover the energy the site uses across the day.
- A battery buffers peaks above the connection limit.
- The connection must still cover the day’s energy, plus losses.
- Measure or estimate the average load before anything else.
The problem
A site has a grid connection, but it is not big enough. Perhaps the permanent supply is not yet energised and only a builder’s supply is available. Perhaps a larger connection is months away, or would cost more than the temporary works justify.
The connection can run the cabins and the lighting. It cannot start the crane, or run the hoist and the pumps together. The usual answer is a generator alongside the grid supply, sized for the peaks and idle most of the day.
How buffering works
A battery sits between the connection and the site. It draws from the connection at a steady rate, within the connection’s limit, around the clock. It supplies the site at whatever rate the loads demand.
When the site draws less than the connection supplies, the surplus charges the battery. When the site draws more, the battery makes up the difference. Seen from the grid, the site looks like a small, steady load. Seen from the site, the supply looks much larger than the connection.
The limit: energy has to balance
This is the part that decides whether the idea works on your site.
The test is a daily energy balance.
Energy available is the connection’s spare capacity multiplied by the hours it is available, reduced for losses in charging and discharging.
Energy used is the site’s average load multiplied by its operating hours.
Two illustrative cases, using round numbers and an assumed 85% round-trip efficiency:
| Case A | Case B | |
|---|---|---|
| Connection capacity free for charging | 60 kW | 30 kW |
| Hours available | 24 | 24 |
| Energy available after losses | 1,224 kWh | 612 kWh |
| Average site load, working hours | 90 kW | 90 kW |
| Working hours | 10 | 10 |
| Energy used | 900 kWh | 900 kWh |
| Daily balance | +324 kWh | −288 kWh |
In case A the site’s 90 kW average is well above the 60 kW connection, yet the arrangement works, because the connection has all night to refill the battery. In case B the same site with half the connection falls short by 288 kWh every day. No size of battery fixes case B; it needs a bigger connection, a generator for part of the day, or less load.
These figures are illustrative arithmetic, not a design. Real projects also need margin for the system’s own consumption, temperature and the busiest days.
Peaks still have to fit the battery’s ratings
Passing the energy test is not enough. The battery system must also be able to deliver the peaks. Sustained demand has to sit within nominal output, 193 kW / 214 kVA for the Ampd 200 and 386 kW / 428 kVA for the Ampd 400, with short peaks inside the time-limited overload ratings.
Charging is limited too. The units accept up to 205 kW and 410 kW, but on a constrained site the connection will be the limit long before the unit is.
Source: Ampd 200 IEC brochure, Rev 1.3, dated 29 April 2026; Ampd 400 IEC brochure, Rev 1.3, dated 29 April 2026. Manufacturer-published figures for the IEC variant, not independently tested. Specifications can change, so the current data sheet for your territory is confirmed in a project proposal.
What to find out first
- The connection’s capacity. In kVA, or amps per phase, and its voltage. The reference units are IEC variants for 380 to 415 V three-phase supplies.
- How much of it is free. Cabins, lighting and other loads already on the connection reduce what is left for charging.
- Hours of availability. A connection that is live around the clock can recharge overnight.
- The site’s average load. A week of metered data is ideal. An equipment list with realistic operating hours is a workable start.
- The peaks. The largest loads, how long they run and whether they coincide.
If you cannot measure the average load, say so. Estimating it from the equipment and the working pattern is part of the technical review.
When the connection is not enough on its own
If the balance is negative for part of the project, the options are:
- Add a generator as a reserve. It charges the battery during the heaviest phases and stays off otherwise.
- Shift load. Move flexible loads, such as heating or charging, to hours when the connection has spare capacity.
- Increase the connection. Sometimes a modest increase tips the balance.
The arrangement can change as the job moves through its phases. See construction site batteries for how loads shift during a project.
Next step
Start a site power assessment with the connection size and a description of what exceeds it. You will get a likely approach and a list of what an engineer needs.
Product figures on this page: Ampd 200 Brochure IEC Rev 1.3, dated 29 April 2026; Ampd 400 Brochure IEC Rev 1.3, dated 29 April 2026. Manufacturer-published, not independently tested.
Questions buyers ask
Can a battery make a small grid connection act like a bigger one?
For short peaks, yes. The battery supplies demand above the connection limit from stored energy and recharges between peaks. It cannot raise the average power the site can draw. If the site’s average demand exceeds what the connection supplies, the battery will run down.
How do I know if my connection is big enough?
Compare the energy the connection can supply in a day with the energy the site uses in a day, allowing for losses. If supply exceeds demand, buffering can work. If not, you need a larger connection, a generator as well, or lower demand.
Do I need permission from the network operator?
Connecting equipment to a grid supply may involve the connection agreement and the network operator’s requirements, which vary by country and connection. That is confirmed with the supply partner and your electrical contractor for the specific site.
What if the connection cannot keep up for part of the project?
A generator can be added as a reserve for the heaviest phases, charging the battery when the connection alone is not enough. The arrangement can change as the site’s demand changes.