Quick answer
Use the export limit in the DNO agreement, not the size of the solar inverter or battery PCS. The site cannot export more than that limit, whether the power comes from solar, the battery or both. Model any solar that must be turned down and any spare solar the battery can store because of that limit.
Start with the export limit in the DNO offer or connection agreement. That is the most electricity the site can export to the grid at any one time, whether it comes from solar generation, electricity discharged from the battery, or both.
If the site cannot use the rest for itself or store it in the battery, the inverter has to reduce solar output. Make sure you don’t include that reduced output in the savings or export income.
Work from what leaves the site
Use the export figure in the DNO connection offer or agreement. Do not take it from the solar inverter rating or battery PCS size.
The G100 export control system measures the current flow level at the connection point. For each half-hour, work out:
Net export = solar generation + battery discharge − site demand − battery charging
If that figure goes over the DNO limit, the control system has to bring export back within the agreed limit. Depending on how the site is set up, it can reduce battery discharge, use spare solar electricity to charge the battery, reduce solar inverter output or send electricity to another site load.
Say the site has:
- 500 kW of solar output available at that point
- 220 kW of site demand
- a 100 kW export limit
- a battery that can charge at 150 kW
Without the battery, the site uses 220 kW, exports 100 kW and the inverter has to reduce 180 kW of available solar output.
With the battery charging at 150 kW, the site still exports 100 kW, but the inverter only has to reduce 30 kW of available solar output.
The site can only use the battery this way while the battery has room to store more electricity and can charge fast enough. If the battery is full, charging too slowly or holding capacity back for a later peak, tariff period or trading event, the inverter will still have to reduce some solar output.
The same site export limit applies to electricity exported after battery discharge. If the solar is already sending out 80 kW under a 100 kW limit, the battery can only add another 20 kW at that point.
EREC G100 sets the technical rules for customer export and import limitation schemes.
Show how much spare solar the battery saves
Run at least three cases through the proposal:
- the site as it is now
- solar only with the G100 limit applied
- solar and battery with the same limit applied
For each case, show:
- solar used directly on site
- solar sent into the battery
- electricity exported
- solar that had to be turned down
If the site uses solar electricity to charge the battery that would otherwise have been lost, that electricity had no export income attached to it. If the site uses solar electricity to charge the battery that the client could have exported and sold, take that lost export payment off the savings the client gets when the site uses the stored electricity later.
Half-hourly data is fine to use for your main financial model, but be aware that it can hide shorter export peaks. If the client’s solar array is large compared with the site load or export limit, use 5-minute or 15-minute data where you can get it. If you only have half-hourly data, allow for more solar generation to be reduced than the model first shows.
GridVolt’s simulator can apply the DNO export limit across the solar-and-battery model and show how much otherwise-reduced solar electricity the battery can store. Only count the saving or export income the client can get from those kWh later, after battery losses and any export income the client has to give up.
Related questions
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