Quick answer
Choose more inverter or PCS power when the battery needs to charge or discharge faster. Choose more usable battery capacity when it needs to keep going for longer. Check the site’s biggest power need, how long it lasts, the connection limits and the extra saving or trading income before paying for more kW or kWh.
Choose more inverter/PCS power when the battery needs to charge or discharge faster. Choose more usable battery capacity when the battery needs to keep charging or discharging for longer.
For example, if the client needs the battery to reduce a short demand peak, charge quickly during a low-price half-hour, go for more inverter/PCS power. But if they need their battery to cover a longer peak, store more solar for later use or keep enough reserve after trading, go for more usable battery capacity.
Work out the inverter/PCS kW from the biggest power requirement
The inverter or PCS kW rating tells you the maximum amount of power the battery system can charge or discharge at one time.
Here’s an example. Suppose the site is drawing 420 kW from the grid during a peak, and the client wants to cap that draw at 300 kW. The battery system needs to discharge at least:
420 kW − 300 kW = 120 kW
Other situations where more inverter/PCS power might help include when the battery needs to:
- Hold the site’s draw from the grid below a target during a short, high peak, including EV charging peaks
- Charge fast enough to use spare solar before the site reaches its export limit
- Charge within a short low-price window
- Stop or reduce a planned charge by a larger amount when a trading instruction pays the site to lower demand
- Discharge into the building quickly enough to reduce grid purchases during a high-price period
- Export more power where the export agreement and DNO limit allow it
Look for a bigger inverter/PCS in situations where the inverter/PCS reaches its kW limit while the battery still has plenty of charge left.
For example, the battery may still hold 60% charge while the site is drawing more from the grid than the agreed target allows. In that case, the battery has enough stored energy, but the inverter/PCS cannot discharge it fast enough.
Check the connection limits before adding more inverter/PCS power. A larger inverter/PCS does not change the site’s import limit, export limit or G100 export limit. Only the DNO can do that. For example, a 300 kW inverter/PCS cannot export 300 kW if the site has only 100 kW of spare export headroom.
Work out the usable battery capacity in kWh from how long the battery has to keep charging or discharging
The usable battery capacity level tells you how long a battery can keep charging or discharging at the power level you need.
If a site battery needs to discharge at 120 kW for 90 minutes, the battery must deliver:
120 kW × 1.5 hours = 180 kWh
The battery system therefore needs at least 180 kWh of usable energy for those 90 minutes before allowing for:
- Minimum state of charge: The lower charge limit the BMS, inverter or controller keeps the battery above
- Conversion losses: Electricity lost as the battery charges and discharges
- Held-back capacity: Any stored energy the client wants to keep for backup, another peak or a later trading window
Other times usable capacity might help are when the battery needs to:
- Cover a long site peak
- Charge when electricity is cheaper and discharge later
- Store midday solar for use later
- Cover several peaks or trading windows before there is time to recharge
Check what the manufacturer’s quoted capacity means before you use it in the proposal.
It may mean total nameplate capacity, usable battery-side capacity or usable AC energy after inverter losses. If you use the wrong figure, the battery may not have enough stored energy to cover the peak, solar-storage period or trading window included in the quote.
Run various options through the same model.
Compare a few inverter/PCS kW and usable battery kWh combinations against the same site data. Use the same tariff, solar generation, import and export limits, reserve settings, export rate, trading fees and trading income estimate in every calculation.
Take care not to count the same kWh twice. If stored electricity is used for peak shaving in a half-hour, it cannot also be used for solar savings or trading income in that same half-hour.
Compare the options before you add more inverter/PCS power or more battery capacity.
Keep adding inverter/PCS power if the battery still has charge left but cannot discharge fast enough. Add usable capacity if the battery runs out of usable energy before the peak, expensive period or trading window ends.
Stop adding either when the extra inverter/PCS power or battery capacity costs more than the extra bill saving or trading payment the client is likely to receive.
The rule of thumb is simple: add kW when the battery cannot charge or discharge fast enough, and add kWh when it cannot keep going long enough.
Related questions
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