Quick answer
Add more kW when the battery cannot charge or discharge fast enough. Add more usable kWh when it can deliver enough power but runs out too soon. Check short site peaks as well as half-hourly averages, then compare the real battery and PCS packages to see where extra size stops improving the client’s return.
Add more kW when the battery can't charge or discharge fast enough. Add more usable capacity when it runs out before the peak or tariff window ends. The half-hourly data should tell you whether the site needs more kW, more kWh or a bit of both.
When the job needs more battery power
Go for more kW if there are sharp peaks on site, like a bank of EV chargers all pulling at once at a depot, or when the battery has to charge inside a tight window, for example a three-hour overnight cheap rate before an early-morning peak.
Say the client wants to keep import from the grid below 300 kW, but the site reaches 380 kW for half an hour. During that half hour, the battery will need to discharge at about 80 kW. A 40 kW power conversion system (PCS), the unit that turns the battery's DC into usable AC, can't cover the full peak on its own, even if there is plenty of stored energy in the battery.
You may need more power if the battery has to:
- Catch sharp spikes off the machinery, refrigeration or chargers
- Keep the site below an agreed import limit
- Soak up a short burst of spare solar before it spills to export
- Recharge inside a short cheap-rate window
- Recharge quickly enough before the next peak
Check the charge side as well as the discharge side. A larger PCS will not help if the site only has 50 kW of spare import capacity overnight. You can hit the same problem if the DNO limit, the export limit or the solar inverter caps the power available.
A PCS can only put out up to its rated power. A 40 kW PCS delivers 40 kW at most, whatever the battery holds. So what sets the PCS size is the highest power the site draws at once, not its average.
Half-hourly data shows you the average load on a site over each thirty minutes, and not its highest level reached. For example, a motor or a charger bank might run for only two or three minutes. This has little to no effect on the half-hourly average but it still sets the highest level of power demand reached across the site.
So, you have to size the PCS to that level and not the average. If you size it from the half-hourly figure, you might fit a PCS that's too small. To avoid undersizing, meter the site at a shorter interval for a week or two to find the real peaks. Then size the PCS to those peaks.
When the job needs more usable capacity
Go for more kWh when the battery has the power but can't keep discharging for long enough.
Using the same 300 kW import target, suppose the site stays at 340 kW for three hours. The battery only needs about 40 kW of discharge power, but it needs around 120 kWh of usable energy before you allow for losses and reserve.
You may need more capacity if the battery has to:
- Cover a long afternoon or evening peak
- Store several hours of surplus solar
- Shift load across a long expensive period
- Deal with repeated peaks before it can recharge
- Keep some charge back for backup
Work from the usable kWh, not the nameplate - GridVolt’s simulator does this automatically. The usable share is the battery's depth of discharge (DOD), the manufacturer's figure for how much of the nameplate you actually get, often around 90 percent. Then check what's left after the minimum state of charge, the client's reserve and conversion losses.
Now compare the actual battery and PCS combinations you can buy at that size, and the full cost of putting each one in. More power may mean you need to recommend a larger PCS, switchgear, cables and connection work. More capacity may mean recommending another cabinet, more space and extra civils.
GridVolt's simulator does all this for you. Just give it the nameplate and the DOD, and it works out the usable capacity for you. Then it runs the PCS and battery combinations against the same half-hourly data, tariff and solar profile. Keep adding kW until the battery can charge and discharge fast enough for the client’s needs.
Then, add enough kWh so it can keep doing the required work for long enough. Stop adding kW or kWh when the next PCS or battery cabinet no longer improves the payback, NPV or IRR the client cares about.
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