Quick answer
Use one half-hourly battery schedule for all savings. Each kWh can only do one job at a time, so electricity used to cut site imports cannot also be counted as export or trading income in the same period. Make the separate savings lines add back to one clear annual total.
To stop double-counting savings in your proposal, build one half-hourly schedule for the battery, and use it to calculate the savings from solar self-consumption, peak shaving and trading.
Don’t model each one separately. In the proposal, you are only selling one battery, with one state of charge, one PCS limit and one amount of usable energy. If you use 50 kWh to reduce site imports in the morning, don’t count that same 50 kWh again as trading export later or as stored energy for the afternoon peak.
Record what happens to each kWh that goes into or comes out of the battery
For every half-hour, record the following figures:
- How much solar electricity charges the battery
- How much grid electricity charges the battery
- How much discharged electricity supplies the site
- How much discharged electricity leaves through the export meter
- Site import and export
- State of charge
- Battery losses
- Any usable kWh the controller reserves for trading, backup or peak shaving
Record one destination for every discharged kWh - this will be the site or the export meter.
Let’s say the battery discharges 100 kWh during one period. If 80 kWh from the battery goes to the building and 20 kWh leaves through the export meter, apply the saving from buying less grid electricity only to the 80 kWh. Apply the export or trading payment only to the 20 kWh. Do not apply both values to the full 100 kWh.
The same rule applies across the rest of the day, so you’ll need to carry the battery’s state of charge into the next half-hour. If a supplier, aggregator or Virtual Trading Party uses 50 kWh from the battery for trading at lunchtime, that 50 kWh is no longer available to reduce the site’s afternoon peak unless the battery charges again before then.
That is why the model behind the proposal has to record one use for each stored kWh in each half-hour.
Make the separate savings add up to one annual total
Start with the client’s current electricity bill. If the proposal already includes solar, first calculate the bill with solar and no battery. Then add each battery use one at a time so you can see what each one adds to the annual saving.
Add them in this order:
- Solar self-consumption and tariff shifting
- Peak-charge reduction
- Contracted trading
After each step, check how the bill or trading statement changed. That tells you what that battery use has added to the client’s annual saving and helps you avoid counting the same discharge, export or peak reduction twice.
Use the client’s actual bill, supplier contract and trading agreement to check five things:
- The import charges after battery operation
- Any maximum-demand or capacity charge from the new site peak
- Trading payments from the measured volumes or availability in the agreement
- Grid charging, battery losses and solar export income given up
- Aggregator fees, revenue share and the cost of grid electricity the site imports to recharge the battery after trading
One battery discharge may reduce both the imported kWh and a separately billed kW peak. That is a genuine saving on two different charges on the bill. Do not use a blended rate here. It may already include the peak cost, so if you add a separate peak saving on top, you count the same saving twice.
In GridVolt, you can model site savings and trading through the same battery schedule. Before you put the figures into the proposal, check that the bill savings, export income and trading income add up to the annual saving in the proposal.
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