How do I prove the battery savings after installation?

Question

Accepted Answer

Agree with the client how you will calculate and report the saving they make after installation.

Start with the site’s actual import and export in each half-hour. First, use the battery AC meter to add back any electricity the battery discharged into the site. This is electricity the site would have imported without the battery.

Then, take off any electricity used to charge the battery. This extra import would not have happened without the battery charging.

The result is the no-battery estimate for each half-hour. Now, apply the client’s tariff to the no-battery estimate and to the actual metered import and export. The difference is how much the battery saved the client. For the post-installation report, show the saving separately from trading income and extra running costs.

Make sure the meter readings line up

The supplier bill only shows how much electricity the site imported from the grid or exported to the grid. It does not show what the site would have imported or exported without the battery.

For the savings report, use readings from all the places that affect the calculation:

the grid import/export meter at the connection point
the battery’s AC meter
the solar system where it affects battery charging
the power requested by the controller and the battery AC power recorded at the meter
the battery state of charge
battery faults and downtime

Before handover, ask the installation team to confirm that the current transformers are installed the right way round, set to the correct ratio, connected to the right phases and showing import and export with the correct signs.

Also ask them to run a controlled charge and discharge test. If the battery discharges at 40 kW, the site import should fall by roughly 40 kW after you allow for any change in the site load or solar output. That test should pick up reversed CTs, wrong multipliers and clocks that do not match.

Record the battery state of charge at the start and end of each reporting period as well. If the battery has less stored energy at the end of the month than it had at the start, part of the saving shown for that month came from electricity that was already in the battery before the reporting period began.

Work out what the bill would have been without the battery

For every half-hour, add back the electricity the battery discharged and take off the electricity used to charge it.

If you treat import as a positive figure, use this calculation.

Estimated import or export without the battery = actual import or export + battery discharge − battery charging

If the site imports 120 kW on average during the half-hour while the battery discharges 40 kW into the site, it would have imported 160 kW without the battery.

If the site imports 180 kW on average during the half-hour while the battery charges at 50 kW, the site load without that charging was 130 kW.

Apply the client’s real tariff to both sets of half-hourly figures. Include the import rates, export payments and any peak, capacity or network charges that change because of battery operation.

Show the client these four figures.

The saving on their electricity bill
Net trading or flexibility income
Software, aggregator and extra maintenance costs
The net saving after those costs

Use the aggregator’s settlement statement for trading income. Do not estimate trading income from the amount the battery discharged.

Also ask whether anything changed on site during the month. New machinery, extra shifts or longer opening hours can change what the site would have used without the battery.

GridVolt’s controller records site and battery data so you can compare what actually happened with the no-battery estimate. Before you use the saving in a contract or performance claim, make sure the client can see the raw half-hourly data, tariff and calculation behind the figure.

How do I prove the battery savings after installation?

Make sure the meter readings line up

Work out what the bill would have been without the battery

Related questions