A common question we receive from battery owners is “why didn’t my battery charge yesterday?”

The expectation is that the battery will be used every day, or that the combination of the installed solar and battery will be used to match a resident’s household energy use and minimise the energy purchased from the grid. The answer is associated with the logic of using your battery to minimise your energy costs and maximise the value of your energy sales. With the BESY Energy Platform, your battery is set up to respond to price signals rather that the signal of your household load.

The simplest example is on a cloudy day where solar production across Norfolk Island is limited, and the diesel generators are required to stay on all day. This results in the tariff staying at 82c/kWh all day. For a resident with a solar and battery system, what solar energy they don’t use themselves will be sold into the grid at 82c/kWh. There is no incentive to store energy in the battery, so the battery will be idle. This outcome preserves the life of the battery for days when it can add value and avoids the round-trip energy losses of battery operations.

A day with bright sunshine provides the opposite outcome. High solar production on Norfolk Island allows the diesel generators to be turned off, and the lower tariffs to apply. At a low tariff of 5c/kWh or 20c/kWh, a resident with a solar and battery system will seek to store as much energy as they can rather than selling surplus solar energy into the grid at low the low tariff. Effectively, their battery will buy electricity at the low tariff and save this energy until the diesel generators are turned back on and the tariff returns to 82c/kWh. For every kWh of storage, the battery owner will make either 77c or 62c (less losses). A typical resident with a 10kWh battery will benefit $7.70 or $6.20 on days when the tariff is 5c/kWh or 20c/kWh respectively. Of course, every resident will also benefit from the low tariffs applying during the day.

In reality there are plenty of days between these two examples. These include days where the diesel generators are not turned off for long enough for a residential battery to be fully charged, and days where the tariff only falls to 41c/kWh. On these days, a battery will be partially used or potentially not used at all.

A final note about why we do not use batteries for load following on Norfolk Island, whereas this is typically the right thing for people to do in other locations. On Norfolk Island, there is no difference between the tariff for buying or selling electricity at any point in time.The dynamic tariffs apply equally to purchases and sales of electricity. In other locations, residents pay one (high) tariff for the energy they use, but receive a low feed-in tariff for the solar energy they export to the grid. This gap provides the financial incentive to minimise the energy they purchase from the grid.

Norfolk Island Beach
Solar panels with blue sky background