Residential Battery Storage

First off how does it work? Residential battery energy storage is most commonly coupled with solar PV installations, the main advantage of this pairing is the ability to take the energy generated by PV during the day and store it to power your home in the evening and overnight.

Since Irish residential customers don’t currently benefit from a residential feed in tariff (this is when you get paid you for surplus energy you produce at home via technology such as solar panels and export it to the grid) it makes sense to try and use as much of the electricity solar panels are producing as possible. Battery storage makes this all possible.

During the day the excess energy your home is not currently using goes into charging the battery, so instead of sending the excess electricity back to the grid, you store it in a battery that can then discharge and power your appliances when the sun isn’t shining.

However, battery storage is not the only option when it comes to storing the excess energy generated by solar PV, a diverter can also help to make use of that excess electricity. A diverter is designed to monitor the Solar PV generated power and the electricity demand in the home, if there is an excess generated, the Diverter will use that ’spare’ generated power to heat your water.

Installing a Diverter will cost about €400, whereas installing a battery can run up to €7,000 for a 5kWh unit.

Adding a Diverter would increase your savings, with some of the unused excess going to heating water. However, a Battery Storage Unit is the most efficient way maximise the energy yield from your Solar PV panels. Of course, heating water is only beneficial if that water is actually used.

Industry often uses the rule of thumb that savings are doubled when solar is combined with battery storage and a day/night meter. This is because the battery has a second trick up its sleeve, energy arbitrage. This is when you purchase and store energy at a time when the cost of electricity is low and subsequently selling or using that stored energy during a time when the cost is high. This strategy is called energy arbitrage and is the most direct method for profiting from energy storage.

How effective battery storage is depends very much on the energy production/consumption profiles of the home, if your demand profile matches up nicely with your power generated from your solar PV, then a battery will be of less of a benefit for you. It is in situations where the demand profile does not align with the power generated from the solar PV that a battery can be of great benefit.

The image below shows the energy flow data from a residential home in the south of Ireland with roof top PV, the yellow line is the power generated the PV system, the green is the energy flows into/out of the property, anything below the x-axis is power that has been exported to the grid. Although the rate of self-consumption was ok for the first half of the day, after 13:00 much of the electricity generated by the solar PV was exported to the grid.

Over a longer period you can see the cumulative impact of this, over 800 kWh of electricity has been exported from this home during the first 8 months of 2020, about 44% of the total energy generated. At €0.18/kWh this is the equivalent to €144 for the first 8 months of 2020. This is a year where the homeowner spent far more time that normal at home during the middle of the day.

At the start of October this homeowner installed a diverter to try and improve their rate of self-consumption, once a significant amount of data has been gathered, we’ll share the results of this decision and compare it to how a battery might have faired instead.