Battery Inverter

A Battery Inverter is separate from the Solar Inverter.

A Home Storage Battery stores electricity in the form of Direct Current (DC) electricity. However your home and the grid work on Alternating Current (AC). You need a dedicated inverter to convert AC into DC when charging up the battery, and to convert DC into AC when discharging the battery.

Because the Battery Inverter is linked to AC in this way, it is usually referred to as “AC Coupled.”

AC Coupled inverters – ideal for retro fit

If you have an existing Solar Array, and you are benefiting from Feed in Tariff (FiT) payments, you can have a battery storage system added by using an AC Coupled Inverter. An AC Coupled Inverter and battery storage solution can be added to any existing solar array.

What does the AC Coupled inverter do?

Imagine it is a sunny day. Your solar panels are delivering 2.5 kW of power. At that moment your house is only consuming 1 kW of power. The AC Coupled inverter will direct excess energy into your battery for later use – in this case 1.5 kW.

To do this the inverter measures the flow of electrons using something called a CT sensor. A Current Transformer (CT) sensor measures alternating current passing through a wire. If you are adding an AC Coupled inverter as a retrofit, you’ll have 2 CT sensors:

1. A CT will be placed on the Live wire that comes into your house after it has passed through your SMETS2 Smart Meter. This measures the flow of electricity into and out of your home from the grid.
2. A CT placed on the Live wire that is exiting your String Inverter. This measures the flow of electricity from your Solar Array

The inverter manages the flow of electricity automatically and works with the battery to do this.

Response Times.

Imagine your house is using 1 kW of electricity. When you turn on the kettle, your energy demand spikes up to 3.5 kW (the average kettle uses 2000W or 2 kW). Since you are connected to the electricity grid, the kettle simply draws the power from the grid, and the kettle instantly turns on. The CT sensor measures the increase in flow of electricity. There is then a delay between the inverter telling the battery to discharge an additional 2 kW, and the flow of power then initiating and finally reaching the full 2 kW. In other words the battery needs time to “ramp up” and then further time to “ramp down” once the kettle has finished boiling.

The response time of an Inverter & Battery is usually up to 5-10 seconds, but can be less than one second.

Combined Total Output.

With an AC Coupled system, the output from the battery is in addition to the output from the String Inverter.

Let’s say your solar array is delivering 2 kW of power on a sunny Easter Sunday. It is time to cook the Easter Sunday lunch. You’ll have the oven on for at least two hours and a couple of rings of your stove on to cook the veg. Your total power demand will be about 5 kW.

Your Solar array is maxed out at 2 kW. So your battery kicks in to provide the additonal 3 kW. And therefore you have a total of 5 kW available.

                2 kW (solar) + 3 kW (battery) = 5 kW (total demand)

With a Hybrid inverter this is not the case. You are limited to the maximum power output of the Hybrid inverter.

A standard Hybrid Inverter will have a maximum specified output in the range of about 3 kW – 4.6 kW. Therefore anything over and above the maximum power output of the Hybrid inverter will need to come from the grid.

Let’s put this another way. You own a 3.6 kW Hybrid inverter. It is very sunny and your 4 kWp Solar Array is generating three quarters of its potential output – 3 kW. Your battery is full. In the Easter Sunday Lunch example, although the battery has lots of energy available to help power the oven, it will be limited by the maximum output of the Hybrid Inverter, and will only supply an additional 0.6 kW.

                3 kW (solar) + 0.6 kW (battery) = 3.6 kW (maximum output of Hybrid inverter).

You can see that we are short by 1.4 kW.

Independent operation.

This is a key advantage of a AC Coupled Inverter – it is independent of the Solar array. The other related benefit is that if there is a problem with the Solar array and the String Inverter is offline for some reason, that doesn’t affect the AC Coupled inverter. It will continue to operate as normal.

Solar panels

Solar panels are excellent value for money, delivering a very competitive return on investment.