There are two main components in a battery storage system:

• The inverter / charger
• The battery itself.

And there are two ways to connect the battery system to your electricity supply: DC or AC Coupling. We’ll look at each in turn.  Then we’ll examine the principles of battery usage.

Battery inverter

The battery is connected to the consumer unit (fuse box) in the property by an inverter. The inverter controls the electricity flowing to & from the battery.

When the inverter is sending electricity to the battery (charging it) the electricity can either come from your solar panel system, or from cheap rate electricity.

When the inverter is drawing electricity from your battery, electricity flows into your home (via the consumer unit). This can power your house, your car, or be exported to the grid, taking advantage of smart tariffs which pay you a premium to export at certain times of day.

A DC (direct current) inverter is used as a direct interface between a DC source (such as a solar array) and the battery. It converts the variable DC input voltage to the precise required DC charge voltage required by the battery.

An AC (alternating current) charger provides a DC charge from an AC source (e.g. the mains grid, or AC supplied by the solar PV inverter).

For our primer on the difference between AC and DC click here.

DC vs AC coupling

Since we have some components in the system (battery & solar panels) using DC and the household electrics and the grid using AC, we have 2 principal options for how we connect the system together.

AC coupling – connect the battery charger to the AC side of the network.

A typical AC coupled schematic is as follows:

In this case the charger is known as an ‘inverter / charger’, converting AC back to DC to allow storage in the battery. The battery inverter / charger is separate from the PV inverter. An AC coupled system is typically installed where there is an existing solar array, and the battery is being installed as an additional component.

With an AC coupled system, the power from the solar PV is additional to the power from the battery.

What this means is that the first electricity to be used is that coming from the solar PV. If that is insufficient, then the battery will make up the difference. In the scenario where your house is drawing 6 kW of power (lets say you are cooking the Sunday roast), if the Solar PV is generating 1.5 kW, and your AC Coupled Battery can discharge at 4.5 kW, you can actually satisfy the 6 kW requirement without drawing power from the grid.

With a DC coupled system, unless you have a 6 kW hybrid inverter, you’ll be unable to satisfy this requirement.

DC coupling – connect the battery charger to the DC side of the PV system.

A typical DC coupled schematic is as follows:

DC coupled systems have a single unit acting as PV inverter and battery charger (a hybrid inverter) making them ideal for new PV installations.

Control of the Battery

The Battery Management System (BMS) controls the battery. The BMS is connected via clamps to the main electricity cable coming into your house from the grid, and the electricity cables coming from your solar PV system (if you have one).

The clamps allow the BMS to monitor the flow of electricity around your property. For example, it can see how much solar electricity is being generated. It can also tell if electricity is being imported from the grid.

Charging your battery with solar panels

In the daytime, if there is more solar electricity than the house needs, the BMS will sense this and divert the surplus solar electricity to charge the battery. The BMS doesn’t let this excess solar electricity escape to the grid.

If, on the other hand, there is not enough solar electricity to charge all the devices turned on in the house, then the BMS will tell the battery to discharge electricity to meet as much of the shortfall as possible.

As long as there are devices turned on in the home, calling for electricity, the battery will keep discharging until it is almost empty. It will then wait until there is surplus solar electricity again in order to recharge.

Charging your battery with cheap, off-peak electricity

It’s also possible to charge your home battery storage with off-peak electricity.

With a smart tariff, the price of your electricity will vary at different times during the day. This is especially useful in winter, when you can charge your battery up overnight when prices are low, and discharge during the day when prices are high.

DC coupled systems have a single unit acting as PV inverter and battery charger (a hybrid inverter) making them ideal for new PV installations.

Power Output

A key consideration is how quickly the battery can discharge its power.

For instance:

• The Solax Triple Power 5.8 kWh battery can discharge at a standard rate of 2.8 kW, but can surge for short periods up to 4 kW.
• When operating with 2 slave battery modules, the combined battery has a standard output of 8.6 kW and can surge up to 12 kW.

However, to access this power the battery needs to be matched with an appropriate inverter. For instance, the inverter might be rated for only 3.7 kW, in which case it can never discharge at a rate above 3.7 kW, no matter how many battery modules are attached.

To put this in context, your oven might be turned on and using 2kW of power. If you turn on the kettle, this will draw an additional 2 kW of power. Added to all the background power usage around your home, you’ll likely be using 5 kW of power at that moment. Whilst your battery may be able to handle this rate of power discharge, unless your inverter is rated for 5 kW, then you’ll need to draw from the grid for the time it takes the kettle to boil.

GivEnergy batteries

The GivEnergy battery range is comprehensive. The smallest battery is the Giv-Bat 2.6 which is a relatively inexpensive way to install a battery system.

All the GivEnergy batteries can be stacked with additional modules. So whilst you might choose to invest initially only in one 2.6 kWh battery, you can add up to a further 4 modules (for a total of 5 modules) giving a total of 13 kWh.

The largest domestic battery is the All in One which is an AC Coupled battery, delivering 13.5 kWh of storage, and stackable up to 80 kWh.