Guide to solar storage batteries (2024)

What is a solar storage battery?

A solar storage battery is just a really big rechargeable battery similar to a mobile phone battery. It is much larger though, commonly storing enough electricity to charge your mobile phone 2000 times or do ~6 full loads of washing.

When your solar panels are generating more than you are currently using, the battery stores that spare electricity. Later on, whenever your house needs more than your panels are producing, the battery will supply that extra power to your house.

Optional extra, costing €2,400 - €2,800

Battery storage is an optional extra for your PV solar panel system, and adds roughly €2,400 - €2,800 to the total price.

Solar Battery Savings Calculator

Below are the figures for a very typical installation here in Ireland. Do feel free to play with the numbers though to adjust to your own circumstances to see if a battery adds up for you.

Yes, this calculator works for any battery from any solar installer, not just us, so please feel free to pop numbers from other quotes in here too and see if a solar storage battery is a good idea for your home.

Main Benefit: Use more of your own solar electricity

Adding battery storage to your solar PV system allows you to save any unused solar electricity to be used later on.

Most domestic solar installations produce more power than you can consume at certain times (as solar generation tends to be fairly steady, whereas household requirements go up and down a lot sometimes in the space of minutes). Without battery storage, the excess power flows back to the grid. It is not wasted, but you do not get to use it yourself.

• For an average home solar PV installation here in Ireland, you might use 65% - 70% of the power generated yourself, with the rest flowing back to the grid.

• If you add a battery, that might increase so your house uses 85% - 90% of the solar electricity you generate.

If you are away during the daytime

Solar panels generate the most electricity during the middle of the day. Even for people who are at home most of the time, most households use most electricity during the mornings and evenings, when the items that use the most electricity go on, such as kettles, ovens and showers.

Your solar battery can store the excess production from the middle of the day for usage later on.

If you are home during the daytime

Solar panels produce nice steady power. But households tend to peak and trough a lot when you are home. Flick on the kettle, and your house might be using triple the amount of electricity just for those 2 minutes it's turned on.

A battery will charge and discharge many times during the day. It will jump in to help supply the kettle if needs be just for those 2 minutes, then will start recharging straight afterwards if you've excess electricity, ready for the next cup of tea.

Typical domestic electricity usage - lots of spikes

Here is a graph of electricity consumption for one of our team member's houses that has solar installed. You can see how the consumption really jumps up and down. This would be a quite normal pattern for an average house in Ireland.

Typical solar production - smooth curve

Compare that to an example day's solar output from the same house (again in Ireland). You can see the production is much more steady.

The battery in this system would store power during each trough, and then release it on each peak.

Extra Benefit: Store night rate electricity to use during the day

Most battery systems can be set up to charge up on cheaper night rate electricity every night, for you then to use that during the day.

We would definitely recommend anyone who gets a battery to have a day rate / night rate electricity tariff from their supplier, as this allows you to get double the benefit from your battery.

How to get double the benefit from your battery.

Most households have two peaks of power usage - in the morning, and in the evening.

• Charge your battery overnight on night rate electricity (normally about half-price)

• Use that full battery to run your morning routine - that's great, it now costs you half what it used to.

• Charge up the battery during the day from your solar panels.

• Use that full battery during the evening, so it's empty and ready to be reloaded with half-price power overnight.

The daytime recharge can vary a lot - in the long summer days, you might get extra solar use where the battery doing more cycles. Dark winter days though might mean your house uses most of the power as it's produced from the panels. But, with the nighttime charge on cheap electricity you can do 365 days / year.

On Electric Ireland's latest smart rate tariff (as of 23rd Aug 2023) that would save you an extra €314.52 per year

What does a solar storage battery cost?

Adding a 5kWh solar storage battery (the most common size) to your system would add between €2,400 - €2,800 to the total cost of your system.

That seems expensive?

A solar storage battery is one of the more expensive parts of a solar electricity system. However, do note a typical battery for solar storage has about 2000 times the capacity of a mobile phone battery.

This extra cost is made up of three main parts:

• The cost of the battery itself

• A more expensive inverter (called a "hybrid inverter") is roughly €900 - €1,100 more than a "string inverter" (that's the more basic type that only connects solar panels to your house's electrical supply

• A few other small bits too make up this overall cost (mounting, wiring, more labour, etc.)

This means that replacing / adding a battery in the future is much cheaper than the total initial cost.

Are grants available for solar storage batteries?

No. There used to be grants for solar power storage batteries, but the SEAI solar grant system was changed a few years ago so the grant depends on the size of your solar panel array only.

How much power can a solar storage battery hold?

The most common size storage battery size for a house in Ireland is 5kWh. That could boil an average kettle for 2.5 hours.

Can this store a full day's generation for evening use?

Generally no, but it would depend on the size of your solar PV system, battery and time of year.

An average 3-bed house might be able to generate 20+ units (kWh) of electricity during a sunny summer's day, so a standard battery could store 25% of this. That said your house will be using a good proportion of that power during the daylight hours anyway.

During a dark December day that same solar system might generate just a handful of units, and then yes the battery would store it all.

Are there alternatives to a Solar Storage Battery?

Yes. If the aim is to make sure you get to use as much of your solar electricity as possible, then there are some other alternatives to consider.

Hot water diverter

A hot water diverter sends any excess electricity to an immersion heater in your hot water tank. Only once your tank is fully hot would any excess then be diverted out to the grid.

Responsive electric car charger

Certain types of electric car chargers can be connected to your solar PV system (for example some made by Zappi EV chargers). The electricity your solar panels produce firstly goes to run your home, then any excess will be diverted to charge your car. Only when your car is full would excess be diverted out to the grid.

Electric car batteries are typically very large, so often can soak up a few days excess electricity even on a sunny summer's day. If you have an electric vehicle, and are at home most of the time, this can be a great use of excess electricity.

Please note as yet you cannot use your electric car battery to run your house, this is a one-way system just to charge your car.

How does a solar battery work with solar panels? How it is controlled?

The inverter is the brains of the operation. The solar panels and batteries both plug into the inverter, which controls the whole operation.

The inverter will tell the battery to discharge if it detects that your house is going to import electricity from the grid. And, it will tell the battery to charge if it detects that your house is producing too much solar electricity and exporting.

How physically big are solar storage batteries?

They tend to be approximately the size and shape of an old VHS video recorder or a desktop PC tower.

Normally, they are placed in your attic right next to the inverter.

Is a solar battery the same as a rechargeable battery?

Yes, a solar battery is much the same as a rechargeable battery, they work the same way, except a solar battery is much larger than most other batteries.

Are there changes to the rest of the equipment to accommodate a battery?

Yes, you need a different type of inverter called a hybrid inverter.

The inverter is like the brains of the whole operation connecting everything together - the solar panels, your house's electrical circuits, and a battery. A standard inverter (also known as a "string inverter" or just "inverter") is cheaper, but can only connect solar panels to your house's electrical circuits. A hybrid inverter is more expensive, but can also connect to a battery.

Hybrid inverters are roughly €900 - €1,100 more expensive than standard inverters.

Can I retrofit a battery or add more batteries to my pv solar system later on?

Only if you had a hybrid inverter fitted initially. Most domestic photovoltaic solar installations that do not have batteries have either a standard inverter or micro-inverter fitted, which cannot take a battery (you can learn more about solar inverter types here ).

If you do wish to have a solar PV system fitted to your home now, then a battery added later, do make sure you request a hybrid inverter be fitted and the space left to add the battery.

Adding more batteries later on

It is generally possible to add extra battery storage capacity later on as long as you have our hybrid inverter in the first place. It will though depend on your system and the space available.

Will my solar batteries run my house in the event of a power cut?

The short answer is no.

ESB require that the inverter cannot supply electricity in the event of a power cut, as this could back feed into the electricity network and cause risk to people working on the grid to fix the lines. This means the system is required to shut down when there is a power cut.

This is also true of the solar panels - they are required to automatically shut down during a power cut.

What is meant by a battery backup?

A backup battery is a different setup to a solar storage battery.

Most hybrid inverters can provide a backup supply similar to a UPS. The backup side from the inverter can supply electricity to dedicated circuits that are not connected to the grid in the event of a grid failure. This is more for industrial purposes, you generally do not get these dedicated circuits that are not connected to the grid in household settings.

Are solar batteries dangerous?

The good news is that solar batteries are quite safe and have built-in functionality to prevent any issues. Solar batteries have the same technology as the batteries in your phone, laptop or tablet.

How long do solar batteries last?

The life expectancy of solar batteries is between 7-10-years. You will certainly need to replace your battery once during the lifetime of your solar PV system.

Please note replacing a battery is much cheaper than the initial install. Much of the cost of an initial photovoltaic solar installation with battery storage is due to things like needing a more expensive hybrid inverter, wiring and mounting, none of which will need replacing.

Where are solar batteries made?

Most of the top batteries in the market are made in China, with some other manufacturers coming from the US and Germany.

How long is the warranty for a solar battery?

Most batteries come with a 10-year warranty or a cycle life dedicated to that battery. 6000 cycles is a typical figure (meaning 6000 full charges followed by 6000 discharges). Often though you need to register your battery with the manufacturer to get the full time, so we'd definitely recommend that you do that. For example, it's common for a battery to come with a 5 year warranty, then you get the extra 5 years if you register.

The manufacturers do expect a battery do degrade over time, and figures like having a minimum of 70% capacity still usable at the end of its warranty period are common to be considered within limits - they would only replace your battery if it's below this limit. Again though the rules vary by manufacturer so please do look at your warranty documentation.

What is a cycle in a battery?

If you can imagine your phone battery going from 100% fully charged to 0%, then charging back up to 100%. This is classed as one "cycle".

What is a BMS?

A BMS is a "Battery Management System" and is a crucial element in batteries. It monitors the battery's voltage, current and temperature. It protects the battery in the event of a fault.