Solar Batteries
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Solar Storage Batteries 101 Last Updated: 5 October 2022

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.

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 ~€3,600 - €4,000

Battery storage is an optional extra for your PV solar panel system, and adds roughly €3,600 - €4,000 to the total price.

Main Benefit: Use more of your own solar electricity

Adding battery storage to your solar PV system allows you to saves 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 goes 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 who 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.

a graph showing the peaks and troughs of electricity consumption in a home in ireland
Typical domestic electricity consumption for one day - a pattern of peaks and troughs

 

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.

graph showing the electrical output of solar panels on a roof on a home in Ireland
Typical solar generation in Ireland for a day of steady weather - a smooth pattern

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

What does a solar storage battery cost?

Adding a 5kWh solar storage battery (the most common size) to your system would add between €3,600 - €4,000 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. Though do note a typical battery for solar storage have about 2000 times the capacity of a mobile photo battery.

This extra cost is made up of three main parts:

  • The cost of the battery itself

  • A most expensive inverter (called a "hybrid inverter") which is roughly €1,000 - €1,200 more than a "string inverter" (that's the more basic type that only connects solar panels to your house electrical supply

  • A few other small bits too which make up this overall cost (mounting, wiring 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 that the grant depends on the size of your solar panel array only.

The Economics - Should I get a solar storage battery?

The economics of a storage battery are an interesting question. It does depend on your system and usage. We can gladly do a quote for you both with and without a solar storage battery so you can compare the numbers for your own scenario.

A real solar storage battery example, from Ireland

Here's the data calculated from a real pv solar installation with a storage battery here in Ireland.

Battery Size 5.1kWh (this is the most common battery size)
Solar Installation Size 4.5kWp (3 bed semi-detached house)
Electricity stored then released 1,660kWh per year
Annual Savings €398.46 (assuming electricity price of 24 cents / kWh)
Battery paid for itself in six years

With an average battery adding between €3,600 - €4,000 to the initial cost of PV solar panel system installation, this battery here in question would have paid for itself in about six years.

What about the new feed-in tariff? Does that change the economics?

Yes. The new feed-in tariffs mean that a battery will take a bit longer to pay for itself.

It used to be that any excess electricity was given away to the grid for free. Then, if you needed more electricity later on you have to buy it from the grid at full price. A battery meant that you can just keep that surplus power, to save having to buy it back later.

Now, with the domestic solar feed-in tariff, you get paid a little bit for the electricity you give to the grid. It's a low rate (roughly 20% to 50% of the buying price, depending on your supplier). A battery now saves you the difference between the low selling rate and the high buying rate.

In the example above the battery would now take roughly 8 - 12 years to pay for itself when factoring in the new feed-in tariff.

Guide numbers only - take with a pinch of salt

Please do take all predictions on solar storage battery usage and savings with a fair pinch of salt - both the numbers on this page and any we put in direct quotes for you. The benefits you get entirely depends on your own pattern of electricity usage, so it can be notoriously hard to predict. Your own results might be significantly better or worse.

That said we wanted to share some real numbers with you, so at least you've a ballpark guide as to what to expect from adding battery storage to a domestic solar panel installation. We hope it helps.

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 been 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 charger can be connected to your solar PV system (for example some made by Zappi EV chargers). The electricity your solar panels produces 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 then 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 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.

What battery technologies are there?

There are 4 main types of battery technology that are used with solar PV.

  • Lead-acid batteries

  • Lithium-ion batteries

  • Nickel-based batteries

  • Flow batteries

Lithium-ion batteries are the most common battery technology installed in Ireland due to their long cycle life, safety and depth of discharge.

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 is 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 €1,000 - €1,200 more expensive that standard inverters.

Can I retro-fit 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 the 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 their 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 similarly 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 are 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 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.

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.

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