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If you’ve decided to go solar, either at home or at your business premises, you will have invested in a roof or ground mounted solar PV installation hoping to maximise your returns. But what do you do when the sun doesn’t shine, it’s night time or there’s a power cut? Until recently, solar battery storage hadn’t developed sufficiently to be commercially viable but that has now changed.

Battery storage and back-up systems enable you to store your surplus solar energy generated during the day. Whenever there’s a demand for power – day or night, rain or shine – you can tap into your battery stored solar power, reducing your electricity costs and minimising your dependence on the grid.

So, what should you be looking for when choosing a battery storage system for your solar installation? To help you make an informed decision to purchase any battery storage product, arm yourself with as much knowledge as possible and ask these 5 key questions.

1. Is there a difference between battery chemistries?

Batteries for solar energy storage are typically made with one of three chemical compositions: lead acid or lithium ion, and saltwater. This renewable energy company, for instance, offers a range of battery storage systems consisting of SMA or Fronius battery controllers and lead acid or lithium ion deep-cycle, long life batteries that come with 10 year warranties.

Lithium ion batteries are lighter, smaller and have a longer lifespan than lead acid batteries but are more expensive. Saltwater batteries are a relative industry newcomer and not yet as established as other technologies. Saline solution electrolytes take the place of heavy metals, which makes these batteries easily recyclable.

2. What does ‘capacity’ mean?

Measured in kilowatt hours (kWh), the capacity it the total amount of electricity a solar battery is able to store. That said, batteries should never fully discharge, so the important number is its ‘usable capacity’, not ‘total capacity’. Tesla’s Powerwall 2, for example, has a capacity of 14kWh but it’s sold as 13.5kWh usable capacity – the battery won’t ever drain completely to prevent damage.

Many home solar batteries are designed to be stackable, meaning you can add multiple units to get extra capacity.

3. How important is the battery’s power output?

The capacity tells you how powerful the battery is but it gives no indication about how much electricity can be provided at any one time – this is the power output, which is measured in kilowatts (kW). Some solar batteries can only deliver 800W, which is no good if you want to boil a regular, 2000W rated kettle.

A battery system with a high capacity but low power output would be able to deliver electricity for a long time, but may not be powerful enough to run your household appliance. Low capacity and high power output, conversely, means that the battery could satisfy all the electricity needs in the building, but only for a short time.

It’s important to check the power rating of the battery storage system you are thinking of buying, so that you don’t end up drawing energy from the grid even though there’s energy in your solar battery.

4. How long will the battery last?

A solar battery will charge and drain (‘cycle’) on a daily basis daily, with the ability to retain a charge gradually decreasing with use – much like a mobile phone. It will have a warranty that guarantees a specified number of cycles (or years of use). Since performance will naturally degrade over time, most manufacturers also guarantee that your battery keeps a certain amount of its capacity for the duration of the warranty.

The typical lifespan for a solar battery is 5-15 years, meaning you will need to replace it at least once to keep up with the 15-30 year lifespan of your solar PV installation. That said, just as solar panels are now lasting longer, it’s expected that further development in battery technology will lead to solar batteries following suit.

5. What’s the price per kilowatt hour (kWh)?

It’s important to compare prices for battery storage before you make your final purchase decision. Some companies offer a price per kWh that’s lower than current grid prices, with the ability to bulk buy future electricity and fix your costs for the lifecycle of the battery storage system.

Some systems’ price per kWh is higher than current grid prices, which effectively means you are wasting money on expensive electricity supply. Also bear in mind that the better the quality of the storage system, the more likely it is that it will last beyond the warrantied time frame.