Batteries are now often the most expensive part of your solar system and choosing which battery technology suits you can be tricky.
What type of battery you need depends on the circumstances.
If you want your batteries in your living space they need to be sealed batteries. If you can have the batteries outside, or in a ventilated box, then regular 'wet' batteries (Lead Acid or NifE) will be fine as long as the application is static. It is possible to have wet batteries inside but, as with all batteries, you will need to locate them in an enclosure with good air flow and an air vent to allow any gases to escape to outside. If the application is mobile such a road going vehicle or boat that experiences rough conditions it is strongly recommend not to use wet batteries as the electrolyte which contains dilute sulphuric acid can be expelled from the top of the batteries due to the movement of the vehicle or boat.
Also consider weight when selecting your battery and give some consideration to passive temperature control - keeping them cool in the summer and warm in the winter.
A rough rule of thumb is between 100AH and 200AH of batteries, in a 12 Volt system, per 250W. (So if in 24 Volt system would be 50AH to 100AH and so on)
The amount of Amp hours of battery capacity you choose needs to be able to be maintained by the size of your solar system. After this it's personal preference on how much you want to spend and how much redundancy you want to have.
We have created a battery comparison chart to help you decide which type of batteries you should use. The chart below shows various battery banks using different types of batteries and it give you an estimate of the amount of usable kWhr storage you will get. We then look at the cost and number of years the battery should last in daily use. We have taken the lower estimates of years on all batteries, and then look at the cost per kWhr and cost per kWhr per year.
All batteries types will last longer if not heavily discharged daily (70-100% discharge). We would advise not designing any system that will deeply discharge your batteries daily. Most systems should be designed with a number of hours or days of autonomy to cover days when there is less sun, it would be at these times when you drained the battery more, rather than daily.
Why do batteries have different ratings? All lead acid batteries have C5, C20, C100 ratings to stand for the amount of time the battery is discharged over. A C5 rating means you if discharge the battery in 5 hours it will have the quoted AH capacity over that discharge period. If you discharge the same battery over a longer period for example 20 hours (C20) it will have more AH capacity as the nature of the chemistry allows the battery to impart more energy over a longer time span. So why are these different? As a comparison consider your car, if you fill up the fuel tank and then drive at 50mph for the full tank you will get about as many miles as possible on that tank. But if you drive at 90mph for the full tank you will only get about 1/2 - 2/3rds of the distance.
We hope this chart will be useful to guide people in their choice of batteries.
|798ah bank 24v||554||665||798||50%||333||7,980||8||£2,160||£271||10||£27.07|
|1452ah bank 48v||1008||1210||1452||50%||605||29,040||29||£6,840||£236||10||£23.55|
|LiFePO4 12V 100ah||100%||1280||1.28||£615||£480||10||£48.05|
|LiFePO4 24V 200ah||100%||5120||5.12||£2,500||£488||10||£48.83|
|LiFePO4 48V 200ah||100%||10240||10.24||£4,650||£454||10||£45.41|
Salt Water Aquion
(no longer manufactured)
|Aquion S-Line 2Kwhr Stack||100%||2,000||2||£1,400||£700||15||£46.67|
|Aquion 5 x S-Line 10Kwhr||100%||10,000||10||£7,000||£700||15||£46.67|