A battery does one thing for a solar array: it moves generation from the middle of the day, when you are probably not using it, to the evening, when you are. That raises the fraction of your own generation you consume, and since consumed electricity is worth several times exported electricity, that is where the money is. Everything else about batteries follows from that single sentence.
Solar panels and battery: how storage makes solar worth more
Without storage, a typical household exports a large share of what its array makes, because the array peaks at midday and the house peaks at seven in the evening. A battery captures the surplus and releases it later, so units that would have earned a low export rate instead replace units you would have bought at the import rate. The value of the battery is therefore the gap between those two rates multiplied by how much it shifts, which is why batteries look better when import prices are high and export rates are low.
Sizing a battery
The useful size is set by the evening and overnight consumption you want to cover and by how much surplus the array actually produces on an average day, not by the biggest unit available. Oversizing means paying for capacity that sits empty most of the year; undersizing means exporting surplus you could have kept. Look at usable capacity rather than nominal, because they differ, and at the continuous power rating, because a battery that holds plenty but can only discharge slowly will not run a shower and an oven at once.
A solar and battery package, a hybrid, or AC coupled later
A battery installed at the same time as the array normally uses a hybrid inverter that handles both, which is the tidiest and usually cheapest route. A battery added later is commonly AC coupled, with its own inverter alongside the existing one, which works well and adds a conversion step. Either is legitimate. What matters is that the combination is a supported one, that the warranty positions are clear, and that the existing inverter is not at the end of its life when you bolt a new asset to it.
The reasons to buy one that are not financial
Backup during a power cut is not automatic: most installations disconnect from the grid when supply fails unless they are specified with a backup circuit, so if you want that, ask for it by name and expect it to add cost and complexity. Independence from tariff swings, the ability to charge cheaply overnight on a time of use tariff and then run the house on it, and simply using more of your own generation are all real reasons. They are worth being honest about, because they are often the actual motivation and they are not the same as payback.
Solar battery dimensions: cm on the datasheet, and where the box can go
The size that matters to an installer is the kilowatt hour rating and the size that matters to you is the cupboard. Home batteries divide into two shapes: a wall hung unit roughly the proportions of a large radiator, and a floor standing tower that can be stacked as capacity is added. Both are heavy enough that a wall unit needs a masonry or properly noggined wall rather than plasterboard. Ask the installer for the datasheet dimensions in centimetres for the exact model quoted, because they vary widely between makes and a quote rarely names them. Then check the clearances the manufacturer requires above, below and to the sides for ventilation, the temperature range the warranty depends on, and whether the unit is rated for outdoors if the only space is on an outside wall. A garage, a utility room or a plant cupboard is the usual answer; a loft is usually the wrong one, on both heat and the job of carrying it up there.
Questions people ask about solar battery storage
Do I need battery storage for solar panels?
No. An array works without one and exports what you do not use. A battery raises the share you use yourself, which is worth money when import rates are much higher than export rates, and it costs money and has its own life.
What size solar battery do I need?
Enough to cover the evening and overnight consumption you want to shift, and no more than the array's typical daily surplus can fill. Compare usable capacity rather than nominal, and check the continuous power rating as well as the size.
Can a battery be added later?
Yes, usually AC coupled with its own inverter. Fitting it with the array using a hybrid inverter is normally tidier and cheaper overall, so it is worth deciding before installation even if you buy later.
Will a battery keep the lights on in a power cut?
Only if it is specified with a backup supply. Most grid connected systems shut down when the grid fails for safety reasons, so backup is a specific option to ask for rather than something a battery gives you by default.