Should you add a battery to an old solar system?

Melbourne is full of small solar systems installed between 2009 and 2012. If you have one and you are now thinking about storage, the answer is often “not yet” — and sometimes “not that”.

The array has to be able to fill the battery

This is the constraint people miss. A battery does not create energy. It stores surplus your panels generate and your house does not immediately use.

A 1.5 kW system in Melbourne might produce around 6 kWh on a good day. Your house will consume most of that as it is generated. The surplus available to charge a battery could be a couple of kilowatt-hours — and much less in winter.

Put a 10 kWh battery on that and it will rarely fill. It will sit partly charged, cycle shallowly, and deliver a fraction of the saving you were quoted. In the worst case the system charges it from the grid, which converts your investment into a slightly expensive way of shifting when you buy power.

If your array cannot fill the battery, expand the array first. We would rather tell you that than sell you the bigger job.

Check your feed-in tariff before you touch anything

This one can cost real money.

A small number of Victorian households are still on legacy premium feed-in tariffs from the early schemes, at rates far above anything available today. Those arrangements are valuable and generally cannot be reinstated once lost.

Modifying or expanding your system can end that arrangement permanently.

If you are on a premium tariff, read the terms before changing anything. Sometimes the right advice is to leave the existing system completely untouched and install a separate system alongside it. It is not the neatest engineering, but if it preserves a tariff worth hundreds a year, it is the right call.

AC-coupled or DC-coupled

If your array is worth keeping and you do want storage, there are two routes.

AC coupling adds a separate battery inverter alongside your existing system. It works with almost anything, leaves your current inverter in place, and is the usual retrofit answer. There is a small efficiency penalty from the extra conversion step.

DC coupling replaces your existing inverter with a hybrid. More efficient, tidier, fewer boxes on the wall — but it means discarding an inverter that may still have warranty left.

The deciding factor is usually the age of your existing inverter. Inverters typically last 8 to 12 years. If yours is approaching that, replacing it with a hybrid makes sense. If it is three years old with warranty remaining, AC coupling is almost always the better choice.

Is the old system even safe?

Install quality during the 2010-2012 rush was inconsistent. Two things are worth checking on any system of that vintage:

Rooftop DC isolators. Water ingress into certain enclosures was a widespread problem and has caused fires. If yours has never been inspected, it should be.

Panel and connector condition. Mismatched connectors and degraded seals are common on older installs.

A health check on a fifteen-year-old system is worth doing regardless of whether you add storage.

A sensible order of operations

1. Check whether you are on a premium feed-in tariff. If so, stop and get advice.
2. Have the existing system inspected, particularly the DC isolators.
3. Work out your actual overnight consumption from interval data.
4. Compare that against your array’s realistic daily surplus.
5. If the array cannot fill a sensibly sized battery, expand the array first.
6. Then, and only then, size the storage.

Skipping to step six is how people end up disappointed.