What wears out, and when

People buy these systems expecting them to last, and mostly they do. What surprises them is that the parts wear out at very different rates, so something needing replacing in year twelve is not a sign that anything went wrong.

Here is the rough shape of it.

The short version

Solar panels barely wear out

Panels have no moving parts, and what happens to them is a very slow decline, not a failure. They lose roughly half a per cent of their output a year, so after twenty years a panel is still doing about ninety per cent of what it did when new.

Manufacturers warrant this: typically twenty-five or thirty years, guaranteeing something like 80 to 87 per cent of the original output at the end of it. That is a performance warranty, which is unusual and tells you how predictable the decline is.

In practice panels are usually removed because the roof needs redoing, not because they stopped working.

The inverter is the one to budget for

The inverter turns the panels' electricity into the kind your house uses. It is electronics working hard every day, warm, often in a loft, and it is the part of a solar installation most likely to need replacing.

Ten to fifteen years is the usual expectation, against panels that will run for twenty-five. So over the life of an array you should expect to buy one more inverter. Set against the cost of the system it is not a large sum, and it lands far more comfortably as a line in your budget than as a surprise in year twelve.

Microinverters — one small unit per panel instead of a single central box — are warranted longer, but there are as many of them as there are panels and they are on the roof, so a replacement means scaffolding.

Heat pumps last about as long as boilers

Fifteen to twenty years is normal, which is much the same as a good gas boiler, and it is the compressor that sets the limit.

One thing shortens it considerably: cycling. A heat pump that is too big for its house cannot turn down far enough on a mild day, so it starts and stops repeatedly all winter, and every start is wear. This is one of the reasons oversizing matters beyond the running cost.

A ground source system is different in a useful way. The heat pump itself lasts the same fifteen to twenty years, but the buried pipework — which is the expensive half — has no moving parts and is expected to last fifty years or more. So the ground loop outlives two or three heat pumps.

Batteries decline instead of failing

A home battery does not stop working one day. It gradually holds less, in the way a phone battery does, just far more slowly because it is treated much more gently.

Warranties are typically ten years, or a number of charge cycles, or a total amount of energy passed through it — whichever comes first — with a guaranteed remaining capacity at the end, usually 70 or 80 per cent.

Read which of those three limits applies to you. A battery cycled once a day for solar will reach a cycle limit at a predictable rate; one also charging from a cheap overnight tariff cycles harder and gets there sooner.

What needs doing

Solar: almost nothing. Rain does the cleaning, provided the roof has enough pitch for water to run off. Worth a look once a year for moss creeping up the bottom edge or bird droppings, which are opaque and can affect more than the panel they land on.

Heat pumps: an annual service. Keeping the outdoor unit clear of leaves, checking system pressure, cleaning the strainer, and checking antifreeze where there is any. Manufacturers often make the warranty conditional on it, so check whether yours does.

Unvented hot water cylinders: annual attention from somebody qualified for unvented systems. This is a separate job from the heat pump service and is routinely forgotten until something goes wrong.

Batteries: nothing you do. The management system looks after them. Keep the area ventilated and at a sensible temperature.

Planning for it

If you are working out whether something pays for itself, one inverter replacement somewhere around year twelve is the realistic thing to include. Everything else on the list either outlasts the payback period comfortably or, like the annual service, is a small running cost, not a lump.