Battery degradation: what you lose each year

A home battery holds less at the end of its life than at the start. That is expected, it is warranted, and it belongs in any projection of what storage saves you. A twenty year forecast that holds battery capacity flat is not a forecast.

Two separate processes are at work, and they are worth telling apart because they respond to different things.

Cycle ageing and calendar ageing

Cycle ageing comes from use. Every charge and discharge takes a small amount off the cells’ capacity. More cycles, more loss.

Calendar ageing comes from time. Cells lose capacity sitting still, influenced by temperature and by the state of charge they are held at.

This is why warranties are written the way they are: a guaranteed remaining percentage after a stated number of years or a stated amount of use, whichever comes first. The manufacturer is covering both processes, and the “whichever comes first” tells you they expect either to be able to end the warranty.

What real warranties actually promise

The two ends of the market illustrate the shape. Tesla’s Powerwall warranty for this region guarantees 70% of original capacity at ten years, with unlimited cycles when the battery is used for solar self consumption, but a throughput cap in other operating modes. GivEnergy offers a twelve year warranty on approved residential installations, conditional on periodic health checks through the battery’s life.

Read the pattern, because it recurs across brands: the year figure gets the headline, while the fine print defines how hard you may work the battery and what fraction of day one capacity the manufacturer is actually standing behind. A “ten year warranty” on a unit warranted to 70% is, precisely, a promise that the battery may lose up to three tenths of its capacity inside the warranty and that is considered normal.

Turn the numbers on yourself:

• Cycling once a day is about 3,650 cycles a decade. Charging cheaply overnight in winter and from the roof in summer can approach two cycles a day, which doubles throughput and reaches any cycle or throughput cap in half the time. If you plan to work the battery as an arbitrage asset, check which operating modes the warranty actually covers.
• A battery at its 70% floor is not dead, but it is smaller. The 6 kilowatt hour unit from our sizing example holds about 4.2 usable kilowatt hours at that floor: still most of a quiet evening’s use, or well short of a heavy one. Whether the end-of-warranty battery still covers your evening is a question you can answer now, with your own overnight consumption figure.

Depth of discharge is part of the design

The reserve a battery keeps back, rather than discharging to empty, exists to slow cell degradation. That is why usable capacity is lower than nominal capacity: you are being protected from yourself.

It also means a system quoted on nominal capacity is quietly promising you energy the management system will not let you have. Compare on usable capacity, always, and price quotes per usable kilowatt hour rather than per advertised one.

Round trip efficiency is separate

Degradation is capacity lost permanently. Round trip efficiency is energy lost on every cycle, in conversion and heat.

Both reduce what you get. They are different deductions and both belong in a projection:

• Degradation shrinks the size of the bucket over the years.
• Round trip loss taxes every fill and empty, immediately and forever.

Confusing them flatters projections in a specific way: applying only a round trip percentage makes year fifteen look like year one with a small haircut, when in reality the bucket itself has shrunk.

What actually shortens battery life

Within your control, roughly in order of influence:

Deep, frequent cycling. More throughput, faster cycle ageing. Somewhat unavoidable, since throughput is also how it earns money: a battery preserved by never cycling is a battery that never pays for itself. The balance to strike is cycling for value, not for its own sake.

Temperature. Heat is hard on cells. A battery in an unventilated loft has a harder life than one in a cool garage, and lofts swing far outside the comfortable range in a UK summer. Where the installer proposes to put it is worth discussing before the day, not after the cabling is run.

Sitting at very high state of charge. Holding a battery full for long periods contributes to calendar ageing. Decent management systems handle this for you; it is worth asking whether yours does.

Putting it in a projection

The correct treatment is to reduce usable capacity year by year along the warranted curve, and recompute what the battery can shift each year on that reduced capacity. Our calculator does exactly this: capacity declines through the projection and the battery’s contribution is recalculated on the declining figure, which is why its savings curve dips rather than gliding.

The wrong treatments, both common:

• Holding capacity flat and applying an arbitrary discount at the end.
• Ignoring it entirely and quoting year one savings across two decades.

If your projection runs past the battery’s expected life, a replacement belongs in it at a realistic cost. Leaving it out flatters the result at exactly the point where the numbers get large: the later years, where a twenty five year total does most of its accumulating.

Ask the right question of a quote

Not “how long does it last”, which invites a reassuring answer, but:

“What remaining capacity does the warranty guarantee, after how many years and how much throughput, in the operating mode I will actually use, and how many cycles will my usage put through it?”

That is answerable, checkable against the warranty document rather than the brochure, and it turns the warranty from a comfort blanket into an input you can model. Then put that input, with your own rates and consumption, through the ROI calculator and see what the battery earns across its real, declining life rather than its advertised one.