Does cycling a home battery every day wear it out?
A home battery on a smart tariff fills and empties every day, sometimes more than once. Owners worry that they are using it up. By the end you will know how lithium cells actually age, what the warranty does and doesn't promise, and why a battery bought to shift energy should be worked rather than nursed.
What a cycle is
A cycle is one full charge and one full discharge: 13.5 kWh in and 13.5 kWh out on a Powerwall. It doesn't have to happen in one go. Charging from 40 % to 90 % and back four times moves the same energy as two full cycles, and cell chemists count it that way, as equivalent full cycles. So "a cycle a day" really means "the battery's capacity once a day, in total".
Depth of discharge is how far down each trip goes. A cycle from 100 % to 0 % is a deep cycle; from 80 % to 50 % is shallow. For most lithium chemistries, shallow cycles cause less wear per kWh moved than deep ones, and the very top and very bottom of the range are where the wear concentrates. Manufacturers already keep a margin at both ends that you never see. The 100 % in the app is not the cells' true 100 %.
Two kinds of ageing
Batteries lose capacity in two separate ways, and it helps to keep them apart.
Calendar ageing
Cells degrade with time whether or not they are used. Chemical side reactions slowly consume lithium and thicken the layers inside each cell. This happens faster when the cell is warm and when it is held at a high state of charge. A battery that sits at 100 % in a hot garage all summer ages faster than one cycling gently at 20 °C, even though the second one is doing more work.
Cycle ageing
Each charge and discharge also causes a small amount of mechanical and chemical wear, from the electrodes expanding and contracting as lithium moves in and out. This is the part that scales with use. It is worse at high currents, at extremes of charge and at extremes of temperature.
The important consequence is that a home battery which is barely used still ages. Over ten years the calendar part is often as large as the cycling part, so the idea that you can "save" the battery by not using it is mostly wrong. You pay for the calendar ageing whatever you do.
NMC and LFP
The Powerwall 2 uses nickel-manganese-cobalt cells, usually written NMC. The Powerwall 3 uses lithium iron phosphate, LFP. In general terms:
| NMC (Powerwall 2) | LFP (Powerwall 3) | |
|---|---|---|
| Energy per kilogram | Higher | Lower, so the unit is heavier for the same kWh |
| Cycle life | Good | Typically longer |
| Sensitivity to sitting full | Higher | Lower |
| Cold charging | Needs warming below freezing | Also needs warming, somewhat more so |
Neither chemistry is fragile in a home battery. Both are managed by a system that limits current, holds back the extremes and heats or cools the pack. LFP's longer cycle life is real and is one reason the industry has moved towards it for stationary storage, but a well-managed NMC pack still lasts well beyond the point where the rest of the installation needs attention. See Powerwall 2 versus Powerwall 3.
What the warranty promises
A home-battery warranty typically guarantees that the battery will still hold a stated percentage of its original capacity at the end of the warranty term, usually ten years. The terms often differ by how the battery is used: a battery cycled only on solar self-consumption may be covered without limit on energy throughput, while one that also grid-charges, exports or takes part in a grid service may have a throughput cap in kWh, after which the warranty ends even if the years haven't run out.
The exact percentage, the term, the throughput figure and which uses fall into which category vary by model, by region and by the date you bought it. Read the warranty document for your own unit, which your installer supplied and which the manufacturer publishes for each model and market. If you are planning to cycle hard on a tariff, the throughput cap is the number to find, and then work out how many years it lasts at your daily kWh.
A battery bought to shift energy should be cycled
Suppose your battery would lose a small fraction of its capacity to calendar ageing over ten years even if it never cycled, and a similar fraction more from cycling once a day for those ten years. Not cycling it saves the second fraction, at the price of never doing the job it was bought for. Every cycle skipped is a day's saving not made. See how much a home battery saves.
A battery bought to sit at 100 % for power cuts is a different purchase with a different logic. A battery bought to store cheap energy for dear hours earns its money by cycling, and the wear from doing so is part of the cost of that saving, not a reason to forgo it. Two cycles a day, as on a tariff with several cheap windows, is more wear again, so the second cycle has to be worth more than its share of the ageing, which on most tariffs it is.
What is worth avoiding is pointless cycling: filling from the grid before a sunny day, or bouncing between 95 % and 100 % all afternoon because the rules keep changing the reserve. Those cycles wear the battery and save nothing.
Temperature, and sitting full or empty
Temperature is the factor you have most control over at installation and least after it. A battery in a garage or on a shaded north wall spends most of its life in the band it likes. One on a south-facing wall in full sun, or in a loft, runs warm every summer afternoon and ages faster. Moving one is an installer's job, but it is worth knowing when choosing a location.
Sitting at either extreme for a long time is the other thing to avoid. Holding 100 % for weeks, for instance with the backup reserve set to 100 % all winter, is the pattern the chemistry likes least. Holding it empty for weeks is also poor, and the management system will draw a little from the grid to prevent it. The daily pattern of a smart tariff, full in the morning and low by bedtime, is fine. A holiday setting that leaves the battery at a middling charge for a fortnight is better still.
What ten years looks like, and when to worry
In relative terms, expect a small loss in the first year or two, a slow and steady decline through the middle years, and a gradual steepening towards the end of the warranty term. The end-of-warranty guarantee is a floor, not a forecast; most units land above it. A battery that reports 13.5 kWh at installation and a kWh or so less after several years of daily use is doing what the chemistry says it should.
What is not normal is a sudden step. A capacity that drops by a noticeable chunk over a few weeks, a battery that can't reach 100 % when it used to, or one that runs from full to reserve far faster than its history suggests, points to a cell group or the management system rather than ageing. That is the time to look at the manufacturer's diagnostics and talk to your installer. A slow drift over years is the battery doing its job; a cliff is a fault.
In SunHarvest
The Energy page keeps a daily history of what went into and out of the battery, so the cycling your tariff actually produces is visible: two fills a day on Cosy, one on Go, and days on Agile when it hardly moved. That is the record to check against a throughput cap in the warranty. The AI planner works a capacity estimate from the largest state-of-charge swing seen in recent history, a rough way to watch for the slow drift, and the change log on Control › Powerwall names the rule behind any reserve change, so pointless bouncing between reserves is easy to spot and stop.
General information, not financial or electrical advice. Tariff terms, prices, warranties and connection rules change and differ by supplier, region and installation; check the current documents for yours. SunHarvest is not affiliated with Tesla or Octopus Energy.