Storage products won’t last forever – read the small print
Actually, there are three questions here:
- How long will the battery last?
- What is the warranty period?
- What does the warranty say about ‘degradation’?
Home batteries tend to be pretty robust. No-one has a crystal ball, but the well-made batteries will last at least 10 years. The really high quality ones should last as long as 15 years.
Degradation
Battery warranties for home storage systems typically last between 5 and 10 years, though the quality of the warranty matters more than the length.
The key term to look for is ‘degradation‘ – this describes how well the battery retains its capacity over time. Just as a mobile phone battery gradually holds less charge after a few years, all lithium batteries degrade with use.
For example, a 5 kWh battery with a “10 years / 20% degradation” warranty means:
- If the battery fails within 10 years, the manufacturer will replace it free of charge.
- After 10 years, the battery is guaranteed to retain at least 80% of its original capacity – so in the worst case, your 5 kWh unit performs like a 4 kWh battery. If capacity falls below that, you’re entitled to a replacement.
Cycles
Many warranties include a cycle limit, for example “10 years or X cycles, whichever comes first.”
A cycle is one full charge and discharge (roughly 0-100%). For example, if your warranty says “10 years or 3,000 cycles,” heavy daily use could reach the cycle limit before the time period expires – effectively shortening your warranty coverage.
The gold standard is a warranty that offers unlimited cycles within the stated term.
Throughput
Another limitation to watch for is throughput – the total amount of energy discharged over the battery’s lifetime.
For example:
“10 years or 30 MWh of throughput, whichever comes first.”
Since 1 MWh = 1,000 kWh, that’s 30,000 kWh total.
If you typically discharge around 10 kWh per day, you’d hit 30,000 kWh after about 3,000 days (roughly 8 years) before reaching the full 10 years.
Battery usage
Finally, check what the warranty says about how the battery can be used.
Some warranties apply only when the battery is used for solar energy capture, not for grid charging or export.
If you plan to use your battery both for solar storage and off-peak charging, check whether this affects your warranty terms.
For example, a warranty might allow “unlimited cycles” for solar use only, but reduce the guarantee to 40% degradation if mixed with grid charging or export.
Summary
Always read the fine print before buying. The most useful warranty is one that:
- Offers 10 years coverage,
- Allows unlimited cycles and throughput, and
- Supports your intended uses (solar, off-peak, or grid services).
If a manufacturer’s warranty doesn’t align with how you’ll actually use your system, it’s worth comparing other models before making a decision.
