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.
Warranty periods generally vary from 5 years to 10 years.
However, what the warranty document says about degradation is key. Degradation refers to how well the battery can hold its charge over time. We are all familiar with the mobile phone that runs out of battery really quickly after two years. That’s degradation.
A 5 kWh home battery storage warranty might say: 10 years with 20% degradation. That means:
- If the battery stops working before 10 years is up, you will get a new battery for free, and
- At the end of the 10-year period, the battery is guaranteed not to have lost more than 20% of its storage capacity. It will act like a 4 kWh battery in the worst case scenario. If it loses more than 20% of its capacity, then you are entitled to a battery replacement, as the warranty terms will have been broken.
However, battery warranties are sometimes qualified. In other words, they say “10 years, as long as…”
A common “as long as” that creeps in is: as long as you don’t exceed a certain number of cycles.
A cycle is when your battery charges to 100% and then discharges to 0% (or close to 0%). That’s one cycle.
Your warranty might say “10 years or 3,000 cycles, whichever comes first.” You might find that you meet the cycle limit before you get to the 10 years. A bit sneaky.
If your battery warranty says “unlimited cycles”, that’s ideal.
Another area to watch out for is throughput.
This is how many units of electricity are discharged from the battery in total. The warranty might say “10 years or 30 MWh of throughput, whichever comes first.”
By the way, a 1 MWh is 1,000 kWh (see our Energy Terminology page).
So to make sense of the throughput figure, you need to:
- Calculate how many kWh of electricity you are likely to get out of the battery per day (let’s say 10 kWh),
- Times the warranty throughput figure in MWh by 1,000 to get a figure in kWh (e.g. 30 MWh x 1,000 = 30,000 kWh),
- Then divide the throughput kWh figure by the amount of electricity you are likely to get from the battery per day (30,000 kWh divided by 10 kWh per day = 3,000 days).
In the example above, 3,000 days equates to just over 8 years. So, again, the 10-year headline warranty figure is not all it’s cracked up to be. You might only get 8 years, even using the battery in a normal way.
The final area of small print to read covers what you use your battery for.
Batteries are versatile and can:
- Capture excess solar electricity
- Be charged with off-peak electricity from the grid
- Provide back-up power to your home in a power-cut (some models)
- Export electricity to the grid (some models)
Your warranty may restrict you to certain uses.
For example, it might say you can have unlimited cycles and unlimited throughput, as long as the battery is only used to capture excess solar generation and then discharge it to the house.
If you also use the battery for off-peak charging, as well as solar capture, then the warranty might weaken. It could say for combined off-peak charging and solar use, instead of 20% degradation after 10 years, it is 40% degradation.
In summary, you need to read the warranty carefully and consider how you will be using the battery, now and in the future. If the warranty does not make sense for you, look at other manufacturers’ models.