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Most home batteries are warranted to retain 60–70% of their original capacity at year 10. Tesla and Sungrow guarantee 70%; Enphase guarantees 60% over its longer 15-year term. That means a 10 kWh battery may deliver closer to 6–7 kWh of usable storage by the end of its warranty period. Gradual capacity loss is normal for lithium chemistry.
The fade is slow and predictable. A battery sized correctly from the start will still cover your evening load in year ten.
Two mechanisms drive degradation in lithium batteries:
Cycle ageing: each charge and discharge cycle causes a small amount of irreversible chemical change inside the cells. Manufacturers cap this with a throughput limit: the total kilowatt-hours a battery is warranted to move over its life. Hit the cap before the year term and the warranty ends there. See battery warranties explained for brand-by-brand throughput figures.
Calendar ageing: the battery degrades with time regardless of how much it cycles. Heat accelerates this. A Sungrow SBH on a west-facing Mandurah garage wall ages faster than one in a shaded, ventilated spot. Perth's climate makes siting a real consideration, not an afterthought.
Both mechanisms run simultaneously.
A 10 kWh battery at install:
| Year | Typical retained capacity | Usable storage (approx.) |
|---|---|---|
| 1 | ~98% | ~9.8 kWh |
| 5 | ~90% | ~9.0 kWh |
| 10 | 60–70% (warranty floor) | ~6.0–7.0 kWh |
Year 1 and year 5 figures are illustrative projections; the year-10 row reflects the manufacturer warranty floor of 60–70%.
Most households find the battery still covers their evening load comfortably at year five, and covers it at year ten.
Because the battery shrinks over time, sizing purely to today's need leaves no headroom. Specify a battery somewhat larger than current daily usage so that even at reduced capacity in later years it still covers the load you need it to cover.
The right buffer depends on how you use it: a household doing one shallow cycle a day ages a battery slowly; one cycling it hard every day will hit the throughput cap sooner. See how to size a battery for a Perth home for a method that works through your actual usage before settling on a size.
Your installer can configure several parameters that affect long-term capacity:
State of charge limits: keeping the battery between roughly 10% and 90% rather than cycling it fully to zero and 100% reduces calendar and cycle stress. Most systems let you set a minimum and maximum charge level.
Charge power limits: charging more slowly generates less heat inside the cells. If your solar array is large and the battery would otherwise fast-charge every day, a power cap helps.
A backup reserve: if blackout protection matters to you, hold a minimum state of charge so the battery is never empty when an outage hits. Read how battery backup actually behaves before setting this.
Perth summers push ambient temperatures well above manufacturer comfort zones for calendar ageing. A battery mounted on a shaded south- or east-facing wall, or in a ventilated enclosure, runs cooler than one bolted to a west-facing garage wall in direct afternoon sun. See battery placement and siting for the specific location rules.
AS/NZS 5139 clearance rules, garage vs outdoor walls, and what installers check on-site.
A step-by-step method covering evening usage and seasonal AC demand.
Activation events credit 70¢/kWh on top of DEBS exports, and signing up is a WA rebate condition.
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Money-relevant figures on this page are checked against primary sources. See how we check our facts.