One of the most common questions in solar backup projects is: “How long will the battery last?” There are two different answers hidden in that question:
- Runtime: how many hours the battery can power selected loads during an outage.
- Lifespan: how many years the battery can serve before capacity degrades and replacement becomes sensible.
Your runtime depends on connected loads. Your lifespan depends on how hard the battery is cycled over time. This guide focuses on lifespan. If you are estimating outage runtime, use the pricing calculator and compare with Battery Capacity & Life Basics.
Runtime vs Lifespan (Why They Get Confused)
A larger battery usually gives more outage runtime. But it can also increase battery lifespan in years because the same daily usage is spread across more storage capacity. In practical terms, a battery that is cycled shallowly tends to age more slowly than one cycled deeply every day.
That is why battery sizing should be based on both:
- Required outage experience (comfort during blackouts)
- Expected cycling pattern (occasional backup vs frequent daily cycling)
Cycle Life Explained
Cycle life is the number of charge/discharge cycles a battery can deliver before it drops to a stated remaining capacity (often 70–80%). Modern LiFePO4 batteries used in backup systems are commonly marketed in the range of ~6,000 to 7,000 cycles under test conditions.
Those numbers are useful, but they are not a direct “years of life” promise. They depend on test conditions such as temperature, charge rate, discharge rate, and depth of discharge. Real-world life is usually a combination of cycle aging and calendar aging.
Equivalent full cycles matter more than charging events
Two half-discharges roughly equal one full cycle in aging terms. So a site that lightly cycles a battery twice in a day may age it similarly to one deeper cycle per day. This is why usage pattern and load management matter just as much as the battery label.
Calendar Aging (Even When Outages Are Rare)
Batteries age over time even if they are not heavily cycled. This is called calendar aging. Heat, time spent at very high state of charge, and poor ventilation can all accelerate capacity loss.
For clients who mainly want backup during outages, calendar aging can become the main life limiter. For clients who cycle batteries daily (for load shifting or frequent outages), cycle aging becomes the bigger factor.
What Most Affects Battery Lifespan in Real Installations
- Temperature: Heat is one of the strongest battery-life reducers. Good placement and ventilation matter.
- Depth of Discharge (DoD): Repeated deep discharges are harder on batteries than shallower cycling.
- Charge/discharge rate: Pulling or charging too hard generates heat and stress.
- System settings: Inverter/BMS limits, charge voltages, and cut-off points affect long-term degradation.
- Installation quality: Cable sizing, terminations, and firmware configuration affect performance and thermal behavior.
- Usage pattern: Daily cycling, multiple outages, and heavy simultaneous loads all change battery stress.
In short: the same battery can have very different lifespans in two different homes or facilities, even if the hardware model is identical.
Warranty, Replacement Planning, and What to Ask
Battery warranties are usually the most practical planning reference for clients. Many lithium backup batteries are sold with 5–10 year warranties, often with conditions around correct installation, operating environment, and supported inverters/settings.
When comparing options, ask for:
- Warranty duration and what capacity threshold is covered
- Whether the warranty is pro-rata or full replacement
- Required commissioning records and maintenance documentation
- Supported inverter models and approved configuration settings
- Expected expansion path (can more modules be added later?)
How Iselle uses this in design
We treat battery selection as a service-level decision, not just a product choice. That means sizing storage around outage priorities, expected cycling, and expansion plans, while keeping settings and protection aligned with warranty requirements.
For a practical starting point, use the estimate calculator to compare runtime across battery sizes, then use this guide and Battery Basics to understand long-term trade-offs before survey and final BoQ.