Battery Degradation Modelling — 25-Year Performance for FE College Solar+Storage
How to model battery capacity retention over 25 years for UK FE college solar+storage projects, and what depth-of-discharge limits to specify.
A commercial LiFePO4 battery in 2026 typically warranties 70% capacity retention at year 10 with 10,000+ cycle life. But what does that actually mean for the FE college Salix energy savings calculation over 25 years? Here’s how to model it properly.
What battery degradation looks like
LiFePO4 (lithium iron phosphate) batteries — the standard chemistry for FE college solar+storage in 2026 — degrade through two mechanisms:
- Calendar ageing — capacity loss over time regardless of cycling, driven by SEI layer growth and electrolyte decomposition
- Cycle ageing — capacity loss per charge-discharge cycle, accelerated by deep discharge cycles, high temperature, and rapid charge rates
For typical FE college use patterns (1-2 cycles per day at 80% depth of discharge maximum, plant-room ambient temperature 15-25°C), capacity retention typically follows:
- Year 1: 99% of rated capacity
- Year 5: 92-95%
- Year 10: 80-87% (warranty floor 70%)
- Year 15: 70-78%
- Year 20: 62-72%
- Year 25: 55-68%
By year 12-15, most commercial installations plan for cell replacement (typically £50-£80/kWh installed) to restore capacity. Some installations operate degraded batteries to year 20-25 without replacement.
What to model in the Salix calculation
For the Salix Decarbonisation Loan energy savings calculation including battery:
- Years 1-8 (loan period): Model 95% average capacity over the repayment period
- Years 9-15: Model 80% average capacity for the post-repayment value period
- Years 16-25: Model 70% average capacity, OR plan cell replacement at year 12-15
Conservative modelling at these levels typically still delivers Salix-eligible economics for FE college projects.
Depth-of-discharge (DoD) specification
The single biggest design lever on battery life is DoD limit:
- 80% DoD: standard for commercial installations, 6,000-8,000 cycles to 70% capacity
- 70% DoD: safer for cycle life, 8,000-10,000 cycles, ~10% less usable capacity per cycle
- 90% DoD: aggressive, 4,000-5,000 cycles, faster degradation
For FE college installs we recommend 80% DoD as default — balances cycle life against usable capacity. Don’t go below 70% (too much usable capacity lost) or above 85% (cycle life degrades sharply).
Cell replacement vs full system replacement
At end-of-warranty (typically year 10-12), the corporation faces a decision:
- Cell replacement only — replace battery cells while retaining BMS, inverter, enclosure. Typical cost £50-£80/kWh installed. Common path.
- Full system replacement — replace battery + BMS + possibly inverter. Typical cost £100-£140/kWh installed. Rare unless inverter is also at end-of-life.
- Operate to year 20-25 without replacement — accept declining capacity, run battery for diminishing value. Sometimes the right answer where downstream use case has shifted.
Plan for cell replacement in year 12 in the Salix calculation; the recovered capacity supports continued operation to year 25.
Temperature management
Battery cycle life is highly temperature-sensitive:
- 15-25°C: rated performance
- 5-15°C: modest reduction in available capacity
- 25-35°C: accelerated degradation (~2x faster ageing per 10°C above 25°C)
- above 35°C or below 0°C: rapid degradation, possible safety implications
For FE college installs, plant-room location with adequate ventilation is essential. Avoid roof-space or attic mounting where summer temperatures exceed 35°C.
Monitoring battery health
Performance monitoring should track:
- State of Health (SoH) — current rated capacity vs original (typically updated monthly by BMS)
- Cycle count — cumulative full-equivalent cycles
- Average DoD — actual depth of discharge per cycle
- Capacity retention curve — projected against warranty floor
- Temperature exposure — time above 30°C in plant room
Decline against warranty floor triggers manufacturer warranty claim before the formal year-10 measurement.
Practical sizing for 25-year operation
For a typical FE college sixth-form-college 80 kWh battery installed at year 0, the 25-year usable energy profile:
- Years 1-8: ~75 kWh average usable per cycle (94% × 80% DoD)
- Years 9-12: ~68 kWh average usable (85% × 80% DoD)
- Year 12: Cell replacement at £4,000-£6,400 (£50-£80/kWh × 80 kWh)
- Years 13-20: ~76 kWh average usable (post-replacement)
- Years 21-25: ~70 kWh average usable
Cumulative usable energy over 25 years: approximately 530,000-600,000 kWh (assuming 1.2 cycles/day average). At typical 22p/kWh self-consumption value, lifetime battery contribution: £117,000-£132,000.
Against initial battery cost (£8,000-£11,200) + cell replacement (£4,000-£6,400) = £12,000-£17,600 total — strong economics over the asset life.