Battery degradation as cash flow

Short answer #

Reduce shifted energy each year by assumed degradation and discount the resulting benefit. Do not equate a capacity warranty with expected performance.

Important variables #

• Investment after support
• Useful energy, efficiency and degradation
• Energy prices and tariff
• Horizon and discount rate

Worked example #

2% annual degradation leaves a 10 kWh battery at about 8.17 kWh after ten years before other losses.

Break-even: What changes the result? #

Test the main driver as a cautious, central and optimistic range. Also check whether the ranking reverses at a crossover or remains robust across the range.

Common mistakes #

• Treating subsidy or tariff as certain
• Omitting losses and degradation
• Confusing simple payback with return

Sources & assumptions

• KfW Bankengruppe — Heizungsförderung ab 21. Juli 2026 (2026-08-17)
• RATIOXA — Methodology and formulas (2026-08-17)