More battery capacity is not automatically better

Short answer #

Only actually shifted kWh creates value. A larger battery may reduce cycles per kWh; test capacity sizes against the same load and generation profile.

Important variables #

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

Worked example #

10 kWh at 200 full cycles shifts 2,000 kWh; 15 kWh at 140 cycles only 2,100 kWh. The extra investment serves little more energy.

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)