A battery can reduce electricity purchases without recovering its installed cost during its useful life. Compare two otherwise identical situations: your solar installation with a battery and the same installation without one. Savings already delivered by the panels must not be counted again as battery savings.
Start with consumption, not the catalogue
Gather twelve months of electricity consumption and solar generation, ideally at hourly or shorter intervals. Identify when surplus generation can be stored and subsequently used. A large battery offers little financial value if it is mostly empty in winter and seldom discharged in summer.
Separate energy entering the battery from energy delivered back to the home. Round-trip efficiency relates these quantities over a complete cycle. Always state whether an annual estimate refers to energy charged or energy delivered. Mixing the two overstates savings.
A fully hypothetical example
Assume 2,000 kWh of solar energy charged each year, 90% round-trip efficiency, an import price of €0.25/kWh and an export value of €0.06/kWh. These are teaching assumptions, not current market tariffs.
The battery delivers 1,800 kWh, avoiding €450 of purchases. However, the 2,000 kWh stored could otherwise have earned €120 through export. The gross annual benefit is therefore €330 before fees and degradation. At an installed cost of €5,000, simple payback would be about 15.2 years if all assumptions remained constant.
What simple payback leaves out
This ratio ignores declining capacity, replacement, financing and changing electricity prices. Build conservative, central and favourable scenarios. Vary actual energy shifted, the price difference and recurring costs in each. If one small assumption change reverses the conclusion, investigate that input before proceeding.
Do not automatically add solar savings, tariff arbitrage and grid-service income. The same stored energy and reserved capacity may not be available for multiple uses at the same time.
Questions to ask before accepting a quote
- Does the price include installation, protection equipment, commissioning and inverter changes?
- What usable capacity is guaranteed, for how long and under which conditions?
- Does the calculation deduct the export income you give up?
- Are there recurring software, maintenance or service charges?
Our approach is to validate savings against your own data before comparing hardware. France’s ADEME also recommends considering demand shifting and the environmental effects of additional storage. The cheapest quote is not necessarily the strongest proposal; a calculation you can inspect is more useful than a promised return. Local tariffs and contract conditions need separate verification outside France.
Sources and transparency
Sources document the principles discussed. Examples, checklists and calculations are educational tools from BatteryIncome, not quotes or performance promises. Contract terms and offers need to be checked when you assess a project.