A storage system installed in 2019 is now on its second inverter firmware and its last battery cycle. The inverter is fine. The mounting is fine. The grid connection is certified and the feed-in tariff is registered. Replacing the whole system throws away all of that.
The Cost Comparison Nobody Disputes
| Full new system | Battery retrofit | |
|---|---|---|
| Hardware | 100% | 35–55% |
| Installation | 2–3 days, two people | 4–8 hours, one person |
| Grid-code re-certification | Required in most EU markets | Not triggered |
| Feed-in tariff re-application | Usually required | Unchanged |
| Scaffolding / roof access | Often required | Not required |
| Downtime | 2–3 days | Half a day |
The Compliance Argument
Replacing a battery does not change the grid connection, so no new grid-code certification is triggered.
This is not a loophole. It follows from how the regulations are written. The grid connection is certified as an assembly of inverter plus protection settings plus declared behaviour under fault conditions. A battery presents a DC voltage and a charge/discharge current limit. Replace it with one that presents the same voltage window and speaks the same protocol and the certified assembly is unchanged.
Replace the inverter and you start again: new test report, new network application, new commissioning witness. In Germany that is weeks. In Italy it can be months.
What “Same Voltage Window and Protocol” Actually Requires
- Voltage window: a 48 V system expects roughly 42–58 V. A 51.2 V LFP pack sits inside that. A high-voltage system expects a specific range, typically 200–800 V, and the replacement must not exceed it at full charge.
- Protocol: CAN or RS485, with the same message set. This is where most retrofits fail — not on voltage but on a proprietary CAN dialect.
- Charge profile: the inverter’s absorption and float voltages must be adjustable to LFP values. Most are; some early hybrids are not.
- Current limit: the BMS must be able to tell the inverter to reduce current, or the inverter must be settable to a fixed limit below the BMS trip point.
How to Check Before You Buy
- Photograph the inverter nameplate: make, model, firmware version
- Photograph the existing battery label: chemistry, nominal voltage, Ah, protocol
- Ask the supplier for a written compatibility statement naming your inverter model and firmware
- Ask what happens if it does not communicate on arrival — who pays for the return
- Confirm the BMS current limit is below the inverter’s maximum charge current
A supplier who will not put compatibility in writing before the order is a supplier who has not checked.
When Retrofit Is the Wrong Answer
- The inverter is more than 12 years old and spare parts are gone
- The inverter uses a closed protocol with no published message set
- The existing system was undersized for the load and the customer wants more power, not more runtime
- The grid connection itself needs upgrading
We will tell you which group your system is in before you order. That is cheaper for both of us than a return shipment.
Send Us Your Inverter Details
Make, model, firmware version and a photo of the existing battery label. We confirm compatibility in writing.