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Lithium packs lose roughly 3–5% of usable capacity per year, so a 2020–21 install is typically down to 75–80% State-of-Health by 2026. The fastest field test is to fully charge, then run a measured load (an oil heater, a kettle, your fridge) and time how long the pack carries that draw before the inverter throttles. If you're getting under 80% of nameplate runtime, the pack — not the inverter — is the bottleneck and an ECB-registered installer should be quoted for replacement.

Plan on roughly 1 kWh per hour of essential evening load — fridge, LED lighting, Wi-Fi, TV and a phone charger sums to about 0.5–0.8 kWh/hr; a single oil heater on its own adds another 1.5–2 kWh/hr. A 5 kWh usable pack covers a 4-hour Stage 6 evening without the heater; double that if you want winter heating included. Size on usable kWh after the SANS 62619 derate, not nameplate gross.

Mixing chemistries on the same bus is a fire and warranty risk — most inverter manufacturers void cover the moment a non-approved battery is detected on the CAN-bus. The safer pattern is to retire the lead-acid bank entirely and reconfigure the inverter's charge profile for lithium, which an ECB-registered installer will do as part of the swap. Expect to replace the inverter too if it predates 2019 and lacks a lithium-compatible firmware.

SANS 62619 is the South African standard governing the safety of secondary lithium cells and batteries used in industrial and stationary applications. It covers thermal runaway containment, internal cell short-circuit testing, and overcharge protection — all of which separate residential-grade lithium from the cheap repurposed EV cells sometimes sold as `home backup`. Insist on a SANS 62619-certified pack named on the quote, with the certificate available on request.