Quick answer
Batteries undershoot when they are undersized for the essential loads connected to them, when the depth-of-discharge reserve is set conservatively, or when age has reduced usable capacity. Lithium degrades roughly one to three percent a year, so a battery that comfortably covered an outage three years ago genuinely covers less now. Hidden parasitic loads are the other common answer.
Common causes
- Capacity undersized for the essential loads actually connected
- Depth-of-discharge reserve set high in the BMS, holding back usable capacity
- Battery ageing, at roughly one to three percent per year for lithium
- Hidden parasitic loads such as a geyser element or underfloor heating left on the backed-up side
- Longer or more frequent outages than the system was sized for
- Cold ambient temperatures temporarily reducing available capacity
What you can do now
- 1List what is actually on the backed-up circuits and measure the draw from the inverter display
- 2Look for high-draw items that should not be on essential loads at all
- 3Check the state-of-charge cut-off and reserve settings in the app or BMS
- 4Compare current runtime against when the system was new, which quantifies degradation
- 5Track consumption over a week rather than judging from one bad outage
When to call a professional
Either add capacity or right-size what the battery is being asked to carry — and check the second before paying for the first. An installer can review the BMS settings and the essential-loads split, and very often the fix is moving a circuit off the backed-up side rather than buying another battery module.
Typical cost in Johannesburg
5kWh add R45,000–R55,000
Indicative ranges only. We provide written, itemised quotes before any work.
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