Before anything is bought, the question is which of four things changed, and each leaves a different trace.
Consumption went up
A new appliance, an old one behaving differently, or something left on. The daily energy-out figure answers it directly, and a clamp meter attributes it circuit by circuit. This is the most common cause and the cheapest to fix.
Charging went down
A shaded panel, a dirty or failing module, a controller that stopped tracking, a DC-DC charger no longer triggered, or simply the season. Compare the daily energy-in figure against the same month last year rather than against last week.
Something is awake that should not be
A standing drain measured with everything supposedly off. Over a long idle period it is the difference between a full bank and a disconnected one, and it is invisible to every instrument except a clamp meter or a monitor.
The battery actually aged
The last thing to conclude, not the first. It is established by a counted discharge from full to the system cut-off at a known load, compared against the original usable capacity — not by a voltage reading and not by the calendar.
Then, and only then, decide what to buy
If consumption rose, reduce it or add charging. If charging fell, fix the charging. If the bank is genuinely worn, replace it — and check whether replacing one battery in a series bank is something its maker permits, because in most cases it is not.
What people ask before they order
Is a bigger battery ever the wrong answer?
Usually, when the real problem is charging. A larger bank that is never refilled makes the shortfall arrive later rather than removing it, and it costs more to replace.
How do I separate consumption from capacity?
Compare the daily energy out against what it was, and the energy in against what it was. A rise in one and a fall in the other point at completely different components.
Last reviewed 16 September 2026