Tubular batteries are still sold hard, mostly on the purchase price. Over the life of a solar installation that is the wrong number to compare.
Usable capacity is not rated capacity
A battery's rating is not what you can take out of it.
With a tubular battery, drawing more than about half the rated capacity shortens its life sharply. So a 200 Ah tubular battery gives you roughly 100 Ah of usable energy if you want it to last.
With lithium, nearly all the rated capacity is usable. A 100 Ah lithium unit gives you close to 100 Ah.
Which means a 100 Ah lithium battery and a 200 Ah tubular battery do about the same work — and that is before considering how long each survives.
Working out the energy
Capacity in kilowatt-hours is voltage times amp-hours, divided by a thousand:
- 25.6 V × 100 Ah = 2.56 kWh
- 51.2 V × 100 Ah = 5.12 kWh
Those are the two lithium sizes we fit. Inverter capacity decides which: up to about 4.2 kW we use 25.6 V units, above that 51.2 V.
Lifespan
A tubular battery in daily cycling use is generally finished in three to four years, sooner if it has been run flat or left unwatered.
A good lithium battery is rated in thousands of charge cycles, which in normal domestic use is eight to ten years. It also degrades gradually rather than failing outright, so you lose runtime slowly instead of losing the system one evening.
The ten-year comparison
Over ten years you would expect to buy a tubular bank roughly three times. You would buy lithium once. Even where the tubular bank is substantially cheaper per purchase, buying it three times is more expensive — and that is before the maintenance.
Maintenance and siting
Tubular batteries need topping up with distilled water, they give off gas, and they need a ventilated space away from living areas. Neglect the watering and the life collapses.
Lithium needs nothing. No watering, no ventilation requirement, no gassing. It is far lighter and can be wall-mounted, which matters in a house with no spare room to give over to a battery bank.
Charging
Lithium accepts charge faster, which matters more than it sounds. On a short winter day, or a day with cloud, the array may only have a few good hours. A battery that can take that charge quickly is a battery that is actually full by evening.
Where tubular still makes sense
If the budget genuinely will not stretch, and the system is small, and somebody will reliably maintain it. We will tell you so rather than pretend otherwise. But for an installation expected to last twenty-five years, lithium is the sounder purchase, which is why it is what we fit.
Rules we do not bend
Units in one bank must match in brand, voltage and capacity. A mixed bank performs like its weakest member and fails early, so we will not install one — not even if the mismatched unit is free.
Current prices and sizes are on the lithium batteries page, and the hybrid systems page explains how the battery fits into the rest of the system.