Does this calculator work for tubular batteries?

Yes. Switch the toggle to tubular and the voltage list changes to 12V, 24V and 48V nominal, and the usable figure drops to 50 percent of the rating. Lithium is worked at 12.8V, 25.6V and 51.2V and at 90 percent.

Why is the usable figure lower than the energy stored?

No battery is emptied completely. Lithium is taken to about 90 percent of its rating before the BMS stops it. Tubular is held at about 50 percent, because taking lead acid lower than half shortens its life sharply.

What does at the socket mean on this calculator?

It is the usable energy after inverter losses of about 10 percent. That is roughly what your appliances actually receive, and it is the only one of the three numbers that reflects what you get to spend in the house.

What voltage should I enter for a 48V battery?

For a lithium pack choose the 48V system option, which uses 51.2V, the real nominal voltage of sixteen cells in series. For tubular choose 48V. If your label shows something else, use the other voltage box and type it in.

Ah to kWh Calculator for Lithium Batteries

Volts multiplied by amp hours, divided by 1000, gives you kWh. A 280Ah battery at 12.8V holds 3.58kWh. Enter your own figures below and the calculator does the rest, including how much of that you can actually use.

Work out your battery kWh

Enter the voltage and the amp hours printed on the battery. You get the energy it actually holds.

System voltage

Amp hours (Ah)

Common sizes:

Energy stored

3.58 kWh

12.8V × 280Ah ÷ 1000 = 3.58 kWh

3.23 kWh

Usable (90% DoD)

2.90 kWh

At the socket

Enter a voltage and an amp hour figure above zero.

Three different numbers, and sellers quote whichever one suits them. The first is what the pack holds. The second is what you can safely take out. The third is roughly what reaches your sockets after inverter losses.

The assumptions, stated plainly so you can check them. Lithium is worked at 90 percent depth of discharge, which is what a normal BMS will actually let you take. Tubular is worked at 50 percent, which is the figure its own cycle life rating is quoted at. Both are then reduced by 10 percent for inverter loss, which is a fair average for a pure sine wave unit under real load. Your own kit will vary a little either way.

Not sure whether the battery you have been quoted is really that size?

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How the conversion works

Volts multiplied by amp hours, divided by 1000. That is the whole formula. A 280Ah lithium pack at 12.8V holds 3.58kWh, which is why it is sold as a 3.6kWh battery.

The part that catches people is the voltage. Amp hours on their own tell you nothing, because the same Ah figure at a different voltage is a completely different battery. Always check what voltage a pack is before you compare it with anything else.

Common conversions

Battery	Working	Energy
12V 280Ah lithium	12.8 × 280 ÷ 1000	3.58 kWh
25.6V 100Ah lithium	25.6 × 100 ÷ 1000	2.56 kWh
51.2V 280Ah lithium	51.2 × 280 ÷ 1000	14.34 kWh
51.2V 314Ah lithium	51.2 × 314 ÷ 1000	16.08 kWh
12V 200Ah tubular	12 × 200 ÷ 1000	2.40 kWh
24V 200Ah tubular	24 × 200 ÷ 1000	4.80 kWh

Why lithium and tubular are not compared on this number alone

A tubular battery holds the energy in the table above, but you cannot use all of it. Take a lead acid battery much below half and you shorten its life sharply. Lithium runs down to about ninety percent of its rating without complaint. So a 2.4kWh tubular battery gives you around 1.2kWh in practice, while a 2.56kWh lithium battery gives you around 2.3kWh. That is the comparison that matters, and it is the one the calculator above shows you.

Where to go next

how to convert Ah to kWh, for the working behind the sum and how to read two quotes against each other.
lithium vs tubular batteries, if the two figures above surprised you.
what size of lithium battery you need, now that you know what a kWh figure means.
what size inverter battery for my house, the free sizing guide, which walks the whole calculation for a real house.