Why a 100 Ah battery is not 100 Ah
Power & Off-Grid Published October 7, 2026
Two batteries sit on a shelf. Both say 100 Ah. One is lead acid, one is lithium iron phosphate, and the lithium one will run your cabin for most of a day longer.
Nothing on either label is wrong. The number on the side is the capacity in the cells, and what you are allowed to take out of them is a different number entirely.
Depth of discharge is the whole story
Every chemistry has a point past which discharging does lasting damage. Depth of discharge is how far down you can go before that starts, expressed as a share of the nameplate.
Flooded and sealed lead acid are conventionally designed around 50%. Lithium iron phosphate runs at 80 to 90% comfortably. So the same 100 Ah becomes 50 Ah of real energy in one case and 80 to 90 in the other, and that gap carries straight through every other calculation you do.
What it does to a bank
Take a modest off-grid load of 1 kWh a day and ask for two days of reserve.
On lithium at 80%, the bank needs 2,500 Wh, which is about 208 Ah at 12 V. On lead acid at 50%, the same job needs 4,000 Wh, which is 333 Ah at 12 V. Same loads, same autonomy, 60% more battery, and lead acid banks are heavier per amp hour to begin with.
That is before cycle life enters it. Lead acid gives a few hundred cycles at its rated depth where lithium iron phosphate gives thousands, so the cheaper bank is also the one being replaced sooner. The solar battery calculator applies the right depth of discharge for the chemistry you pick and reports amp hours at 12, 24 and 48 V, which is the comparison that actually matters when a supplier quotes one voltage and your inverter wants another.
Amp hours are not a unit of energy
This is the trap behind most mismatched comparisons. An amp hour only means something once you know the voltage it sits at.
A 100 Ah battery at 12 V holds 1.2 kWh. A 100 Ah battery at 48 V holds 4.8 kWh. Four times the energy, same headline number, and they are routinely listed next to each other in search results. Convert to watt hours before comparing anything, every time.
The second deduction nobody budgets for
Depth of discharge is the deduction people eventually learn about. Round trip efficiency is the one they usually miss.
Energy is lost going into the battery and coming back out, and more is lost in the inverter converting DC to AC. Lead acid round trips at roughly 80 to 85%, lithium iron phosphate at 90 to 95%, and a decent inverter adds its own loss on top. Put 1 kWh of solar in and you do not get 1 kWh out at the socket.
Sizing that ignores this comes up short in exactly the conditions you built the system for, which is a run of dull days when nothing is being replaced.
Discharge rate matters more on lead acid
Lead acid has one more quirk worth knowing. Its rated capacity assumes a slow discharge, usually over 20 hours. Pull it faster and the available capacity falls, which is Peukert’s law.
A kettle or a well pump on a lead acid bank will take more out of it than the arithmetic suggests. Lithium is far flatter in this respect, which is part of why it suits spiky loads. The same effect governs how long a UPS really lasts under load, and the UPS runtime calculator models it directly.
What this means at the shop
Compare banks on usable watt hours, not on nameplate amp hours. Usable watt hours is nameplate amp hours, times voltage, times depth of discharge, and it is the only figure that describes what you will actually be able to use.
Do that and the price comparison changes shape. Lithium iron phosphate usually costs more per nameplate amp hour and less per usable kilowatt hour over its life, which is the comparison worth making, as long as you are honest that the cheaper option is also the one you will be lifting out of the cupboard again in four years.
Questions people ask
What happens if I go below the recommended depth of discharge?
Nothing dramatic on the day. The cost is cycle life, and it compounds. Lead acid taken to 80% regularly can lose most of its expected life, which is why the 50% figure is treated as a design limit rather than a warning line.
Are amp hours comparable between voltages?
No, and this is a common and expensive mistake. 100 Ah at 12 V is 1.2 kWh while 100 Ah at 48 V is 4.8 kWh. Compare watt hours, which is amp hours times volts, whenever two banks run at different voltages.
Does cold weather change usable capacity?
Considerably. Lead acid loses a large share of its capacity near freezing, and lithium iron phosphate cannot be charged below freezing at all without a heater, though it will still discharge. An unheated outbuilding is a real design constraint.
Is a bigger bank always safer?
Up to a point. An oversized bank that never cycles deeply is kind to the batteries but expensive, and on lead acid a bank that is chronically undercharged because the array cannot fill it will sulphate and fail early.