Battery Lifetime Cost
The cheaper battery is often the more expensive one. Put two side by side and see which actually costs less over its whole life.
What this works out
The price on the invoice tells you very little. A battery that costs half as much but wears out in a third of the time is the expensive one.
So this ignores the purchase price and asks a different question: across everything this battery will ever give you, what does each unit of electricity end up costing? That is the figure the two columns produce, and the lower one wins.
Nothing is assumed. Every number below is one you type in, and the figures already filled in are placeholders to show the shape of it. Change them to your own. The currency is whatever you are thinking in, since the answer is a ratio.
How the sum works
Over its whole life, each kWh of battery gives you cycles × how much you use kWh of electricity. So the purchase cost of each one you get back is price ÷ that number. A battery costing twice as much that lasts four times as long is half the price per unit.
Losses are deliberately not applied to that figure. Efficiency does not change how much the battery gives out. It changes how much you have to put in to fill it. So it belongs in the charging cost, at electricity price ÷ efficiency, and nowhere else. Counting it twice is the commonest mistake in these sums, and it always flatters the more efficient battery.
Upkeep is spread across the life, at price × rate × years, where years is cycles ÷ uses per year. How often you use it does not change what a unit costs on its own. It changes how long the battery lasts, and therefore how many years of upkeep get paid before it is retired.
What this leaves out. The cost of borrowing, the way capacity fades before the end, what the battery is worth as scrap, having to replace a shorter-lived one halfway through a project, and any difference in what each needs by way of installation, cooling or fire protection. Any of those can change the answer. This is the arithmetic everyone can agree on, not a substitute for a proper project model.
This tool contains no price data. Every figure in it is one you have entered, and the numbers filled in by default are illustrative placeholders rather than published figures for any product, chemistry or supplier. The Battery Ledger does not publish, hold or redistribute commodity, cell or system price data. Results are arithmetic on your own assumptions and are not financial, investment or engineering advice. Check any figure independently before relying on it commercially. To report an error in the method, email editor@thebatteryledger.com.