Two batteries with the same list price can end up costing twice as much as each other by the end of their lives. It is not a paradox: it is what happens when you compare the purchase price instead of the cost of the energy that that battery will actually be able to store and deliver over time.

It is the comparison that installers find themselves making every time they present a quote, often in front of a client who only looks at the bottom line. Here we try to list the four numbers that really matter, and at the bottom you will find a calculator to do it for your own.

The list price does not state how much energy costs

You don't buy a battery to own it: you buy it for the amount of energy that will pass through it before it stops being useful. The correct metric, therefore, is not the euro per kilowatt-hour nominal, but the euro per kilowatt-hour actually cycled over the system’s lifetime.

To work this out, you need four pieces of data, which any reputable technical specification sheet will provide.

1. The rated capacity

The stated kilowatt-hours. It’s the figure everyone looks at, and it’s also the least informative of the four, because on its own it doesn’t tell you how many of those kilowatt-hours are actually usable.

2. Depth of Discharge (DoD)

The percentage of capacity that can actually be used without damaging the cells. A 10 kWh battery with a DoD of 80% provides 8 kWh per cycle, not 10. This is the difference between the rated capacity and the available capacity, and when comparing two products, the 20% may represent the actual value.

3. The number of cycles

How many full charge and discharge cycles can the battery withstand before reaching the end of its useful life? Please note: A number of cycles without the residual capacity to which it refers is meaningless. Saying “6,000 cycles” is like saying “it lasts a long time”: the right question is 6,000 cycles – up to what percentage of remaining capacity?

4. Residual capacity at the end of its service life

The percentage remaining once the stated number of cycles has been used up. This serves two purposes: to make the number of cycles comparable, and to estimate the average energy actually available over the system’s entire lifetime, which lies halfway between the initial and final capacities.

The formula

Taken together, these four figures give the total energy that the battery will cycle over its lifetime:

Total energy = capacity × DoD × cycles × average degradation
where the average degradation is the average of 100% and the declared residual capacity.

And that brings us to the key figure:

Cost per cycled kWh = price ÷ total energy

It’s a calculation that takes just thirty seconds and turns quite a few comparisons on their head. Here’s the calculator: enter the details of the battery you’re considering on the left, and compare them with those of ours on the right.

What does an installer look for, apart from the number?

The cost per kilowatt-hour, taking peak and off-peak rates into account, is the correct way to compare two products, but it does not tell the whole story. There are three factors that are not included in the formula and which carry as much weight – or even more – than that figure.

Safety. It’s the first question end customers ask when they’re told that the battery is installed in the house or garage. Sodium-based chemistry doesn’t use lithium, cobalt or nickel, and that changes the conversation.

Support. A battery that lasts twenty years only makes sense if the manufacturer answers the phone for twenty years. It’s worth asking where the customer service centre is and who runs it.

The guarantee. It’s not the stated lifespan, but what exactly it covers and who honours it: the manufacturer, the importer or the distributor.

In summary

Comparing two storage systems based on their list price is like comparing two cars based on the cost of a full tank of petrol. The figure to put on the table, in front of the customer, is how much each kilowatt-hour that battery will deliver over its lifetime costs. In most cases, it is also the figure that makes an option that appears more expensive actually the better value for money of the two.

Are you an installer and would you like the confidential price list or a comparison of your figures? Please contact us via the installers’ section.