"Is lithium worth it?" is usually asked as a price question and answered as a capability question. Both answers are legitimate, but they are different calculations, and mixing them produces the confident nonsense you see in forum threads.
This page handles the narrow version: on replacement cycles alone, how long does the extra cost take to come back? It deliberately ignores everything lithium does better, because those benefits are worth different amounts to different owners and cannot be reduced to a single number.
Why this site does not pre-fill the prices
This site does not sell batteries and publishes no prices of its own. Battery pricing in Australia moves, varies by importer and channel, and is quoted differently depending on whether installation is included. A price printed here would be wrong within months and would be treated as authoritative anyway.
So the calculator starts with placeholder values that you should replace with figures from your own quotes and the manufacturer's own datasheet. The arithmetic is the useful part, not the defaults.
Compare usable energy, not nominal capacity
The most common error in this comparison is comparing nominal amp-hours. A lead-acid bank is conventionally sized so that a typical discharge uses a modest share of its rated capacity, because deep discharges shorten its life sharply. A LiFePO4 bank is normally specified to use much more of its rated capacity.
The consequence is that a lead-acid bank delivering the same usable energy is physically larger, heavier and more expensive than its nominal rating suggests. If you compare a 400Ah lead-acid bank against a 400Ah lithium bank, you are not comparing like with like, and the comparison flatters lead-acid.
Work out the usable energy you need first. Then price the two banks that deliver it.
Compare cycle life at the same depth of discharge
Cycle-life claims are only comparable when they are measured the same way. A figure quoted at 50% depth of discharge is not comparable with one quoted at 80%, and neither is comparable with one measured at a different temperature or discharge rate.
Take both numbers from the manufacturer's published datasheet, at the same stated depth of discharge, or do not use the comparison at all.
What this calculation leaves out
Everything below can matter more than the payback figure, and none of it is included:
- Weight. A lithium bank of the same usable energy is substantially lighter. On a performance yacht or a trailer boat this can be the whole argument.
- Charge acceptance. Lithium accepts high current for most of the cycle rather than tapering early, which shortens generator runtime and makes alternator and solar charging more productive.
- Voltage stability. A flatter discharge curve means inverter loads and electronics see a more consistent supply.
- The cost of the rest of the system. Lithium may require a DC-DC charger, alternator protection, a different shore charger, new cabling or BMS integration. If your quote includes those, include them on the lithium side of this calculation.
- Installation. A conversion is rarely just batteries.
Do not use this alone
A payback figure is one input into a decision that also involves safety, compliance, insurance, weight, space and how you actually use the boat. If the payback number is the only reason to change, the honest answer is often to keep the lead-acid bank until it dies.