How this is calculated
Cost per stored kilowatt-hour is the purchase price divided by every kilowatt-hour the bank will ever hand back: nameplate kWh × depth of discharge × cycles × round-trip efficiency. Efficiency belongs in there because heat losses are energy you paid for and never saw.
Worked example
A 10 kWh lithium iron phosphate bank at 400 per nameplate kWh costs 4,000. Taking 80 percent out gives 8.0 kWh a cycle; Victron rates 2,500 cycles at that depth; round trip is 92 percent. So 8.0 × 2,500 × 0.92 = 18,400 kWh delivered, and 4,000 ÷ 18,400 = 0.22 per stored kWh.
Hold the price still and switch to AGM: 5.0 kWh a cycle at 50 percent depth, 600 cycles, 80 percent round trip. That is 2,400 kWh, or 1.67 per stored kWh — same money in, seven and a half times the cost per unit of work.
Use the cycle count published at the depth you actually run, and price against nameplate capacity, since that is what you are billed for. Size the bank first and pick chemistry second — the off-grid battery bank calculator returns the nameplate kilowatt-hours for the box above.
Quick answers
Which battery chemistry lasts the longest?
Of those with published home-scale datasheets, lithium iron phosphate: 2,500 cycles at 80 percent depth against 400 for AGM, both from Victron.
How do you compare cycle life fairly?
Read the depth attached to it. Victron’s AGM Deep Cycle is rated 400 cycles at 80 percent, 600 at 50 and 1,500 at 30. One battery, three headline figures.
What is round-trip efficiency and why does it matter?
The share of stored energy you get back. Victron publishes 92 percent for LiFePO4 and 80 percent for average lead-acid, dropping to 50 percent or worse above 80 percent state of charge.
Can any of these charge below freezing?
No lithium cell should. Battery University gives 0 to 45 °C (32 to 113 °F) as the permitted charge range, and charging below freezing plates metallic lithium onto the anode. Lead-acid has no cut-off, only lost capacity in the cold.
Is sodium-ion ready for a home bank?
Not on the published record. The International Energy Agency put sodium-ion at under one percent of global lithium-ion production in 2025, and the striking figures in circulation come from CATL’s Naxtra announcement.
What a cycle count is really telling you
Every cycle figure is a laboratory result with a test condition attached, and the conditions differ. EVE’s LF280K cell is rated 6,000 cycles, but that test runs it from 3.65 V to 2.50 V at 0.5C under a 300 kgf clamp at 25 °C. Victron’s 2,500 cycles is the same chemistry at 80 percent depth in a finished battery.
Heat shortens all of them. EVE’s cell drops from 6,000 cycles at 25 °C to 2,500 at 45 °C, and Victron gives its AGM range 7 to 10 years of float life at 20 °C, 4 at 30 °C and 2 at 40 °C.
Temperature is where the chemistries genuinely split
Lead-acid takes a charge at any temperature it survives, giving less back as it cools. Nothing breaks.
Lithium is the opposite. Push current into a cell below 32 °F and metallic lithium plates onto the anode, permanently and silently, which is why Victron sets its charge floor at 41 °F while the bare EVE cell allows 32 °F. Discharging stays permitted down to −4 °F.
Lithium is the opposite. Push current into a cell below 0 °C and metallic lithium plates onto the anode, permanently and silently, which is why Victron sets its charge floor at +5 °C while the bare EVE cell allows 0 °C. Discharging stays permitted down to −20 °C.
Sodium-ion is the outlier worth watching: CATL claims 90 percent usable capacity at minus 40, the one reading where Celsius and Fahrenheit agree, for its Naxtra cell announced in April 2025. If independent testing confirms it, cold unheated buildings get a different answer.
An unheated shed at 20 °F will refuse a lithium charge on a bright morning with the array producing. Heated enclosures and self-heating batteries fix it, and both cost energy.
An unheated outbuilding at −7 °C will refuse a lithium charge on a bright morning with the array producing. Heated enclosures and self-heating batteries fix it, and both cost energy.
Weight, space and fire behavior
A Victron 12.8 V 200 Ah lithium battery is 44 lb for 2.56 kWh; a Victron AGM 12 V 220 Ah is 143 lb for 2.64 kWh, half of it off limits day to day.
A Victron 12.8 V 200 Ah lithium battery is 20 kg for 2.56 kWh; a Victron AGM 12 V 220 Ah is 65 kg for 2.64 kWh, half of it off limits day to day.
Fire behavior separates the two lithium rows. Battery University puts thermal runaway onset at 270 °C for lithium iron phosphate and 210 °C for NMC, which is most of the reason fixed home storage moved to iron phosphate while phones and cars kept the nickel blends. The trade is set out in the LiFePO4 against NMC comparison. Lead-acid has no cell-level runaway and a different hazard instead: hydrogen given off while charging, which ventilation solves.
In the United States a residential unit is expected to be listed to UL 9540, with the battery under UL 1973. NFPA 855’s residential chapter covers 1 to 20 kWh per unit; above that it counts as commercial. The same chapter caps the total in a utility or storage space at 40 kWh, and 80 kWh in a garage or detached structure. Wiring falls under Article 706 of the National Electrical Code.
In Europe the equivalents are IEC 62619 for industrial lithium cells and IEC 61427 for solar cycling duty, which Victron’s OPzS Solar range is built to. Ventilation for a vented lead-acid bank follows EN IEC 62485-2, which sets the hydrogen limit and the airflow to hold it.
What each one is actually good at
| Chemistry | Best use | Poor fit |
|---|---|---|
| LiFePO4 | Daily-cycled off-grid and backup banks, vehicles, anywhere weight or space is tight | Unheated buildings with no charge-temperature protection |
| NMC | Weight-critical portable gear and vehicles, packaged home units with active thermal management | Owner-built fixed banks, where the lower runaway threshold buys nothing |
| AGM lead-acid | Standby and rarely-cycled backup, engine starting, sealed installations that must not be watered | Anything cycled daily, where 400 cycles arrives quickly |
| Flooded lead-acid | Large stationary banks where weight is free and someone will do the watering | Sealed rooms, boats, vehicles, absent owners |
| Sodium-ion | Watch it. Very cold sites are the obvious opening | Any bank you have to specify and buy this year |
AGM is a different job rather than a weaker lithium: standby duty that cycles a few times a year never reaches 400 cycles, and there its low price and indifference to cold charging win.
What the market says a kilowatt-hour costs
Pack prices are surveyed; retail off-grid prices are not. These are the floor beneath your quote.
| Benchmark | Figure | Source and date |
|---|---|---|
| Lithium-ion pack, global average, all uses | 108 per kWh | BloombergNEF annual battery price survey, published 9 December 2025 |
| LFP pack, all segments | 81 per kWh | Same survey |
| NMC pack, all segments | 128 per kWh | Same survey |
| Packs going into stationary storage | 70 per kWh | Same survey |
EnergySage’s 22nd Marketplace Report, covering July to December 2025, put installed home storage at 1,074 dollars per kWh, up 3.6 percent on the previous six months, with tariffs on imported cells named as the cause. The gap from 81 to 1,074 is enclosure, inverter, wiring, permits and labor, none of which changes with chemistry.
No European marketplace survey publishes an equivalent installed figure, so treat the pack prices as a floor in euro terms and take quotes for enclosure, inverter, wiring and labor. That part barely moves with chemistry.
Which is the argument for comparing on lifetime cost: a chemistry replaced twice inside a decade buys the labor twice as well.
Sources
Victron Energy datasheets: Lithium Battery Smart, Gel and AGM Batteries (Table 2 on service life against temperature) and OPzS Solar. EVE Power LF280K cell specification, 23 March 2021. Tesla Powerwall 2 datasheet, North America edition. Battery University BU-205, BU-410 and BU-802b. CATL’s Naxtra announcement of 21 April 2025, treated throughout as a maker claim, and International Energy Agency commentary putting 2025 sodium-ion output below 1 percent of lithium-ion. BloombergNEF annual battery price survey, 9 December 2025. EnergySage 22nd Marketplace Report, second half of 2025. NFPA 855 residential chapter. Figures checked 2 September 2026.
Go deeper
Turn a chemistry choice into a bank size with the off-grid battery bank calculator. The LiFePO4 against lead-acid sizing guide works the same trade over a decade, and the LiFePO4 voltage chart covers what these packs read on a meter. The rest is in the storage section.