Work out what size leisure battery you need

No. The arithmetic runs inside your browser on the published dataset it downloads. There is no form submission, no request carrying your numbers and no analytics on them. Nothing you type is stored or sent by this site.

Lead-acid usable capacity = nameplate x 50% depth of discharge. The 50% figure is the depth of discharge the NCC Verified Leisure Battery Scheme uses to rate every class of leisure battery, and Victron describes it as 'generally accepted'. It is an industry convention with a UK scheme's test basis behind it, not a standard. Victron’s wording: “NCC Verified leisure batteries are tested by the manufacturer and verified by the NCC taking into account their minimum capacity minimum and their minimum life cycles at 50% depth of discharge, into 1 of 3 classes.” (checked 16 September 2026).

LiFePO4 usable capacity = nameplate x 80% depth of discharge as the economic working figure. Victron states 80% DOD is used for LFP, and its LFP Smart datasheet publishes cycle life at 80%, 70% and 50% DoD (2500 / 3000 / 5000 cycles to 80% of nominal capacity). Deeper discharge is survivable but costs cycles. Source: “For Lithium-iron-phosphate (LiFePO4 or LFP) which is the safest of the mainstream Li-ion battery types, 80% DOD is used.” (Victron Energy blog: Batteries: Lithium-ion vs AGM (John Rushworth), checked 16 September 2026).

Because the number on the box is often not the 20-hour figure. Banner publishes its 957 51 as 110 Ah at the 100-hour rate, 100 Ah at the 20-hour rate and 75 Ah at the 5-hour rate (Banner Technical Data Sheet – Energy Bull 957 51 (Document DBTBAT00339), checked 16 September 2026). The advertised 100 Ah is 0% above the 20-hour figure. Stating the rate makes batteries comparable; 5 of the 19 indexed batteries publish more than one rate.

No manufacturer in the index publishes a duty cycle, so the checker never assumes one. Where a maker publishes an average, such as Dometic’s 1.1 Ah/h for the CRX 50 at +25 °C ambient, the average is used as published. Thetford is the only maker in the index that names a test standard for its daily figure: “IEC62552-2020 part 3”. Where only a rated (running) current is published, the line is shown as an upper bound and labelled.

Dometic's CRX 50 installation and operating manual publishes 'Rated current 12 V⎓: 5.0 A'. That is the current while the compressor runs, about 4.5 times the catalogue average of 1.1 Ah/h. This checker uses the catalogue average and never lets the 5.0 A figure reach the daily total. Calculators seeded from the manual will tell you to buy a bank roughly four times too big.

No, because no manufacturer publishes it as a number. No manufacturer publishes a numeric lead-acid temperature derate table in the documents read on 2026-09-16 (Victron Gel/AGM datasheet section 8 and GS Yuasa NP Application Manual Figure 7 are graphs only). Brief rule 8: never read a number off a graph. The checker reports the direction of the effect and says the figure is not published.

Where a lead-acid battery publishes only one capacity rating the checker uses Victron's default Peukert exponent of 1.25 and labels it as the manufacturer's approximate default for 'most lead acid batteries', never as this battery's figure. Victron's own caveat that the exponent is 'no more than a rough approximation of reality' is carried with it. Victron: “The default setting in the battery monitor for the Peukert exponent is 1.25. This is an acceptable average value for most lead acid batteries.” (checked 16 September 2026). Where a battery publishes two capacity ratings the checker derives the battery’s own exponent from them instead, and says so.