Everything in this article assumes you have already assessed your daily power requirements (DPR). The article will stipulate a DPR of 1,000Wh (1kWh).

This 1,000Wh will be the base of all our calculations below.

We will assume lithium = the common LiFePO4 and not any other form of lithium chemistry.

• lead-chemistry batteries (FLA, AGM, GEL) are typically nominal 12v and discharged to 50% Depth of Discharge. A 100Ah lead battery holds ~1,200Wh (100Ah x 12v), and one discharged to 50% holds 600Wh (1,200Wh x 0.5). It would take 166Ah of lead battery to give us 1,000Wh usable.
• lithium batteries are typically nominal 12.8v and discharged to 80% DoD. A 100Ah LFP battery holds 1,280Wh, of which 1,024Wh are usable (1,280Wh x 0.8). It would take 98Ah of LFP to give us 1,000Wh usable.

'Dwellers contemplating large currents relative to capacity (“C rates”) might have to oversize the bank to get sufficient throughput, and/or choose a chemistry with lower resistance. FLA are famously stingy with current, AGM good, and Lithium excellent.

The simplest model assumes the bank will be charged then all the loads will be run from the bank.

In real life sometimes loads are running while the system is making power.

• running a 400w when solar is producing 200w means a net load of 200w.
• running a 100w while alternator charging is producing may mean a net load of 0w (a freebie, in terms of battery capacity)

You may want to add a section to your DPR spreadsheet to account for load use while solar is present, etc. In extreme cases only a very small bank may be required if most loads are effectively run off the panel.

Finally we get to size something! We will divide the daily power requirement by the maximum depth of discharge preferable for each battery chemistry to get required battery capacity in Wh. Deep cycle batteries are traditionally rated in Ah (amp-hours), so we will divide Wh capacity by the battery chemistry's nominal voltage. See below.

Lead is typically discharged to 50% DoD as a good balance between performance and longevity.

2,084Wh / 0.50 DoD = 4,168Wh
4,168Wh / 12.0v nominal voltage = 347Ah of lead-chemistry required.

Also see the section on minimum charging current below.

Lithium is typically discharged to 80% DoD as a good balance between performance and longevity.

2,084Wh / 0.80 DoD = 2,605Wh
2,605Wh / 12.8v nominal voltage = 204Ah of lithium required.