Heads up: this tool is intended to simplify my life when helping people with power setups, not to be Holy Gospel. Do your own homework.

The model makes several assumptions:

• Lithium can be discharged to 80% DoD, lead chemistries to 50%
• it uses nominal voltages (12.8v, 12.0v, etc)
• it assumes LiFePO4 rather than Li-NMC, so use Watt-hours for sizing given the option
• it uses PVwatts for insolation averages (hours of full sun equivalent by month
• assumes a steady level of current from alternator, so for short drives use rated current for DC-DC and 0.2C for direct chargining. For longer drives use maybe half of that (0.1C)

the spreadsheet is online in Google Docs. Copy/download if you want to play with it. Areas that are editable are in green.

I don't intend to make it particularly friendly, since I am the intended audience.