Re: Back of an envelope calculations: Perpetuity
"then you can virtually cycle the thing in perpetuity"
No, you can't.
Lithium cells store best at about 66% charge.
They start degrading with time as soon as made even if not discharged. For off grid you need to store daylight energy for night time. So cycling is daily and perhaps 1/3rd discharge for new batteries and after a year or two that might be 50% of capacity as capacity reduces with use (and time for Lithium Ion).
I recently charged a New Old Stock 40 year old Lead Acid cell. It's fine because it was never filled with acid. Sadly today shops store Lead Acid batteries pre-filled, so check the date code!
There are really two main kinds of Lead Acid Gel cells. The UPS/ Alarm / Emergency light float applications and the regular cycling (portable lamps and inverters) were it's discharged daily.
They are more for standby or portable use at lower AH than 40AH
Gel types are poor at high pulse currents compared to wet,
There are several flavours of wet lead acid:
High pulse current (for starter motors). These don't like deep cycle. Typically 30AH to 200AH and 12V.
Industrial UPS standby. There may be a generator too. Much higher power than single PC / Server UPS.
Regular deep cycle (i.e. night time on off grid solar). Typically sold as 2V 200AH or larger capacity.
Only units subject to movement / vibration need fibreglass mat spacers.
They have different plate constructions. Life is shorter on all types if the battery is completely discharged. But this is perhaps more true of Lithium Ion. The NiMH in contrast can be stored discharged (better actually!) and don't have a problem with full discharge.
All types technology reduce in capacity with number of cycles, so for off grid night time use double the kWH capacity needed.