Battery longevity or drain or usage time calculation for 12v 10W solar panel

Chargin Time with no losses:

Consider that we have a 12v, 10W solar panel

So amp = 10/12 = 0.833A
So it 0.8333A/h
So to charge a 12v, 7Ah battery it will take 8.4hrs


Longevity or Drain or Usage time with 100W bulb / load

If we connect a 100W input with no losses the current is 100W/12V = 8.33A

This means you need 8.33Ah capacity for each hour of running.

WIth 7Ah battery fully charged 8.33A/7Ah = 0.84 hours
So the battery can be used to run the 100W load for 0.84 hrs

In practice the losses due to the inverter, heat losses and PF must be added as part of the load. The losses can be from few watts to 150W losses.