daestrom wrote:

Yep. I think you have the basic idea, but in this stuff you have to be
careful to dot your 'i's and cross your 't's. (this group is good at
pointing out mix ups like this). It may seem rather 'anal' at times, but it
*is* important to get the right results ;-)

I agree...

If you're drawing 144 watt-hours *out* of your battery, how many watt-hours
do you have to put in? More, unless you've found the perfect battery.
Assume 80% of the energy you put into the battery gets lost, so to get 144
watt-hours *out*, you should plan on putting 144/0.80 = 180 watt-hours *in*.

I know what you're thinking and you'd get it right. But you worded it
backwards.

Perhaps something like the following is what you meant:
"Assume the battery storage is 80% efficient, 20% of the energy you
put into the battery gets lost"