5 miles?!?!?!? Hell Yeah the grid charger will give you wicked mpg even without lean burn. Since it wouldnt draw much power you could power it with an inverter and deep cycle battery to charge while at work.
I have access to electricity at work at 4 days a week now, unless the generator dies at the other site, then I would only have power 3 days.
I don't think there's a good way to tell how full the Insight's battery is. Even the computer can't tell, so one problem you'll have by topping it off at night/at work is that when you get in it to drive, the computer will be under the assumption that it's at the same capacity as when it was parked and will charge/assist based on its assumed battery level. I'm considering trying it anyway, by putting my grid charger on a timer and only adding a couple of hours of charge.
SOC is somewhat subjective. One way to guesstimate it is by voltage. For the gen 2 109 volts is like 90% 117 is 100% according to the cars SOC meter. Full balance is like 122 volts. So you set the charger to 109 so it stops at that point.
The problem is when you pass that. Granted 122 is absolutely full, the car never reaches that as it only uses x % of the battery. This is why when guys go a maintenance charge it wont register properly til it does a recal.
Plan B is to never fill up the battery or let it do a recal.