Back when I used my old RM tablet at school , I had 2 high capacity batteries and they lasted me the whole 8 hours of use just fine , with another half a battery of use left and no charging required. That was on a circa 2003 tablet with a LOT of mileage on (it was an ex-primary school tablet so it had be used a LOT by children below 11).
Now that was back then and batteries have come a long way since so I don't doubt that a netbook of similar high quality will get a lot of usage. That tablet even had a Wacom screen which is well known as using a LOT of power. It would also be BOILING on the back during the day and would make condensation marks on cold days
. It also used an old era processor which had horrible efficiency. Nowadays processors use much less charge.
This was it here:
RM do offer the Slate nowadays but it is generally only to school contracts but they do offer to private individuals too. If you can get a hold of a Slate with high capacity batteries you are done here really. My school network administrator also handled their contract with RM and he attended the expos they held to show off their new stuff and he said the Slate was fantastic. I do not doubt their new offerings either , for the price , they are top quality machines , built for frequent use. The batteries , even the high capacity ones , were small for their useage (mine was about the size of a spine on an A4 binder in height and about 10cm or so wide so long and fairly thin.
You could have a few of those high cap batteries with the off device charger (which you could get for £10 with my tablet) , and you could charge all the batteries from one mains supply in about 2 hours so you could get weeks of use from a 2 hour charge if you had a few of them since I doubt you use the laptop at all times.