| First off - get a tin of white paint and paint the inside surfaces.
(don't bother using mirrors or silver foil - unless you want specific spot enhancement)
lamps - as all have said - fluorescents will usually give better consumption depending on wattage.
to work out your average time on a halogen,:
20watt 12 volt is 1.7A
given a 13Ahr battery you should have less than 7.5hours worth of light.
Also, your halogens don't take kindly to under running on voltage so you'll probably blacken the lamps if you don't overrun them to 12.8volts+ every so often.
There is an alternative in LED lighting. You'll find em in a few electrical distributors marketed as garden lighting - the newer ones have an LED cluster giving a fair wack of light where you need it, for low running cost and little heat - but can cost between 14 and 39 squid a piece.
OR....buy a wind up torch? | 
Threaded Mode