I and a few others went with a group 51 battery. Its a tight fit, but it works well. Some have opted for the optima or a lawn mower battery. Id like a deep cycle battery, but couldn't wait when mines died. Make sure to pre charge it before installation.
So, 1500 watts? Is that peak or RMS?  Better get some 0 gauge wire, a cap if not a second battery if so.
As far as mpg it depends on your current battery to electrical system needs. If you battery is too small and the charging system is always engaged, it hurts your mpg. Then if you switch it out to a larger battery that is pre charged, it helps as your charging system isn't used to power your electrical crap vs actually being stored in the battery for use later.
Sent from my Autoguide iPod touch app
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Enginer 4 kilowatt PHEV, 3000k 35 watt fogs, Eco bulb highs, 4300k 35 watt low all w/relay kits, DRLs/Rear Wiper removed&rear interior gutted, Sony HU W/front speakers, Tanabe nf springs, 35% tint all around, all LED lamp replacement, 09 fit progress rear sway bar, OEM block heater, full gril block, KN Filter, Honda vent visiors, group 51 battery, home made balancer/grid charger Best/Worse MPG 96/36
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