or maybe the timer switch into the inverters on/off rocker switch? it would be nice to be able to estimate power consumption, i mean, for on/off to have meaning again!
I don't know if it's already been addressed, but make sure your timer is on the battery side, and not the inverter output (110v) side. The inverter has a not-unsubstantial power draw even when it's not hooked up to a load. Most of these timers are mechanical switches, and don't care about the voltage being switched.