I use a tripp-lite inverter, one that is similar to this
Tripp-lite inverter
It's an inverter and at the same time a charger for a car battery. I hook up a deep cycle battery to it and run 2 Tunze DC powerheads to it. It also powers my Aqua controller and my router/cable box. It'll run for about 4 hours. I choose to use this inverter because I could switch out the battery if it stops working and it out perform a regular UPC when it comes to efficiency. However the inverter + battery did cost a little over $450 but it is very reliable.
Steven