DC/AC power converters (sometimes called "inverters") are inexpensive and flexible devices often overlooked by reef hobbyist. Converters that will power 400 W (watts) of devices can typically be had for $35 on eBay.com web site. These devices can be hooked up to an extra car battery or better yet marine batteries used on boats or even directly to a car via a cigarette lighter port if you can park within an extension cord distance. These are ideal for running a heater and a powerhead or two. It can also be used for recharging a mid-sized UPS unit if you cant get the car close enough to the tank. DC/AC power converters are really your only cost effective way for running a heater. Typical aquarium sized heaters in the 100-300 W range will drain just about any UPS unit in a matter of minutes. Obviously, a generator would be ideal but they are priced beyond the reach of many hobbyists and tend to be in short supply when you really need one. If you do hookup a DC/AC power converter to a running car be smart about it: put the parking break on, do not run the car in the garage (deadly fumes) and make sure nobody drives off with your car.
This unit runs cool to the touch, is very quiet, can easily be converted from cigarette lighter to battery terminal feed. It has a 20 A (ampere) fuse with two power outlets and a master on/off switch. The unit weighs less than a pound. Im really impressed with it.
This is taken from "Disaster Readiness" by Richard Durso, published in Advanced Aquarist, August 2002.