My 21 cuft refer draws 8 amps 12V DC so 100 watts. You can figure 4X start up amps so 500 PSW would be enough. I went 1000W so I could power a Sat receiver, DVR, and LED TV in addition to the refer.
I bought them from a battery dealer in Atlanta as seconds... NO Brand. The first ones I bought were only $50 and I tested them by seeing how long they would hold a charge above 12.9V... six months. The latest price was actually $70 each and they weigh 60+ lbs.
*I just went and checked the voltage on two that have been sitting for almost a year and they were both 12.47V so I need to charge them.