Size generator (resolved)

What size generator would I need to keep a tank running during a power outage (or how do I calculate)? I have a 65g using about 500 watts. They seem to be rated for a running watts amount?

2 ecotech radions so about 300 watts
2 ecotech mp10s so about 36 watts
3 koralias nano 425 power heads so about 11 watts
12w skimmer pump
3 small pumps for reactors cobalt mj 900 so about 36 watts
50 watt heater
55 watt return pump
2 Brs dosing pumps negligible watts

Total 500 watts (if that even helps, sorry I'm a CPA not an electrician)

I'm not looking for a generator to live through the apocalypse or run anything else in my house, I'm just wanting a "oh crap power is going to be out for 6 hours to a few days" and I want to run just my tank (I can go elsewhere for living but my tank can't). Can anyone tell me a size range of a generator aka minimum not maximum that would work and save me a Saturday of google research? Thanks!

Edited by Bryce - February 08 2014 at 10:11am

just running 2 tanks listed, and that equipment, if you are wanting to ONLY run your tanks, almost any generator made will do that.  You will need to have a power cord long enough to run to only the tank with the generator outside.

Just to make the math simple every 100 watts = about 1 amp.  The rating on a circuit is 80% of the breaker rating , so on a 20 amp breaker 16 amps is about the max you want to have on the circuit  to prevent it from tripping when there is a peak in the load . Peak in the load could be a microwave, hair dryer etc turning on as it increases the amp load for a few seconds on start up. So you have 500w you want to run, a 600w would be the smallest you should run without any issues  .  you could almost run an inverter off of your car for that small of an amount. Hope that kind helps with the rating system. 

50 watt heater seems pretty weak. If your home goes with out power during the winter I think the heater wouldn't maintain. Also consider running dual heaters in case one fails. Just my 2 cents.