The instructions for these things should give you a good idea of what kind of performance you'll get depending on what you've got plugged into it. I don't think it's going to run a 300 watt heater for very long with 40 Ah battery capacity and the inverter running at close to max power. Add a Powerhead to the mix and you're really pushing the envelope. So I guess it comes down to how long the power outage will be & the total wattage of items you decide to plug into it.
At some point I plan to get some deep cycle batteries and a good inverter as backup power. Also planning on getting some solar panels to charge the batteries. As my budget allows, I'd like to build up a larger solar array with battery bank to run items in my house permanently. Transfer switches are available as well, to automate things. With LED lighting on the verge of taking off & replacing the CFLs everyone's been installing, the plan is to run most of the house lighting with LEDs directly from a DC voltage source, the bank of batteries. No inverter required to run LEDs if you do it right and since they use even less power than CFLs, a properly sized battery bank charged by a solar array should take care of the whole house, perhaps even the tank as far as lighting goes.
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