odenwell said:
I'm sure it's been mentioned before, but modified sine wave inverters are not real good at running inductive loads like a motor. You usually get a lot of chatter and little, if any, water movement. The best inverters for this purpose are true sine wave, but they can be a little pricey.
Actually, IME the modified sine-wave pumps are pretty darn good. Most you can use with the inductive loads used in the hobby without much problem. They claim you can use them on TV and Hi-Fi's w/o interference, but I don't 100% believe that claim. I wouldn't use them to power expensive electronic equipement (i.e., that Pentium 4 computer), but STILL they probably make them a lot better now-a-days. I have a Portawattz, and it's given me no problems with 3 Maxi-jets1200's, a Mag9, an airpump, and heater -- a pretty big load.
Calculate your load by adding up how many watts each device consumes (usually listed on the back as Watts... Remember that [Power = Current * Voltage]... so if just current (Amps) is listed then you can still calculate your Power = Amps listed * 120VAC). Then buy an inverter that can handle 1.5 times your load. Seriously, modified sine-wave inverters aren't that expensive, so buy one that's over-rated for power.
Remember numbers you computed are just for everything once it's running. The START-UP power requirement is a lot more. This is because inductive loads draw a lot of "in-rushing" current when they first start up. Electrons rush into the motor's coil to energize it, and it's not until the coil starts turning that electrical resistance builds up across the coil to reduce current. Because the initial current is so high, the initial power drawn by the motor/pump is high [Power = Current * Voltage]. This initial power draw is called "surge power." So, be sure to by an inverter that's not only rated for 1.5 times the continuous operating power you calculated, but also is able to provide high surge current [Most of the time this will automatically be the case... but, you can see the rationale for buying 1.5 times what you need, as these inverters will also be able to provide higher surge power needed].
For the person who asked about car batteries... They really don't like to be deeply discharged (which is what will happen), and doing so may damage the car batterry. If you have a volt-meter, you can monitor the battery's voltage while it's running the inverter . [Personally, I wouldn't let it get below 10.7 VDC]. You could just leave the car running all the time [do this outside, not in your garage or any place fumes could gather and be dangerous]. Remember to keep the wires from the battery to the inverter short. The wires from the inverter to the pumps can be long. So, you'll need to use heavy-duty [15 Amp] extension cords (like air conditioner or refrigerator cords) to bring that power into the house!
