The first item to consider in buying your inverter is the power that the inverter must provide. To do that you will have to look on the back of each item that will run from the inverter and find the amperage draw or the wattage draw. If the item has the watts listed then no problem just write that info down to be added in the future to you r other items in order to find the total. When all you can find is the amp draw just multiply the amps times the volts and that will give you the watts.Remember that amps times volts works with ALL voltages to give you the wattage used.Once you have totaled up all the wattage you need simply find an inverter which will provide about 50% more than you require .Do that to help compensate for inductive surge requirement caused by pump motors and compressor motors. Now find out the size you need for the battery pack. If the inverter must produce 500 watts all the time then the battery must also provide 500 PLUS the wattage lost from the inverter conversion function. That means you must check the rating of the inverter to discover the loss.Again go back to our WATTS formula. A 12 volt battery witch has 50 amps being drawn from it is providing 600 watts. This gives you an idea of the size required for the battery and the alternator to charge that battery. Because of this high current draw most people use deep cycle batteries with ratings larger than 200 amps for inverter power supplies. These batteries will provide the large currents listed above and can be charged again and again. We have exceptions to this rule however and that is the case of the pick up truck 120 volt inverter supply. These car/truck mounted inverters use the battery and alternator installed. Most modern factory installed alternators produce close to 1000 watts. That means if you a only drawing 500 watts from a 1000 watt inverter your alternator can continue to keep the battery in good shape while the inverter does its work. Never forget that the watts in to an inverter will always be a bit higher than the watts out of an inverter.