The first thing you will need to know is what you are planning on running in this time of emergency. The fewer the items or the smaller the amperage, the smaller or least costly inverter will be needed. I will give a table of items and their average power uses below followed by a simple formula that will give you an idea of the size of inverter you will need.
Stackable Washer/Dryer: 2500 Watts
Electric Chain Saw: 1700 Watts
Drill Electric
Electric Circular Saw: 1600 Watts
Full Sized Microwave Oven: 1400 Watts
Chest Freezer: 1200 Watts
Mini Microwaves: 1000 Watts
1/3 inch HP Submersible Pump: 920 Watts
Reciprocating Saw: 720 Watts
1/2 inch Reversible Drill: 620 Watts
Coffee Maker: 600 Watts
Portable Vacuum: 525 Watts
Electric Garden Tools: 475 Watts
10 Speed Blender: 450 Watts
Orbital Sander: 400 Watts
Home Stereo: 350 Watts
3/8 inch Variable Speed Drill: 320 Watts
20 inch TV/VCR Combo: 300 Watts
12 inch Three Speed Fan:200 Watts
Computer/Printer/Fax: 150 Watts
Quartz Halogen Spotlight: 100 Watts
Mini Stereo/CD Player: 50 Watts
Laptop Computer/8 inch TV: 38-45 Watts
Cell Phone/Camcorder Charger: 10 Watts
14 Volt Cordless Drill Charger: 8 Watts
If the appliance you are planning on using is listed in watts, just go ahead and use that number. If it is listed in amps you just need to use this simple formula: volts (120) X amps (listed on device) = watts. This will give you the amount of watts the appliance will be using. So if you had a 3 amp appliance, and you wanted to find the watts, the formula would be: 120 (volts) X 3 (amps) = 360 watts.
You would then total all of the watts that you will be using and come up with a number. Lets say your total watts usage is 500 watts. If that's the case it would be a good idea to get a 750 watt inverter. Always get a larger size than what you think you will need, it may come in handy.
Your next step will be to determine where your power supply is going to come from. If it is for emergency purposes there is a very good chance that it will be coming from a battery hooked up to your vehicle, or you may be a little better prepared and have a bank of batteries hooked up in a parallel fashion. Since most people reading this are probably unfamiliar with creating a battery bank I will just cover running the inverter from the vehicle. The main thing to know is what type of battery you will be using. It is suggested that you have a deep cycle battery or those that are used for trolling motors or golf carts and that you get as many amp hours as your budget allows. A few years back I was able to go 11 days using a 750w inverter and a 120amp hour deep cycle battery attached to my car. I had to run the car for 10 minutes every 2 hours to run everything that I had hooked up which was a laptop, 13" color tv, direct tv receiver, modem for the laptop, 40w light, and a small box fan. That doesn't sound like much, but I thought I was staying at the Ritz Carlton compared to most of my neighbors.
The last thing you need to know is how long will your power supply (battery) last before it needs recharging. First you will need to know the wattage of all the appliances. Lets say we have 480 total watts of power being used, and we are running our inverter off of a 12 volt battery attached to the car. Next we have to convert this number so that we know how many amps are being used. We do this by taking the watts (480) and dividing it by 12 (volts of the battery) and come up with 40 amps. The formula is: watts / volts = amps. If we have a 120 amp hour battery we would divide 120 (amp hours) by 40 (amps) and get 3 hours before the battery is dead. It is recommended not to drain a battery below 50% of its capacity so you may want to divide the 3 hour in half giving you 1 1/2 hour before starting the car up to recharge the battery.