This question is difficult to answer as it all depends on what you plan to run, and the efficiency of the inverter. I could (almost) write a book in trying to give you an answer !!!
Obviously a 100 watt light bulb will require more current draw than a 25 watt bulb, a newer refrigerator will require less amperage than an older one of equal size. and so on and so forth.
You need to find the amperage draw of each device, this may be found on the appliance or better yet purchase a good ampmeter and find the "actual" amperage draw if each appliance you plan to run off of your inverter.
remember: V (voltage) multiplied by A (amperage)= W (wattage), W/A=V, W/V=A
You then need to realize that no inverter is totally 100% efficient, there will be a loss of power. How much depends on the inverter you purchase. There will also be a loss due to resistance in connections, wiring, ect.
You might also concider that you may want to add or upgrade something later, which might increase your power requirements. Also, some electronic devices will only operate on true sine wave. (most of the lesser expensive inverters are modified sine wave).
There are many places on the web to help you decide on the correct inverter for your needs. Check out the many solar power sites, they usually have a ton of great information on inverters.
