Sizing. Let's start by determining what it is that you want to operate. Here's a table listing some of the more common appliances and their wattage requirements.
 

Appliance	Wattage (approx.)	Appliance	Wattage (approx.)
13" color TV	50	Computer	70
19" color TV	120	Computer monitor	55
Stereo	50	Computer printer	20
VCR	25	Hand mixer	100
Satellite Receiver	20	Blender	350
Heating pad	50	Toaster	1250
Electric blanket	150	Microwave	650-1000
Electric drill	500	Coffee maker	1250 brew, 200 warm

It's obvious that  your microwave requires quite a lot of power. Most ovens will draw 650 - 1000 watts. Starting surges can be as high as 1500 watts. If you want to be able to use your microwave with your inverter, your inverter should be capable of producing at least 1000 watts and be rated for a 1500 watt surge. If you want to be able to use resistance heated appliances, like a toaster oven or electric coffee maker, be sure that the inverter you choose has a high enough rating. Most of these appliances will require 1200 to 1500 watts. They don't have a starting surge, like motors or microwaves, but are hungry for watts. When in doubt, consult the owners manual for the appliance or check the ratings plate for wattage requirements.

Now is a good time to discuss just what these wattage numbers really mean in terms of 12 volt battery power. First, we need to do a little math... Hey, wait... come back here! I promise that it will be simple and won't require you to buy a scientific calculator to figure it out! What we need to do first is figure out how many DC amps the inverter will draw when it's making that AC power for your appliances. Put simply, for every 100 watts of AC power that your inverter is producing, it needs to draw about 10 amps from your 12 volt battery system. For inverters rated at 90% efficiency, the number is closer to 9.25 amps per 100 watts, but for ease of calculation, just assume that 10 amps DC per 100 watts AC... it's easier to figure that way and errs on the side of safety. For those of you who just MUST have the math, here it is:    Watts = Volts x Amps, so Amps = Watts / Volts. For an inverter, operating at 90% efficiency, the conversion can be represented thus: AC watts / 12 volts X 1.11  = DC amps
Example... television drawing 100 watts operating on an inverter will draw about 9.25 amps from your batteries..
100 / 12 X 1.11 = 9.25
The reason that this is important is that you only have a fixed capacity in amp/hours available from your battery bank. A pair of 6v deep cycle batteries or a pair of group 24 RV batteries will have a capacity of about 200 amp hours. That means that if you run that TV from the previous example for about 20 hours, your batteries will be flat! 9.25A X 20 hours = 185 amp/hours.

Still with me? Good! Using the chart on wattage requirements and adding our new-found knowledge of amp hours, lets take a shot at how many amp hours of battery bank we need to supply us for a standard days use.
 

Appliance	Watts	Use Time	Watt Hours	Amp Hours
13" color TV	50	3 hr.	150	15
Satellite Receiver	20	3 hr.	60	6
Computer/monitor	125	2 hr.	250	25
Coffee Maker	1250	20 min	417	41.7
Microwave	1000	15 min.	250	25
		Totals >	1127	112.7

Don't forget that you are also using power from your batteries to run 12 Volt lighting, fans, water pump, etc.

You begin to see that putting in a larger inverter to run bigger loads will require you to have an appropriately sized battery bank and an adequate method for recharging it! You can't hook a 2000 watt inverter to a single battery and cook the Thanksgiving turkey in the microwave! You should get a feel for your intended inverter usage and take into account the size of your battery bank before selecting an inverter. In my personal case, I chose a 750 watt inverter to go with my 2 battery bank. I didn't have room or weight capacity for more batteries, so I went with a smaller inverter. It runs all my electronics, charges power tools, runs kitchen tools, but won't support my microwave or toaster oven. It was a good compromise for my situation. It was also a lot cheaper, as my inverter was about $500, compared to the $900+ that I would have paid out for a 1500 watt unit. And that brings us to the next topic:

Hybrid Systems.  Often, it is more cost effective to purchase a smaller inverter to run the small appliances that you have and use a generator to power the more watt-hungry larger appliances, like the microwave and coffee maker. This will give you quiet power for most uses and minimize the wear and tear on your generator. If you can't manage a full sized battery bank and large inverter, this hybrid approach may work better for you. It is the approach that I chose to use, and it works well for me.

Inverter types  There are 3 main types of inverters. Square Wave, Modified Sine Wave and True Sine Wave. Let's look at the differences:
    Square Wave inverters are the cheapest. They also are usually the least efficient and will not run any of your electronic equipment. The reason for this is that the AC waveform they produce is very different from what you get at a standard wall outlet. Instead of being a sine wave it is a square wave. These inverters are usually functional for power tools and motors, but will not work with your TV or stereo. Definitely not recommended!
    Modified Sine Wave inverters are the middle of the road. They produce an output wave form that is close to a true sine wave and will work adequately with most devices, including TVs and computers. You may be able to see some slight distortion lines on the TV or hear a slight buzz on the stereo, but for the most part, these inverters provide quite acceptable performance. One noted exception is that most laser printers won't work properly with a Modified Sine Wave inverter. They are also middle-of-the-road as far as cost goes.
    True Sine Wave inverters are exactly that. They produce a practically perfect sine wave output. These units will operate even the fussiest electronic device, but they are much more expensive than their cousins, the Modified Sine Wave inverters. Do you need one? Probably not, unless you simply MUST have the best, or are trying to get a highly sensitive piece of equipment to work in your RV.

Battery Charging.  Many of the better inverters designed for RV use are also high quality battery chargers. This is an excellent option, as it allows you to get rid of that old, inefficient converter and enjoy faster, safer 3 stage charging. Most of the major inverter manufacturers offer this either as standard equipment or as an add-on accessory on their inverters. These chargers typically can deliver from 25 to 150 amps of charge current and run very well with generator power, allowing you to quickly recharge your batteries while out in the boonies. These 3 stage chargers will also not boil the water out of your batteries. Instead, they will bring them to full charge and then taper back to a true float charge... these units can safely be left plugged in continuously, unlike the standard converter found in most RVs.

Costs. You can expect to pay about $50 to $100 for a small portable MSW unit. RV designed units start at about $500 for a 750 watt w/charger and go all the way up to units rated at more than 3000 watts and costing several thousand dollars. Price increases with wattage ratings and capabilities. The best thing I can suggest is to do some research. Check the links listed below for inverter manufacturers and remember to size the inverter based on your real needs.