If you're looking to clean large surface areas such as large decks, outdoor pool decks, patios, driveways, and homes, you may want to look into purchasing a gas-powered pressure washer. These are far more powerful and are typically what professional power washers use, although they do make relatively affordable consumer-grade gas-powered power washers. With these, you'll use less time and less water with better results for large surface areas.
You'll also want to look at the motor types available. For electrics, the size and power of the motor can vary greatly. They mostly come in two sizes: small and large and operate with either a universal motor or induction motor, with induction motors typically costing a bit more and offering more power. This will depend largely on your needs, including the surface area of what you're cleaning and the number of times you intended to use your pressure washer.
For electrics you'll also have to determine whether you want a small electric one which typically won't have a built-in detergent tank. A pro for this is that it's so light and can be maneuvered around easily as you're cleaning (great for smaller people to use). But, if you'd like something with a built in tank, you're going to have to go larger. Some models even have dual detergent tanks.
Nozzles are important to look at too. Most pressure washers offer two different types of nozzles so you can ensure how much pressure you want and whether you want to spray a small, concentrated area, or a large, wide area.
You'll also want to look at the type of detergent that is specified by the pressure washer manufacturer before you buy it. You'll want to make sure that you have easy access to purchasing this type of detergent so you don't have to go searching endlessly for the type of detergent your pressure washer requires.
Also before purchasing a pressure washer, you'll want to see if there are warranties that are available. Often times, manufacturers will include free warranties with the purchase or service plans. Other bonuses like free detergent included with the purchase can add up in terms of value too.
Just make sure that prior to your purchase that you go on the internet and check consumer reviews on pressure washers. Consumer reviews can let you know how people just like you would grade the performance of the pressure washer, along with cost, serviceability, parts replacement, etc. It may seem tedious, but checking consumer reviews can go a long way in helping you determine if purchasing a pressure washer is a worthy investment (instead of paying someone to do it for you or renting one), and if so, how much you should invest in one.