A pressure washer that operates between 1300-2300 PSI you will be able to cover almost any task. Anything less and you will not be able to remove most common problems and anything higher can cause damage to materials.
Pressure washing, whether with a high-pressure washer or a power washer, is a great way to clean your home without running water. High-pressure washers allow you to remove unwanted debris and mold from your siding in a matter of minutes. Most experts recommend pressure washing for homes that have shingle siding or vinyl siding, depending on the amount of mildew or mold buildup. It's recommended to do this twice a year typically around fall and spring!
A typical gas or electric pressure washer pumps the water from a garden hose to a high pressure. The pressure washer is hooked to a high pressure hose that when the trigger is pulled the water mixes with air to come out of the nozzle. Typically the water comes out around 75 times that of a regular garden hose. Some pressure washers also have a detergent tank that will mix with the water so when they both come out of the hose at once. The high pressure not only cleans more effectively but wastes less water than a regular hose.
If you're trying to get dirt off a wood fence, then you can use a little bit of soap and water, or baking soda and water. A deck brush will allow you to clean the fence faster. If you don't want to use chemicals you can also try vinegar and water, but may have to scrub harder.