There are many ways to drive a pressure washer pump.  All methods do the same thing, turn the pump.  It is important to use enough horsepower to match the requirements of the pump.

Gas engines are probably the most common method of powering a pressure washer pump.  They are reasonable cost, portable and convenient.  

Many pressure washer manufacturers have settled on Honda as the standard engine for their machines.  Briggs and Subaru have also been used as an alternative to Honda.  Recently there has been an influx of inexpensive engines from China, which have had varied success.   Reliability has been a problem.  While many Chinese made engines look exactly like a Honda, they did not run like a Honda, with major problems showing up after only a few hours of running.  Honda have responded to these knock-offs by going after sellers of any engine that looks like a Honda, using copyright laws.

If you are contemplating purchasing a pressure washer with a gas engine made in China make sure you understand the possible problems.  Also, verify the reliability of the dealer for warranty and parts and service.

While diesel engines are the most reliable and fuel efficient alternative for powering a pressure washer, they are also the heaviest and most expensive.

Most portable applications need a lighter engine so diesel is generally ruled out.  However,  fixed mounted units washers on a wash trailer or wash truck and in-plant applications can be ideal for a diesel engine.  

Diesel engines can be very convenient when used in conjunction with a Hot Water Heater because only one fuel is needed.

If you have the power and the right circumstance then the electric driven washer  is the best.  Electric power is not always as convenient if you have to take the washer to the work.  But, if you are taking the work to the washer then electric driven pressure washer is for you.  The most limiting factor with electric power is the electrical power you have available.  Most homes only have 110/120 volt, single phase power.  With this voltage you are limited to the amperage of the breaker in your electrical panel. 15 amps will run a 1.5 HP motor.  20 amps will run a 2 HP motor.  Most homes have 110/120 volt, 15 amp breakers.  Dryers and electric ranges usually operate on 220 volts.  With some modifications to the electrical system it is possible to install a 220 volt circuit for a pressure washer to plug in. 

The introduction of the "Hobby" big box store electric washer has given every homeowner the ability to own a pressure washer.  These plug into any electric outlet in and around the home.  Don't expect too much from one of these units and be prepared to replace it on occasion.  If you have lots of time and patience then one of these will do the job.

Most commercial buildings and facilities do have higher voltages available allowing for small to very large pressures and flows  In-plant applications are ideal for electric powered washer.  With no fuel or fumes to deal with they can be installed anywhere in a plant. 

A power takeoff (PTO) is simply attaching a pump to a power source used for other purposes.  Many engines used to drive trucks, tractors, boats and generators have the ability to accept a PTO which can be used to drive a pressure washer pump.  The challenge for driving a pressure washer pump is to match the flanges.  PTO manufacturers do not think of marrying a pressure washer pump to their flange so adaptation is necessary.  PTO pressure washers are generally limited to the farming industry where 3 point hitches on tractors, with some adaptation, will work with a pressure washer.  

Hydraulic driven washers can be very convenient in the right circumstance.  Hydraulic pressure washers, while they have a motor, do not have any power.  They rely on hydraulic power from another source to run the hydraulic motor.

Many operations such as boats, barges, trucks, cranes, concrete pump trucks, excavators, tractors, loaders and farm machinery have a hydraulic source available.  Another great application for a hydraulic powered pressure washer is that they can be used in remote and explosive environments where gas engines or electric motors can not be used.