IMHO...
Electric power-washers are quieter than gas models,
and can be used both indoors/outdoors.
However...Gasoline power-washers offers higher psi and gpm.
Therefore: Gasoline PW's are the ones I recommend.
As such; and FWIW:
-Best to purchase a commercial-grade unit...never anything less than a Prosumer-grade.
-HP ,PSI, plus GPM determines the overall
'abilities'.
-Get at least a: 5-8hp; 2600-3000 psi; 2.5-3.0 GPM unit.
-The pump should be of a: "tri-plex design"...
(I like Caterpillar's direct-drive triplex pumps)
-Unit must have (for me anyway): Low oil shutdown/thermal protection for pump & motor.
-4-5 yr. warranty for home usage
-Return/Repair facility network
-Easily accessible accessories...for customization
BUT:
If Gasoline power washers are not, or cannot be: "your cup-o'-tea"...
-Convert the above figures to the highest electric power-washer NO.'s as possible...
-Except for the pump---Make sure to keep it: "tri-plex"!!
Bob