People on here have scolded me by telling me, a hp is a hp, electric or gas motor. 

I do not quite agree as a gas motor will loose power over time faster than an electric motor will.  A gas at 1/2 speed does not have the same power as an electric.  Being that electric always run at the same speed and a gas you can be in error on speed more easily. 

Could you go to a big box store or some place that has quite a number of power washers and see what the mfg. do?

Do they use a 3 hp electric on the same pump as say a 5 hp gas or a 3 hp gas or a 6 hp gas?  That would give you an idea as to the size of pump you can run with a 3 hp electric.  Also note the pulley sizes used.