While I'm writing about the legal stuff first, that's actually the last thing I did.
An Electric Assisted Bicycle must:
have working pedals * motor of 1000 Watts or less * 20 MPH or less * Can go anywhere a bicycle can go except the sidewalk. No riding on the sidewalk. * Just follow bicycle laws regarding helmets and such * Bike paths (like the Burke Gilman) are OK.The next class of vehicle up is a moped:
30 MPH or less * 2HP or less (just shy of 1500W) * can't go on the Burke Gilman or other certain rights of way * needs registration * needs mirrors on left and right * must wear motorcycle helmet and eye protection * no mandatory insurance * don't need pedals anymore (law changed in 2009) * Must follow some archaic 1969 DOT document
As you can see, being a moped (especially creating a home built moped like me) is a much bigger pain in the ass.
For now I'll start with the Electric Assisted Bicycle and then if I'm dissatisfied with performance I'll upgrade components and jump through hoops.
"Electric-assisted bicycle" means a bicycle with two or three wheels, a saddle, fully operative pedals for human propulsion, and an electric motor. The electric-assisted bicycle's electric motor must have a power output of no more than one thousand watts, be incapable of propelling the device at a speed of more than twenty miles per hour on level ground, and be incapable of further increasing the speed of the device when human power alone is used to propel the device beyond twenty miles per hour.
[1997 c 328 § 1.]
Helmets and mirrors: http://apps.leg.wa.gov/rcw/default.aspx?cite=46.37.530
Watts and HP: http://www.bestscooterbuys.com/power.html