Cheap, practical way- install a hub motor kit (9C, Amped, whatever). If you want to go 25 without pedaling, use a 48v10ah battery. My 36v15ah holds me at 21-22 without pedaling on flats. I pedal it up to 24-25 without a lot of effort. 7 miles of riding feels like about 3. Most kits like this are not pedal-assist (meaning you have to pedal if you want to move) but have a throttle that you engage (thumb, half-twist, full-twist) to the degree you want. My thumb throttle makes a slight angle to the grip shift so it kind of gets stuck when fully engaged. I know it's not the safest, but I like the cruise-control nature it affords, especially in the very cold mornings.