Well to go slow you only need very little power. But it goes up exponentially much more like with wheels. So you need to design a boat that is long, thin, light and with a large roof surface area. Basically a catamaran or a power trimaran. Then you want glass solar so it's affordable and lasts 20+ years but that is top heavy. But theoretically with the right design solar panels are cheaper than a sail and less complex and easier to maintain than sail+motor+house battery.

But 4-5 knots (~mph) is quite easy. From my amateur calculations 6-7 knots is already hard. I mean those speeds that are sustainable for 24/7 cruising. If you have a large car battery you can go vrooom for a few hours and then slowly recharge over a few days. But my goal is really a slow but seaworthy condomaran.