…because I heard you only get 75% of the panel’s rating.
I used to think so, but in most cases this approach will not give you the optimal panel size for your needs. If your panel is too small, you will be disappointed with your daily range. If your panel is too big, your bike will be needlessly heavy and cumbersome, making it less fun to ride.
Trying to match solar output to motor power may seem compelling but it is an oversimplification that doesn’t match up with the way an ebike motor is actually used. You will need more power than your solar panel can provide every time you accelerate from a stop, go up a hill or ride in the shade. Conversely, your panel will produce more energy than you can immediately consume every time you coast down a hill, stop at a traffic light, take a break… not to mention the daylight hours before and after your ride. A better approach is to work with average daily watt-hours.
Assuming your goal is multi-day tours with no AC/wall plug/grid/mains battery charging along the way, take your average daily distance and multiply it by 16 Wh per mile (10 Wh per km) to get your daily watt-hours consumed. Next, divide by 3.5 to get your solar panel size in watts. For example: you want to average 60 miles per day times 16 Wh/mi divided by 3.5 gives 274 watt solar panel needed.
This is a starting point based on average values from a couple dozen long distance solar bike tours, including my own. These values hold surprisingly well for tours between mid-spring and mid-autumn across a wide range of latitudes. Round up or down based on your circumstances. If you’re very good about keeping your solar panel tilted toward the sun all the time and/or you have an exceptionally lightweight bike and/or you are a very strong rider then can use a slightly smaller panel. If your panel doesn’t tilt at all and/or you have an exceptionally heavy bike and/or you contribute relatively little pedaling effort, you will need a bigger solar panel to reach your distance goal.
I emphasize long distance because that seems like the most compelling use case for solar panels attached to an ebike. The long-term averaging causes the Wh per unit distance and daily Wh per solar watt (3.5) values to converge across different riders in different locations. This is because you end up with a mix of sunny days and cloudy days but also because there’s a selection filter inherent in these statistics: only riders and machines who prepare for and successfully complete such long tours are included. If you are able to cherry-pick your riding days to only include perfect, sunny days at the peak of summer then you can do more with a smaller panel. If you sacrifice durability and strength to build a fragile, ultralight setup then you’re less likely to complete a multi-month tour.
Bonus tip: You can also run the calculation backwards. Let’s say you’re only willing/able to strap a 100 watt solar panel to your bike. What kind of daily range can you expect? Take 100 watts, multiply by 3.5 to get 350 Wh, then divide by 16 Wh/mi to get 22 miles at that assist level.