I took the bike out for a couple of quick rides and ended up calculating how a 350 watt motor might handle various grades. Mind you, this is not just an Evelo thing, but it would broadly apply to any e-bike with similar power and efficiencies.
I don't have an app to track routes and grades, but I found that Komoot.com will do this.
I took the bike up a nearby hill and hit a grade that Komoot indicates as 12%. Though I haven't verified it yet, I'd say that 12% looks ballpark. The motor power was less than I had hoped for, even in 1st gear with PAS set at 5/5. I had been hoping for sort of a "beach cruiser" experience, even if it was slow, but I found I had to (gasp!) use my muscles to assist! While I didn't stand on the pedals, I was pedaling with a high seat position and it required considerably more effort than I had hoped for on a warm day.
We then took the Dash and one of our folding bikes out to the paved Foothills Trail just west of Buckley, WA and the experience was night-and-day different. I rode up a steady grade of about 4% with PAS 1/5 in 3rd gear with minimal pedaling effort. It lugged a trifle as I was pedaling so minimally, but it still moved along quite nicely. I've also been very comfortable with pedaling up our street, which is at 8% grade.
The difference in these two experiences led me to think about the power required to travel up slopes, leading me to a formula where the power required = mass x gravity x velocity x the sin of the slope. In order to keep it simple, I neglected the effect of aerodynamic drag, where the drag increases with the square of velocity as I was focused on the impacts of grades.
Per Evelo, the motor on the Dash is rated at 350 watts of input power. I allowed a modest additional 0.15 horsepower for a relaxed pedaling effort as per the graph below from the International Human Powered Vehicle Association. This 0.15 HP contribution is roughly mid-range for a two-hour effort-level from an “average human”.
Summing 350 watts of motor input power plus (0.15 HP x 746 watts/HP) multiplied the total times an estimated 75% electrical efficiency and 90% to allow for various mechanical inefficiencies resulted in a total of something over 300 watts of output power. Plugging this conservative 300 watts into the formula with a combined weight of 230 lbs. for the bike and rider gave me the results shown in the second graph below.
In the end, what I found interesting about this exercise was how this combination could easily maintain a satisfying 8 mph or more on slopes up to about 8%. After that, the speeds with this limited pedaling effort are going to be lower. This appears to be in general accord with my observed experience; 4% grade was a joy, and 8% was just fine, but the 12% grade required more effort than I might really want to contribute on a hot day.
However, I also took heart that the speed curve flattens out to the right. With the proper gearing, I think this bike could be just the ticket for our needs. I'll post more about that later on.