Best the way to find out the range to cargo ratio you could look at the specifications of the battery of the cargo electric bike to determine which one goes the furthest with the most cargo onboard, however many different factors influence this final result.
The torque, power, battery capacity, weight of the bike, weight of the rider and other components on the bike.
Considerations When Comparing Cargo Electric Bikes
You can haul far more by trailer than directly on an e-bike, just like with cars. For instance, I haul using a Surly Ted 5′ bicycle trailer attached to an e-bike.
The e-bike is rated for 330 lbs. including the rider, which, since I weigh about 185 lbs., leaves 145 lbs. for hauling. In reality, it’s pretty difficult to get 145 lbs. of stuff on a touring bike and have it still be rideable. Fitting that much on a cargo bike is still a bit of a challenge.
However, my trailer is rated at 300 lbs., and I’ve had occasion to max that out numerous times and verify that it does no harm to the bike (although the limit going uphill for myself and the 250W motor seems to be about 250 lbs.). Lastly, the amount of stuff you carry itself affects your range. Unladen, the e-bike above gets about 1,200 MPGe.
Loaded up, it can get as low as 300 MPGe – still better than most things on the road, but a quarter the usual range of the bike. In short, it’s a highly dynamic thing to calculate with a lot of variables.
It’s the sort of thing that, in a hypothetical business environment, say a delivery company using e-bikes, mathematicians might estimate with differential equations and Kalman filters (perhaps companies like DHL and UPS have already implemented these sorts of estimates, since they operate delivery e-bikes in the EU).
Even then, you would want to do it on the fly with sensor fusion as orders are delivered, since every package delivered or picked up would change the estimate.
Your Best Bet At Calculate Range to Weight
The best you can do is calculate the general case and then pick a bike with what seem to be optimal specs for your use case. I just cited the below calculator from Grintech (a Canadian e-bike kit company) in another answer the day before, but I’ll cite it again because it is applicable here as well.
The calculator only includes components this one company sells, but it is a wide enough variety that it is roughly representative of the market as a whole (and it even lets you input a custom motor):
Average Electric Cargo Bike Ranges
In general terms, a cargo bike with a big battery running 36V, that is lighter than other cargo bikes, has a powerful motor, but is class I, meaning it can only do 20 mph, and high torque, will have an excellent range. If you select a Bafang 500W motor with a 36V, 26.5 Ah battery and input 200W of human power into the calculator above, it simulates a system with 124 km of range (unladen, of course). It’s capable of 36 kph, or about 22 mph, so there’s your class I bike; and maximum torque is around 90 Nm. So now you would need to go out and find a bike like that after reading lots of specs.
The same system above, with with a 72V, 23 Ah battery, only gets 50 km – well less than half the 36V system.
You might keep in mind though that maybe you don’t need 124 km of range. In our case, we have two batteries for both of our current e-bikes, so we have 1000 Wh of capacity for both bikes. I’ve had to swap batteries on many occasions part way through a ride with heavy loads, although I’ve only nearly depleted both batteries once or twice. Another thing to keep in mind is that I kept the default value for gross vehicle weight of 220 lbs. and 26″ wheels. In reality, I weigh 185 lbs. and one of our e-bikes weighs nearly 60 lbs. and has 27.5″ wheels; that drops my range down to 108 km with a 36V, 26.5 Ah system.
If I more closely simulate my battery, which is 36V, 17 Ah by selecting the 36V/16.5 Ah option, it drops again to 67 km. But anyway, you get the idea; play around with it.
One note about drivetrain; we have a Nuvinci shifter, which is great, but avoid belt drives for cargo bikes. They are attractive because they are quiet and maintenance free, but for serious cargo hauling, you will get far more torque out of a chain.