If we agree that the modern ebike motor doesnt offer resistance when not powered then its easy to put in simple understandable terms that the less technically minded can visualise..
My carbon bike weighed 8.6KG ,
my E-Bike weighs at least 18.5KG. but could be 20KG!!
Difference approximate 10KG
A std house brick weighs about 2.2KG
10 divided by 2.2 =4.5
So, a rucksack with four and a half house-bricks in it should simulate riding an e bike without power..
Or, if you are used to a heavier bike then 3 and a half should do it.
Apart from the discomfort of carting this weight on your back, flat riding and downhills wont be much of a problem once you get rolling, but on a hill it would be like riding through treacle perhaps..
But dont forget that the weight that is your enemy up hill, becomes your friend downhill... Until you want to stop that is...