It’s all about weight when going to space. It’s one of the basic parts of rocket science. If you want to send 1 pound into space, you need to add gas to cover that weight, and the weight of the gas you add. Eventually you reach a point where the combined weight of the gas is more than the thrust it provides and you can’t add more. Solar is light, a few hours of no power isn’t a big deal over months or years of operation time, if you are confident things will come back on when there’s light again.