The idea isn't to pay them. Nobody gets paid for using BitTorrent, but it still works, because there are people who have "free" bandwidth (i.e. they weren't using it, and if they don't use it, it's lost, so they don't care).

You often hear the argument that people aren't going to "use" their bandwidth and watch ads, but that's dumb. The ads are to pay for development of the app or the content. The bandwidth is to offset what you downloaded yourself. It's two different things. And if you have unmetered bandwidth you're not using anyway, don't be a miser.

But if you really want to give people something as an incentive to upload, show them fewer ads, and give more credits during peak hours. Now, if you want to see no ads, you have to upload something like 10x what you download, and correspondingly up to 10% of users can get that deal. But if 10% of users take that deal, you eliminate your bandwidth costs and still have 90% of the ad revenue. And if more are willing to take it, it means you can give fewer credits until it balances, or vice versa.

Incidentally, residential solar isn't really a problem because you already have non-uniform demand throughout the day. The longstanding solution is to have "peaker plants" that only come online during peak hours. It's only a problem if you want to replace 100% of generation capacity with solar and have no storage solution. But that doesn't stop you from replacing e.g. 50% of generation with solar, and then running other plants less often and paying the fuel savings to the solar generators.

There are also ways to shift demand around, namely through pricing. Make electricity cheaper during sunlight hours and more expensive between sunset and 8PM and suddenly tons of people are doing laundry midday on a weekend instead of in the evening during peak hours. Which also makes it more economical for people who can't avoid peak hour usage to buy an energy storage system so they can run on stored energy when the power company is charging high rates and then charge it back up when power is cheaper.