If you have to pay for the electricity, it's only economical to mine (Bitcoins) with an ASIC. The CPU and GPU mining is now so slow that you will get very few Bitcoins in average, and the cost of the electricity to run it will be grater than the money you can get from the Bitcoins. (To get a smooth outcome you should probably mine in a pool, but this decrease the expected average result only little.)

What tip volume and value do you envision that tipping fees will make up the main source of income for miners?

The volume of tips would have to increase drastically. In any case, the volume would depend on the price. Slightly off point, it might not be completely unrealistic to imagine a world in which miners do not derive their main source of income from mining. Perhaps miners could running mining programs on SoCs during the day while the miners themselves work other/additional jobs. While the number of miners mining in this way would have to be very large in order for this configuration to be possible, should this be the case, miners wouldn't be nearly as financially dependent on mining, and they could be more immune to fluctuations in the price

Miners need expensive and energy hungry equipment because this is what protects the Bitcoin network.