This blog post from Tailscale suggests battery life is an issue for several of their users:
Thanks everyone for the insightful responses. 
Based on your input, I’m comfortable running the VPN insofar as it will have little to no impact on battery life. As to the ethical question, I feel more comfortable limiting or eliminating ads as much as possible, but I will give this more thought before I make a final decision. The marketers seem to have little regard for the reader’s experience and will push the limits as far as they believe they can. That said, I don’t want to penalize those independent writers who depend on ads for their livelihoods. 
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With respect to the ethical issue:
I have no ethical qualms about using ad-blockers.
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That is truly disturbing. I searched the list and thankfully, ExpressVPN is not one of the VPNs listed. My ethical quandary is resolved. Going forward, I’m always running a VPN and blocking ads. Not to overstate it, but one could argue this is a matter of “self-defense.”
I downloaded MFP a few years ago and used an Apple “hide my email” address to register. About a week later I started receiving email from its parent company at my primary address. A coincidence?
Although I can’t opine on the battery usage, this is not entirely accurate. It is true that the payload for most traffic is encrypted these days, using a VPN actually does a second encryption which includes both the payload and the IP header. This is why your ISP can’t track you. So the VPN results in an additional encryption pass and then adds a new IP header to the whole thing.