ESP8266 Community Forum

Here are some battery choices for ESP8266 based projects: https://diyi0t.com/best-battery-for-esp8266/

You could go with:

Multiple batteries in a easy change pack - one pack charging while the other is running. You could even measure the battery output and get your website to email you when it is close to dyeing. I did this on one of my setups without the emailing.

Batteries with a solar charger but will probably only last over the summer unless you live somewhere sunny

A big battery

If you use deep sleep and only send data data every now and again that will make the battery last a lot longer.

If you get to enthusiastic things get complicated quickly. I had two battery packs with an auto switch over IC and I would take out the flat pack and replace it. It also had a solar panel!