I'm currently playing with ESP's writing to a MYSQL server on one of my websites, the ESP's wake up every ten minutes or so, read the temperature, humidity, and battery voltage, and upload it to a remote server via a PHP script - it also uploads the WiFi network it's connected to, and the signal strength.
So far they seem to be working flawlessly, here's a link to the graphs of the two currently running:
The top graph is an ESP01, actually connected to USB on the PC I'm using at the moment - and as it's an ESP01 the battery voltage is actually the internal supply in the chip. The lower graph, which I built today is a Wemos Mini, running off a Lithium AA cell, connected via a USB charger/protection board (and monitors the actual battery voltage) - the unit is currently sat under the eves of my house - the initial high temperature and low humidity was when it was at in the sun through my attic window. I haven't actually measured the current consumption, I'm just waiting to see how long the battery lasts.
My main problem at the moment is the graphing, I wish to have the X axis labelled in date and time, but I'm struggling getting PHPGRAPHLIB to do it - altering the x-axis interval removes the labels entirely.
I've also got a third ESP running, another Wemos Mini, with a battery charger shield fitted, and that's running from an 18650 battery - as the shield up-converts the battery to 5V, it's presumably taking more current than the bare Wemos version?. This one simply uploads to a CSV text file via PHP on the same server - it's been running since 23rd April, and the battery reading is currently 3.98V. The CSV files is now over 90Kb.