Framing new project: WiFi weather sensor... lowest power components!

Experts,

Just curious if anyone has thoughts on my list of low-power components for an outside weather sensor that uses WiFi to upload results to a web server:

ATTiny84
esp8266

I'm thinking I can power this combo (and sensors below) with very little power, yet have enough inputs for sensors. Is there a uC with lower power in sleep/standby than the ATTiny series?

sensors:
temperature
humidity
bar. pressure
ambient light level
rainfall

The attiny84 only has 8KB flash and 512 bytes SRAM. The floating point code with sprintf alone is probably going to take up 2KB of flash, add the code to talk to the ESP8266 with an AT command set and you are going to run out of flash real quick.

Also, the attiny84 doesn't have a hardware UART so you would have to use (or write your own software UART), which in my honest opinion, is not suited for talking to the ESP8266.

I have an Atmega328p+ESP8266+TCN75A (temp sensor)+BH1750 (ambient light sensor) and I'm already at 8900 bytes, and this is written in pure C (compared to a bloated Arduino C++ code).

The difference between deep sleeps on an Atmega328 to an attiny84 is almost negligible and orders of magnitude miniscule to the power consumption of an ESP8266.

I would at least consider an atmega MCU for such a project.

An ESP8266 is a powerful processor in it's own right, why do you need another processor alongside it?

Most MCUs have a sleep mode where the current consumption is so low that it is negligible.
Important is, that before entering sleep mode, you switch all the peripherals, modules etc. off. Further, don't wake it up more often than is required for your measurements.

It would appear that reframing the question is in order.

What components would be best for a low low low power weather system?

2 MCUs does Not fit.
WiFi does not fit.

ESP32 has a low power processor that can do most of the sensing.

Suggest you start with the basics.
Wemos D1 mini. Or ESP32
BME280
DS18B20
Buy or make a rain sensor.

the BME280 in low power

To get started make all the parts work.

There are some tutorials on how to add a solar Charger

For weather you don't need to send data more than about every 15 minutes.

Get that working
Use deep sleep
Test longevity in battery

While testing read up on the ulra low processor of the ESP32

Read up on the RF and LoRa Options.

Lowest power has a few places for your attention.

Sensing
Local processing and datalogger
Transmission

Ultimately WiFi is not going to be the best for transmission.

While it is true that WLAN is not optimal, you can still get useful results. See #58 ESP8266 Sensor runs 17 days on a coin cell/transmits data to sparkfun.com and ubidots.com - YouTube

The “headline” figures of 425 days are for a transmission every hour.