I m doing a weather system project using arduino uno, bme280 sensor, raspberry pi 3, soil sensor and LoRa gateway also using solar panel. But I can't understand how much bettery size is use in this..for saving charge in a week or long time...please solve this..
Raspberry Pi is one of the worst possible choices for a battery powered project, due to its high current draw. A single Arduino is all you need for a weather station.
See this excellent low power tutorial.
Pi can use from 350 mA in standby to over 1200mA on some of the more powerful models. The arduino uno is more like 20 to 40 mA (without add-ons)
You need to figure out all the mA of the units you are using and add them together.
If it comes to (for example) 1000mA you need 1Ah to run the device for 1 hour, or 24Ah to run it for all 24hours per day. - therefore your battery should be min of 24Ah+
Next, how many hours full direct sunlight do you have? if you have example 10 hours, you need to make more than your 24Ah in 10 hour. Therefore a solar panel at least 2.4Amps out
These are rough estimates, as I have not considered the voltage conversions. Eg dropping 18V solar to 12V batter, and 12V to 5V arduino.
As a rule of thumb you want to go well above these figures to consider dark rainy days, and temperature etc, but this costs more bucks