Measure power of solar cell with an esp8266
To measure the power, you need to measure both voltage and current. The maximum power a solar panel delivers depends on the resistance of the load; different measuring resistors, or temperature drift, etc. will show different power readings.

That said, yes, you can use a resistor to measure current, but it takes a bit of work to make it accurate.

Shorting a single panel is OK; they operate normally at near their short-circuit current anyway. Note that if you have a measuring resistor attached, it won't be short-circuited anymore.

The voltage won't be the same as without a load; the open voltage is higher than the normal operating voltage, and a shorted solar panel will show even less: 0V.


You could try the following: find out the MPP of your small measuring solar panel. It should be in the specification somewhere. It will show the voltage and current at maximum power, or at its normal operating point; terms used may differ.
Use those two values and \$\small R \normalsize = {V \over I}\$ to calculate a resistor value. This resistor will put the panel at or near its MPP. Use \$\small P = V \times I\$ to decide what wattage the resistor should be.
Use an INA260 measuring sensor module or some such in series with the resistor to measure current and voltage; it connects with the ESP8266 over I2C.
The INA260 actually uses a very small resistor to measure current, as you planned to do, but its resistance's value on its own is so small it would almost short the panel which would give you useless voltage readings close to 0V.
I wouldn't bother with the ESP8266's ADC (it is a pain to use and not very good), but use the ESP8266 to read the values the INA260 sends over I2C, and then send them on to wherever you want them.