Show how calculations are made (#861)

Co-authored-by: Franck Nijhof <git@frenck.dev>

components/sensor/pulse_counter.rst

@@ -76,7 +76,7 @@ count the light pulses on a power meter, you can do the following:
         unit_of_measurement: 'kW'
         name: 'Power Meter House'
         filters:
-          - multiply: 0.06
+          - multiply: 0.06  # (60s/1000 pulses per kWh)
 
 
 Counting total pulses
@@ -95,13 +95,13 @@ measure the total consumed energy in kWh.
         unit_of_measurement: 'kW'
         name: 'Power Meter House'
         filters:
-          - multiply: 0.06
+          - multiply: 0.06  # (60s/1000 pulses per kWh)
 
         total:
           unit_of_measurement: 'kWh'
           name: 'Energy Meter House'
           filters:
-            - multiply: 0.001
+            - multiply: 0.001  # (1/1000 pulses per kWh)
 
 See Also
 --------

cookbook/power_meter.rst

@@ -24,6 +24,18 @@ And... that should already be it :)
     :align: center
     :width: 80.0%
 
+.. note::
+
+    Some energy meters have an exposed S0 port (which essentially just is a switch that closes), if
+    that is the case the photodiode can be replaced with the following connection.
+
+    .. code-block::
+
+        S0 ------------ VCC
+        S0 --+-- 10k -- GND
+        .    |
+        .    +--------- GPIO12
+
 For ESPHome, you can then use the
 :doc:`pulse counter sensor </components/sensor/pulse_counter>` using below configuration:
 
@@ -35,7 +47,7 @@ For ESPHome, you can then use the
         unit_of_measurement: 'kW'
         name: 'Power Meter'
         filters:
-          - multiply: 0.06
+          - multiply: 0.06  # (60s/1000 pulses per kWh)
 
 Adjust ``GPIO12`` to match your set up of course. The output from the pulse counter sensor is in
 ``pulses/min`` and we also know that 1000 pulses from the LED should equal 1kWh of power usage.