Add docs for P1 Monitor integration (#19000)

source/_integrations/p1_monitor.markdown

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+---
+title: P1 Monitor
+description: Instructions on how to integrate P1 Monitor within Home Assistant.
+ha_category:
+  - Energy
+ha_release: 2021.9
+ha_iot_class: Local Polling
+ha_config_flow: true
+ha_codeowners:
+  - '@klaasnicolaas'
+ha_domain: p1_monitor
+ha_platforms:
+  - sensor
+ha_quality_scale: platinum
+---
+
+The P1 Monitor integration integrates the [P1 Monitor](https://www.ztatz.nl/p1-monitor/)
+API platform with Home Assistant.
+
+P1 Monitor is a platform that allows you to read the data from your smart meter via the serial port (P1), such as your energy consumption, but also that of gas or a water meter.
+
+{% include integrations/config_flow.md %}
+
+## Sensors
+
+The P1 Monitor platform mainly provides sensors that you can use in your
+[energy dashboard][energy].
+
+**Note** that by default, the gas consumption entities are disabled, so if you want to use them, you need to enable them manually.
+
+### SmartMeter
+
+Read out what your meter readings are for energy consumption/yield, see what your current power consumption is and in which tariff period you are currently.
+
+- Gas Consumption (m3)
+- Power Consumption / Production (W)
+- Energy Consumption Low/High (kWh)
+- Energy Production Low/High (kwH)
+- Energy Tariff Period (low / high)
+
+### Phases
+
+See per phase what your voltage, current and power consumption/production is.
+
+- Voltage phases L1/2/3 (V)
+- Current Phases L1/2/3 (A)
+- Power consumed phases L1/2/3 (W)
+- Power Produced phases L1/2/3 (W)
+
+### Settings
+
+You can use the rates set in P1 Monitor for your calculations in Home Assistant.
+
+- Gas Consumption Tariff
+- Energy Consumption Tariff Low/High
+- Energy Production Tariff Low/High