Andrew Lyman-Buttler

I made a spreadsheet to determine when to use a gas furnace and when to use a heat pump if you have both.

The tradeoff depends on several factors:

1. Heat pump quality and outdoor air temperature: Heat pumps don't create heat, they extract it from outside air using the same method as an air conditioner or refrigerator. If the outside air is colder, the pump has to work against a bigger gradient, so it's less efficient. Each heat pump has a Coefficient of Performance (CoP) that's tested at several different temperatures; this represents how many units of energy you can pump into your house for every 1 unit of energy expended. This ratio should always be greater than 1; you'd expect 1 from a space heater or baseboard heater that just turns electricity directly into heat through resistance.
2. Furnace AFUE: The proportion of chemical energy in the gas that the furnace can effectively convert into useful heat. Higher numbers favor using gas.
3. Price of gas: This tends to fluctuate a lot with demand, and is also obfuscated on a lot of gas bills by the way they break down fees and taxes. In Minnesota, natural gas is usually pretty cheap--so cheap, in fact, that at current prices it never makes financial sense to run my heat pump at any temperature. On the graph, no matter how warm it gets, the unit cost of electric heating (the blue line) never gets below the red "cost of gas" line.
4. Price of electricity: Things get even more complicated if this varies by time of day. Mine doesn't, but if it did, I'd run the spreadsheet twice to generate graphs for peak and off-peak hours.

MISO, which operates the power grid for much of central North America, has a public API that you can use to access real-time production data. You can turn this into a Home Assistant sensor by adding the following code to your configuration.yaml:

rest:
  - resource: 'https://api.misoenergy.org/MISORTWDDataBroker/DataBrokerServices.asmx?messageType=getfuelmix&returnType=json'
    scan_interval: 300
    sensor:
     - name: "Total grid power"
       value_template: "{{ value_json.TotalMW|int }}"
       device_class: power
       unit_of_measurement: "MW"
       icon: mdi:transmission-tower
     - name: "Grid coal"
       value_template: "{{ value_json.Fuel.Type.0.ACT|int }}"
       device_class: power
       unit_of_measurement: "MW"
       icon: mdi:factory
     - name: "Grid gas"
       value_template: "{{ value_json.Fuel.Type.1.ACT|int }}"
       device_class: power
       unit_of_measurement: "MW"
       icon: mdi:gas-burner
     - name: "Grid nuclear"
       value_template: "{{ value_json.Fuel.Type.2.ACT|int }}"
       device_class: power
       unit_of_measurement: "MW"
       icon: mdi:atom
     - name: "Grid wind"
       value_template: "{{ value_json.Fuel.Type.3.ACT|int }}"
       device_class: power
       unit_of_measurement: "MW"
       icon: mdi:wind-power
     - name: "Grid solar"
       value_template: "{{ value_json.Fuel.Type.4.ACT|int }}"
       device_class: power
       unit_of_measurement: "MW"
       icon: mdi:solar-power
     - name: "Grid other"
       value_template: "{{ value_json.Fuel.Type.5.ACT|int }}"
       device_class: power
       unit_of_measurement: "MW"
       icon: mdi:hydro-power

I don't drive much, so unless I have a big trip coming up, I rarely need a full battery. I want to turn the tap on when the electricity is clean, and off again when it's dirty. After collecting data for a while, I decided on cutoffs for how much coal power is too much (or how much wind is enough) and created automations to switch on/off the EVSE (which, in my case, is just a smart plug connected to the wall outlet that the charger's plugged into). I haven't figured out how to "set and forget" this one because of all the seasonal variation in power generation, but there could be a role for the Statistics integration here (for example, a script to detect when coal power generation is in the 10th percentile for a rolling 30-day window).

alias: Turn on EVSE when grid coal usage is low
description: ""
trigger:
  - platform: numeric_state
    entity_id: sensor.grid_coal
    below: 19999.9
condition:
  - condition: numeric_state
    entity_id: sensor.grid_wind
    above: 10000
action:
  - type: turn_on
    device_id: bd7c040ad491346705181910a569143d
    entity_id: switch.evse
    domain: switch
mode: single

alias: Turn off EVSE when coal usage is high
description: ""
trigger:
  - platform: numeric_state
    entity_id: sensor.grid_coal
    above: 24999.6
  - platform: numeric_state
    entity_id: sensor.grid_wind
    below: 8000
condition: []
action:
  - type: turn_off
    device_id: bd7c040ad491346705181910a569143d
    entity_id: switch.evse
    domain: switch
mode: single