We are considering a heatpump for heating and cooling so I am calculating the electrical load for our panel using the instructions here. The list below includes all major appliances and other devices with a total wattage of 23,025W. Without the heat-pump, the calculation would suggest an amperage needed of 63.375, which seems very low.

Calculation: ((23,025-10,000)*0.4+10,000)/240 = 15210/240 = 63.375

The heat-pump with 9600 W would change that to 103.4 AMP.

However, the simple rule-of-thumb discussed in the article suggests that our house needs at least 200-amp will all electrical appliances or even 300 amp with electrical heating/cooling.

Question: Why are the results of the calculation versus the rule-of-thumb so different? I know that my list of electrical devices and appliances is not complete but that does not seem to explain the difference.

** Background: ** Our service is 120A right now so the underlying question is whether adding a heat-pump would require us to update the electrical panel and service adding more than $5k to the costs.

List of electrical devices

  • Induction cooktop: 9600 W
  • Oven: 3100 W
  • Washing machine: 1300 W
  • Dryer: 1000 W
  • Dishwasher: 1200 W
  • Garbage disposal: 720 W
  • Coffee machine: 1750 W
  • Fridge: 180 W
  • Lights: 200 W
  • Microwave: 700 W
  • Toaster: 900 W
  • Mixer 325 W
  • Blow dryer: 1900 W
  • TV: 150 W

Total: 23,025 W

  • 1
    This is NOT the correct load calculation. Try using this one: gensizer.assurancepower.com/… Commented Feb 24, 2022 at 23:25
  • You don't need to list all the smaller 120V loads. Those are accounted for in the Load Calculaton either in the 1500 VA allocations for K/bath circuits, or 3 VA/sqft catch-all. Commented Feb 25, 2022 at 19:32

1 Answer 1


A true load calculation is not:

  • Add up the nameplate values of all your major appliances


  • Add up all your existing breaker sizes

It is a far more complex calculation that includes:

  • Values for some of your major appliances based on the nameplate values (but not necessarily the same values)
  • Certain required circuits at set usage (e.g., kitchen countertop circuits)
  • Standard values for certain common appliances
  • Heating or air conditioning but not both - except with a heat pump where they are one and the same, of course
  • Additional standardized amounts based on the size of the house to allow for lighting and miscellaneous loads

Your listed values are likely high in some respects, low in others and simply missing some other key values altogether.

Upload a picture of your panel and also some more details about your house - total size, what type of water heater, etc. and the pros can come up with some much more accurate calculations.

  • 1
    Thanks! I will create a create a separate question with the pictures and slightly revise this one to keep it useful for others. Commented Feb 25, 2022 at 1:27
  • 1
    I tried to give as much detail as possible here: diy.stackexchange.com/questions/245322/… Commented Feb 25, 2022 at 2:05

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