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A modest tankless takes 75 amps. Serious whole house tankless can draw 150 or even 180 amps. You need to start with a Load Calculation for your house. Those are surprisingly simple, you need

  • the square footage (this will be for a "catch-all" that covers almost all lights and 120V plug-in loads)
  • number of kitchen receptacle CIRCUITS ("2 or 3" is a correct answer, "8 receptacles" is a wrong answer and a waste of your time).
  • number of laundry room ditto ditto ditto (usually 0 or 1)
  • nameplate amps, VA or watts off of all 240V appliances and fixed-in-place 120V high-draw appliances:

The last one means built-in microwave, bathroom heater, dishwasher, disposal, central vac, furnace. NOT refrigerators or fans, they don't draw enough to matter). All are converted to VA (volt-amps). For resistive heaters, watts equals VA. if it specified amps, multiply its stated amps x its stated volts (e.g. 10 x 230 = 2300). Dryers are counted as 5500W unless you know them to be something else.

You don't just add them, they go through a worksheet. This does some "fudge factors" to account for the fact that they won't be all maxed out at once, e.g. reduces the electric oven/range number to reflect practical usage (not all burners are cycled on at once).

You also cross off any loads that won't run at the same time, using only the higher figure. (e.g. electric resistive heat vs. A/C unit. However, the air conditioner probably uses the furnace's air handler, and the air handler is like 95% of gas furnace power.
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