Calculators / Water heaters
Can your panel handle an electric water heater?
Pick the water heater and tell us about your panel and the big things in the house. You get the breakers and wire it needs and whether your service has room.
Updated · How we check our numbers
Your panel can take it
Three 40 A double-pole, 3 runs of 8 AWG
The tankless, 27 kw draws 113 A. With it, your house load comes to about 115 A of your 200 A panel.
- Draws
- 113 A27.0 kW at 240 V
- Breakers
- 3 × 40 A double-pole
- House load with it
- 115 Aof 200 A
- Left over
- 85 A
Estimate for planning only, not professional advice. Check the manufacturer’s instructions, your local code and a licensed professional before you buy, build or install. Use at your own risk. Terms.
Running costHeat pump vs electric vs gas, a year
Tankless, 27 kW: 27.0 kW ÷ 240 V = 112.5 A; Rheem RTEX-27: three 40 A double-pole, 3 runs of 8 AWG
lights and plugs = 1800 sq ft × 3 VA = 5.4 kVA
house loads = 5.4 kVA + kitchen and laundry circuits 4.5 kVA + appliances 7.5 kVA + tankless, 27 kw 27 kVA = 44.4 kVA
counted = 8 kVA + (44.4 kVA − 8 kVA) × 40% = 22.6 kVA
with heat and AC at 100% = 22.6 kVA + 5 kVA = 27.6 kVA
load = 27.6 kVA ÷ 240 V = 115 A of your 200 A panel
before = 70 A; with it = 115 A
Every type on your panel
220.83 load check- Electric tank, 4,500 W18.8 A; one 30 A double-pole (Bradford White allows 25 A), 10 AWG123 A left
- Heat pump water heater (240 V)18.8 A; one 30 A double-pole, 10 AWG123 A left
- Tankless, 13 kW (point of use)54 A; one 60 A double-pole, 6 AWG109 A left
- Tankless, 18 kW75 A; two 40 A double-pole, 2 runs of 8 AWG100 A left
- Tankless, 24 kW100 A; three 40 A double-pole, 3 runs of 8 AWG90 A left
- Tankless, 27 kW113 A; three 40 A double-pole, 3 runs of 8 AWG85 A left
- EcoSmart ECO 27113 A; three 40 A double-pole, 3 runs of 8 AWG85 A left
- Stiebel Eltron Tempra 36 Plus150 A; three 50 A double-pole, 3 runs of 8/2 AWG70 A left
- Tankless, 36 kW150 A; four 40 A double-pole, 4 runs of 8 AWG70 A left
Count your empty spaces: three 40 A double-pole take 6 breaker slots.
113 A is a big share of any house's service. Your utility's service wire and meter have limits too; ask an electrician before you buy.
Most towns want a permit for a new 240 V circuit. This is a planning check, not a load calculation stamped by an electrician.
How we work it out
The circuit. A standard electric tank and a heat pump water heater both have 4,500 W elements on 240 V. Rheem and A.O. Smith list a 30 A double-pole breaker; Bradford White lists 25 A with 10 AWG wire. Electric tankless units are a different scale: Rheem's 27 kW model draws 113 A and needs three 40 A double-pole breakers, each with its own 8 AWG run.
amps = watts ÷ 240 V
The panel. We add the heater to your house with the National Electrical Code's existing-dwelling method (220.83, 120.83 in the 2026 edition), the same check our hot tub and EV charger calculators use: 3 VA per sq ft for lights and plugs, the kitchen and laundry circuits, and every fixed appliance at nameplate, with the first 8 kVA counted in full and the rest at 40%. Heat and AC go on top at 100%. The water heater you have now comes out of the total.
house amps = counted load ÷ 240 V, compared with your main breaker
Under 10 A to spare, we call it tight. This is a planning check. An electrician's load calculation and the permit settle it.
Where the numbers come from
Models
Water heaters
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