Tank vs tankless water heater in Florida: the amperage problem

A tankless water heater heats water as it passes through, without storing it; a tank heater keeps a reservoir hot all day. Per the DOE, a tankless is 24-34% more efficient than a tank in homes that use 41 gallons or less of hot water a day, and only 8-14% in homes that use a lot. In Florida the detail almost nobody mentions is electrical: a whole-house electric tankless draws between 50 and 200 amps, and many panels cannot handle it without a service upgrade that costs more than the heater.

How does each type work?

• Tank (storage). A 40-80 gallon reservoir with electric elements or a gas burner that keeps water hot all day. Simple and cheap; loses heat through the tank walls (standby loss).
• Tankless (demand). Heats only when you open the tap, with very powerful elements or a large burner. No standby loss, but flow is limited to the unit’s gallons per minute.
• Heat pump (hybrid). A tank with a heat pump on top: moves heat from the air into the water. The DOE rates it 2-3 times more efficient than a resistance tank. Explained in heat pump water heater.
• Solar thermal. Roof collectors that preheat the water in a tank. Covered in solar water heating.

Comparison table for a Florida home

All figures are from the DOE; real operating cost depends on your electric or gas rate and how much hot water you use.

Why does an electric tankless collide with the panel?

Because doing in seconds what a tank does over hours takes a lot of instantaneous power. The DOE says it plainly: most electric tankless units require 208 or 240 V and 50 to 200 amps depending on size, and that load (50 A or more) requires evaluating the panel’s capacity so the unit does not interfere with other appliances. In a Florida home with central A/C, an electric dryer, range and oven, the panel may have no margin, and upgrading service is an electrician-and-utility job.

There is a second effect: peak demand. An electric tankless spikes instantaneous draw exactly when you shower, and even if total kWh drop, the current it demands is enormous. If you later add a battery or generator, that peak sizes the equipment.

Heads up: the tankless is sold as “unlimited hot water and savings.” The first is flow, not volume: two showers plus the dishwasher drop the temperature. The second is true mainly if you use little water; the DOE cuts it to 8-14% in high-use homes. And the efficiency gain of an electric tankless is smaller than a heat pump’s. When someone sells it without asking how many amps your panel has, they are not designing for your house.

When does a tankless make sense?

• You have natural gas or propane: a gas tankless avoids the amperage problem and lasts more than 20 years per the DOE.
• Space rules: apartment, small laundry room, house with no garage.
• You use little hot water (one or two people): that is where the DOE’s 24-34% lives.
• You already have a 200 A panel with margin measured by an electrician.
• You want point-of-use units: a small tankless under a distant sink avoids waiting and wasting water, without loading the panel like a whole-house unit.

When an electric tankless does NOT make sense

• A 100-150 A panel already loaded with central A/C and an electric kitchen: the upgrade can cost more than the heater.
• A family of four or more with simultaneous showers: flow splits and savings fall to 8-14%.
• You are going to install a heat pump water heater: more efficient and uses the existing circuit.
• You want the most savings per dollar in an all-electric Florida home: the heat pump wins on numbers.

What to check before you decide

• Your main panel’s amperage and how much is in use (an electrician calculates it in minutes).
• How many gallons of hot water your home uses: the DOE uses 41 gallons/day as the threshold for the large savings.
• Whether there is gas in the house: it changes the whole decision.
• Required flow in GPM: count showers and fixtures used at once, and incoming water temperature.
• Your utility’s rebates: in Florida, Duke Energy publishes one for heat pump water heaters with conditions. And if you are going solar, decide the heater before sizing: it is one of the 3 pillars.