Short answer
For whole-house hot water in a Maryland house, gas tankless is the one that keeps up in winter: about 5 GPM on January WSSC water against about 3.2 GPM from the largest common 36 kW electric unit, which also needs 150 amps of dedicated service and costs roughly three times as much to run. Electric tankless earns its place at a single remote fixture; for an all-electric house, a heat pump water heater is usually the better whole-house answer.
Key takeaways
- On 45°F January WSSC water, a 199,000 BTU gas unit delivers about 5 GPM; a 36 kW electric unit about 3.2 GPM — one shower and a faucet.
- A 36 kW electric tankless unit draws 150 amps at 240 volts, which a 100-amp or original 150-amp service cannot spare.
- At round Maryland rates, electric tankless costs about $660 a year to run, against about $220 for gas tankless and $190 for a heat pump water heater.
- Electric tankless is the right tool at one remote fixture: a powder-room sink needs roughly 9 kW in January, not 36.
- In a house with no gas, the whole-house electric choice is a heat pump water heater, not electric tankless.
On this page
Electric tankless water heaters get quoted for one reason: they avoid the two most expensive parts of a gas tankless conversion, the larger gas line and the new vent. Nothing to run to the meter, nothing through the wall. In a Maryland house that saving usually reappears at the electrical panel, and the performance gap shows up the first cold week of January.
This guide assumes you have already decided on tankless. If you have not, start with tank vs. tankless in Maryland — for many households a tank is still the better answer.
Gas vs. electric tankless at a glance
| Condensing gas tankless | Whole-house electric tankless | |
|---|---|---|
| Heat output | About 189,000 BTU/h delivered | About 122,000 BTU/h at 36 kW |
| January flow on WSSC water | About 5.0 GPM | About 3.2 GPM |
| Supply needed | Larger gas line, about 199,000 BTU | About 150 amps at 240 V |
| Venting | PVC sidewall vent and condensate drain | None |
| Annual cost at illustration rates | About $224 | About $664 |
| Works in a power outage | No | No |
| Service | Annual descale, inlet screen, combustion check | Descale, inspect elements and connections |
| Best use | Whole-house hot water | One remote fixture |
The physics both units obey
Every tankless heater is limited by the same arithmetic: heating output divided by the temperature rise you ask for equals the flow you get. The U.S. Department of Energy’s sizing guidance gives the rough benchmark — a gas unit manages a 70°F rise at about 5 gallons a minute, an electric unit about 2 — and its tankless overview notes plainly that gas-fired units deliver higher flow rates than electric ones.
The reason is output. A whole-house condensing gas unit fires at up to about 199,000 BTU per hour. The largest common residential electric unit is 36 kW, which is about 123,000 BTU per hour. Same arithmetic, about two-thirds of the heat.
If you want to check a quote yourself, the formula is short: gallons per minute × temperature rise in °F × 500 = BTU per hour, and dividing BTU per hour by 3,412 gives kilowatts.
The winter inlet problem
WSSC water comes from the Potomac and Patuxent rivers, so it tracks the seasons. We size for an inlet near 45°F in January and about 70°F in late summer. Raising water to 120°F therefore takes a 75°F rise in winter and a 50°F rise in summer:
| Unit | Output | Summer (50°F rise) | January (75°F rise) |
|---|---|---|---|
| Condensing gas, 199,000 BTU | ~189,000 BTU/h delivered | ~7.5 GPM | ~5.0 GPM |
| Electric, 36 kW | ~122,000 BTU/h delivered | ~4.9 GPM | ~3.2 GPM |
| Electric, 27 kW | ~91,000 BTU/h delivered | ~3.6 GPM | ~2.4 GPM |
A current WaterSense shower runs about 2.0 GPM. In January a 36 kW electric unit covers one shower and a kitchen faucet, and the second shower runs cool. In August the same unit feels generous, which is why we hear “it was fine when we installed it.”
Maryland note: the Potomac-side water most of the county receives is hard — WSSC’s tap water analysis reports show plant averages around 7 to 9 grains per gallon — and electric elements scale just as gas heat exchangers do. Either type needs periodic descaling; neither is a fit-and-forget appliance on WSSC water.
The 150-amp question
A 36 kW unit at 240 volts draws 150 amps. A 27 kW unit draws about 113. These usually land on two or three dedicated double-pole breakers, and the panel has to carry that on top of everything else running on a January morning — which, in an all-electric house, includes a heat pump in defrost and its backup strips.
That is a problem for the housing stock we work in:
- Plenty of 1940s–60s houses in Silver Spring and Takoma Park still run 100-amp services. A whole-house electric tankless unit is not installable there without a service upgrade.
- Original 150-amp services in 1980s houses usually cannot spare 150 amps either once a load calculation is done.
- A service upgrade means a Montgomery County electrical permit and coordination with Pepco or BGE for the new service, and it often costs more than the gas line the electric unit was supposed to avoid.
Running cost
Electric tankless is close to 100% efficient at the appliance. The trouble is the price of the energy. Using the medium-use pattern from the federal efficiency test (55 gallons a day, heated 67°F) at round illustration rates of 20¢ per kWh and $1.90 per therm — substitute your own bills:
| Water heater | Typical UEF | Annual energy | Annual cost at illustration rates |
|---|---|---|---|
| Condensing gas tankless | 0.95 | ~118 therms | ~$224 |
| Electric tankless | 0.99 | ~3,320 kWh | ~$664 |
| Heat pump water heater | 3.5 | ~940 kWh | ~$188 |
Electric tankless costs about three times as much to run as gas tankless, and three and a half times as much as a heat pump water heater. If the reason to go electric is a house with no gas — much of Glen Hills and Travilah in Potomac — the electric option worth pricing is a heat pump water heater, covered in our heat pump versus gas comparison, not an electric tankless unit.
Propane tankless is the other option in those houses. It performs like natural gas tankless, but propane costs substantially more per delivered BTU, and the tank and regulator have to be sized for the unit at full fire. An undersized propane supply makes the unit drop out under load in a way that looks exactly like a control fault.
Where electric tankless is the right tool
Electric tankless is excellent at one job: a single fixture far from the main heater. A powder room at the far end of a long ranch, a basement bar sink, an accessory apartment over a garage. Sized for one fixture, the winter inlet problem disappears.
Run the formula for a powder-room sink at about 1 gallon a minute, raised from 45°F January water to a comfortable 105°F: 1 × 60 × 500 = 30,000 BTU per hour, or roughly 9 kW. A 0.5 GPM aerator halves that. That is a unit a dedicated circuit can carry without touching the service, and it removes both the long wait for hot water and the gallons poured down the drain while waiting.
Which one to choose
- Whole-house, gas available: condensing gas tankless, with the gas line sized for about 199,000 BTU and an annual descale. This is the only tankless option that carries a family through a Maryland January.
- Whole-house, no gas: a heat pump water heater, not electric tankless.
- One remote fixture: a point-of-use electric tankless unit.
- An existing tank that is failing: decide whether you are replacing like for like or changing type before the emergency, not during it. Our repair or replace guide covers how to buy that time.
How we quote it
A tankless installation quote from us shows the January flow your household needs, the gas line or electrical scope that delivers it, venting, permit and the annual service that keeps it there. Gas work in most of Montgomery County is permitted and inspected by WSSC Water and done by a WSSC-licensed master gasfitter. If you already have a tankless unit that runs cold in winter, a tankless repair visit separates a sizing limit from scale or a failing component. If the honest answer is a tank water heater, we will say so. Tell us what you have and what you want it to do.
Sources
- Sizing a New Water Heater U.S. Department of Energy, Energy Saver Tankless sizing by flow and temperature rise; typical 70°F rise at about 5 GPM for gas units and 2 GPM for electric.
- Tankless or Demand-Type Water Heaters U.S. Department of Energy, Energy Saver Gas-fired tankless units produce higher flow rates than electric; point-of-use applications.
- Tap Water Analysis Report WSSC Water Hardness data for Potomac and Patuxent plant water, which governs scaling on heating elements and heat exchangers alike.