Most home electrification journeys begin with replacing a gas water heater with an electric one. Typically, that means installing a Heat Pump Water Heater (HPWH). And for good reason—they’re efficient, widely available, qualify for rebates, and are relatively simple to install. Plus, they don’t take up much space and offer an easy entry point for homeowners looking to decarbonize.

For many households, especially smaller ones with moderate hot water usage, HPWHs are a solid solution. But if you're in a larger home, have high hot water demand, and are truly thinking about future-proofing your home while optimizing energy efficiency and lowering lifetime operating costs, it's worth looking at a more powerful, more flexible alternative: the Air-to-Water Heat Pump (ATWHP).

Let’s break it down.

What's a BTU, and Why Should You Care?

BTU stands for British Thermal Unit, a measurement of energy. One BTU is the amount of energy required to raise the temperature of one pound of water by one degree Fahrenheit. Understanding BTUs gives you a real-world grip on how heating systems compare.

In practice:

1 gallon of water = 8.34 pounds

Raising 60 gallons of water from 50°F to 120°F requires:
60 gallons × 70°F × 8.34 BTUs/pound/°F = 35,028 BTUs

That’s what your water heater needs to deliver to restore a full tank of hot water.

HPWHs: Great for Many Homes, But With Limitations

A typical HPWH produces around 4,000-6,000 BTUs per hour in heat pump mode (models vary). So let’s use a conservative 5,000 BTU/hr for our calculation:

35,028 BTUs ÷ 5,000 BTUs/hr ≈ 7 hours

Now, in reality, you don't need to wait for full tank recovery to get hot water—tank stratification means hot water rises to the top and becomes available sooner. But even partial recovery can become a bottleneck. Throw in a few showers, a dishwasher cycle, and a load of laundry… and suddenly, you’re out of hot water before you’re out of things to wash.

To compensate for high-demand periods, most HPWHs include a backup resistance heating element. This kicks in when the heat pump can’t keep up. But there’s a trade-off: resistance heat is wildly inefficient. Its COP (Coefficient of Performance) is 1, meaning every kWh of electricity gives you just one kWh of heat. By contrast, when the HPWH is operating in heat pump mode, the COP typically ranges from 2.5 to 3.5. That means you're getting 2.5 to 3.5 units of heat for every unit of electricity. Falling back on resistance heating erases that efficiency advantage in an instant.

And it gets pricey:

A 4,500-watt resistance element running for 3 hours = 13.5 kWh

At $0.15/kWh, that’s $2.03 per use

Use that daily and you’re spending an extra $60 a month on your “efficient” water heater. For households with predictable and moderate hot water usage patterns, that might not be a big deal. But for anyone who doesn’t want to worry about running out of hot water, such as large families, frequent entertainers, or people who value the peace of mind that comes with always having hot water, this becomes a real limitation. Many people try to solve this by installing tankless gas water heaters. But that option comes with a significant environmental cost and keeps you tied to fossil fuels.

ATWHPs: Built for Real Life

Air-to-Water Heat Pumps are designed for bigger loads and more demanding scenarios. Even a modest 3-ton system delivers 36,000 BTUs/hr, 6-9 times more than a typical HPWH. Back to our 60-gallon tank:

35,028 BTUs ÷ 36,000 BTUs/hr ≈ 0.97 hours

You’re fully reheated in under an hour and without resistance backup. Larger systems recover even faster. More importantly, ATWHPs can handle space heating and cooling too. With one piece of equipment, you’ve got a whole-home thermal solution.

Need an Analogy?

A HPWH is like a commuter bike: solid, reliable, but a bit slow. An ATWHP is an electric SUV: it hauls more, moves faster, and handles whatever you throw at it. You don’t want to rely on a bike for a cross-country road trip, and you don’t want to rely on a HPWH for a house full of people on a busy weekday morning.

Smarter Heat, Smarter Grid

Add a thermal storage tank (also called a buffer tank) to your ATWHP setup and things get really clever. You can preheat water when electricity is cheaper, like at night or during sunny hours, and save it for later. It’s thermal energy storage that works a lot like a battery, just simpler and more durable.

This isn’t only about comfort. It’s also about resilience. Shifting when you use energy helps the grid, lowers peak demand, and makes your home part of the clean energy future.

By the Numbers

*Full recovery time; usable hot water available sooner due to stratification

Final Take

If you’re looking for the cost-effective way to ditch gas, a HPWH works great. But if you’re planning for the next 10–20 years and want consistent, high-performance comfort, ATWHPs like Aris are in a different class.

More importantly, Aris tackles the bigger challenge. Our hydronic platform replaces your furnace, air conditioner, and old water heater with a single, smart package that offers maximum performance and efficiency.

Running out of hot water is annoying. Let’s think bigger.