Instant Hot Water Tank Sizing Calculator

We’ve all been there: You’re standing at the kitchen sink waiting for the water to get hot. The water is ice-cold, then lukewarm, and then just okay. While you wait, gallon of water go down the drain.

This is why many folks consider investing in a point-of-use electric tank. These little buggers is mounted directly beside your fixture and do away with all that wasted water, and travel time.

How to Choose the Right Size for Your Water Heater

It sounds easy enough until you guess the size wrong and find yourself stuck with nothing but cold water (or your electric bill spikes during the night). It’s about finding a balance between how much hot water you need versus how fast you can recover it from cold.

If you have a big tank, then you’ve got lots of hot water, but when you draw it off, it take an eternity to warm back up. If you have a small tank, then it warms up lickity-split, but if someone in another part of the house flushes the toilet, then you’re out of hot water halfway through showering.

That’s where the calculator at the top come into play. Enter your personal details, such as how often you use water in this way. How much do you use per event? You input your target temperature and the starting temperature of the cold supply. It then crunches the numbers to find that sweet spot.

The secret variable that catches most DIYers out is temperature rise. They get caught up by thinking about the tank size but forget about the temperature of the water coming in to their tank. In a location where the groundwater remains close to forty degrees in winter, warming that up to a hundred twenty degrees is going to take much more energy than if the inlet water is already at sixty-five degrees.

That’s why the tool takes that difference (the difference between those two numbers) into account to show you just what wattage your recovery element has to shares to sustain that flow. And it’ll calculate the first-hour delivery (total amount of hot water you’ve got available before the tank runs dry). That’s much more important than the so-called nominal tank size since it factors in not only the volume being stored but also the water warmed during the course of an hour.

Question 3 is: How many fixtures do you have? Will multiple fixtures be running at once? It’s easy if it’s just a bathroom vanity, but what about a kitchen and someone washing dishes while the dishwasher starts up? You can multiply the draw with the simultaneous fixture input. Suddenly that same little tank good for a single user won’t cut it if both sink are drawing water.

By default, the reference table on the page outlines average draw rates by common fixtures so you’ll have a starting point for your input. While a handwash sink draws just a fraction of a gallon, a laundry or utility sink use can suck more than a gallon in a short time.

Another practical thought involves standby loss. The tank isn’t perfectly insulated. Over time heat will escape from the tank to the surrounding wall or cabinet space. This loss should of been accounted for with a reserve percentage added to the calculator. That way it doesn’t size the tank down to the very last watt, leaving no wiggle room for variation in usage habits.

For most residential applications a ten to fifteen percent reserve is generally fine. Then the water stays hot enough to cover any unexpected draw without wasting electricity constantly re-heating unused water every hour.

The power of your electric recovery elements matter, too, and I’m not just talking about their effectiveness. Depending on the size of your tank and what you use it for (like fast recovery versus a small point-of-use tank), an element could use as little as seven-hundred watts and work with your typical house circuit. Or, if you’re looking at a bigger unit with a quick recovery time, you might be looking at fifteen-hundred watts or higher, typically necessitating a special twenty-four volt circuit.

You don’t want to shell out big bucks for a retrofit when it’s easy enough to check your power load beforehand. With the calculator, you can input your desired recovery rate and see what wattage you’ll need.

In the end though, we’re looking for comfort while wasting nothing. When you turn on the tap, you want hot water right away, not after 30 seconds. Size the tank to match how much hot water you use. Give it enough kick to heat up that same amount quickly and you’ll never waste anything by waiting. It’s all about efficiency through precision.

The right sized tank will give you the hot water you need when you need it; no more, no less. Heat up a bathtub full of hot water that no one uses? That’s just wasting money. It is not efficient at all.