What Size Water Heater Do I Need Calculator

What Size Water Heater Do I Need Calculator

Don’t base your decision on what size water heater to buy based solely on the number of people in your house.  Base it, for example, on whether you want to be able to do laundry and take showers at the same time. This calculator will help you estimate what size gas or electric water heater you should buy based on your actual needs; how much hot water would you like to have available at peak usage.

The calculator will also provide the generic “how many people in your household” answer so that you can compare the results.

Base your decision on what size water heater to buy on the amount of hot water that is used for one purpose before it is needed by another.  If you are doing laundry and also want to take a shower, you may run out of hot water before your shower is over.  Or if 2 people want to take showers one after the other while the dishwasher is running, will there be enough hot water left over for the second person?

The amount of hot water used varies greatly from household to household.  One household might only do all their laundry once a week and take a single 8 minute shower in the evening.  Another household might do laundry nightly and take several longer 12 minute showers each evening.

You should base the size of your water heater on how much hot water you want available at peak use.

How Long Does It take A Water Heater To Heat Back Up

After you’ve used all the hot water, you can expect to wait 30-40 minutes for a gas heater and one to 1 1/2 hours for an electric water heater to heat back up.

Because a water heater fills up with cold water as it is being used, it can only deliver 70% of its water capacity within 20 degrees of its thermostat setting.

How Much Hot Water Is Used For Different Activities?

This is where you start your calculation for picking a new water heater.

How Much Hot Water Do Different Appliances Use?

A dishwasher uses 7 gallons of hot water per load.
A top-loading washing machine uses 25 gallons of hot water per load.
A front-loading washing machine uses 15 gallons of hot water per load.
Shaving for 4 minutes uses 2 gallons of hot water.
Washing dishes by hand for 5 minutes uses 10 gallons of hot water.
About 15 gallons of hot water are used to fill a bathtub.

How Much Hot Water Does A Shower Use?

If your shower head has a flow rate of 2.5 GPM (as most newer ones do) and you normally take an 8 minute shower, you’ll use a total of 20 gallons of water (both hot and cold water).  If you’ve set the shower handles so that 50% cold water and 50% hot water is coming out, you’ll have used 10 gallons of hot water for that 8 minute shower.  The EPA says that most people use 40 gallons of water per shower, so that would mean 20 gallons of hot water are used per shower.

If you buy a 30 gallon water heater and two people want to take a shower after one another, the second person might have to wait as long as 1 1/2 hours before taking their shower.  The same applies if you want to do laundry and take a shower within the same hour using a 30 gallon water heater.

While most recommendations suggest buying a 30 gallon water heater for a household of 2 adults, that means you will never be able to do a load of laundry and shower in the same hour.  Nor will you be able to take 2 consecutive showers.

Residential water heaters come in sizes from 30 to 80 gallons.  If you want a water heater that can supply enough hot water to allow for normal family use, we recommend sizing up your water heater an extra 10 gallons beyond the basic recommendations.

What Size Of Water Heater Do You Need Per Family Size

This is the basic recommended water heater size based on household occupants.  It doesn’t take into consideration how much you need based on actual peak use.

  • 1-2 people need a 30 gallon gas or electric water heater
  • 2-3 people need a 40 gallon electric water heater or 30 gallon gas water heater
  • 3-4 people need a 40-50 gallon electric water heater or 30-40 gallon gas water heater
  • 5 or more people need a 50-80 electric gallon water heater or 40-65 gallon gas water heater

A study showed that 26% of an average home’s daily water consumption occurs during peak usage, with peak hours of 7am and again at 5pm.

If you find that you are routinely running out of hot water, you might consider also installing a tankless water heater.  These small units can be installed inside cabinets or on walls (the Rheem unit pictured below is only 12″ x 8″), and are capable of delivering hot water on-demand with a flow rate high enough to feed a shower or washing machine.

amazon tankless water heaeter


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