If you search the net, you will find several different water heater
sizing guides. That's because there is not a one size fits all sizing
Sizing a water heater to fit your needs is not hard to do. You do not
want to heat a lot of water that you don't need.
On the other hand you need to be sure that you have enough hot water
for you and your family.
One simple equation is to figure ten gallons for every family member
starting at thirty gallons.
30 gallon - three people.
40 gallon - four people.
50 gallon - five people.
The above is a general rule. A family of five or six can survive very well
with a thirty gallon water heater if they space out their needs.
A family of two may be disappointed with a thirty gallon tank if they
tried to take long showers back to back.
The chart below will give you a general idea of how much hot
water you would use for a given activity.
Average gallons of hot water per use:
Using the chart above as a guide, estimate how many gallons of
hot water will you need during a peak hour.
Add anything that you may use hot water for that's not on the list that you
would want to use during the peak hour.
Now that you know the maximum hot water usage per hour that you need
for your home, you'll need to purchase a water heater that will cover
New storage or tank type water heaters have an energy guide label.
Look at the top left corner of the label and you will see the "first hour rating."
This is how much hot water the heater will produce in one hour starting
with a full tank of hot water (see pic below).
Use this label when water heater sizing.
This label is on a 40 gal. electric water heater.
As you can see the first hour rating is 40 gal.
First hour ratings are depending on heat source, size of heat source and capacity.
Other 40 gal. electric water heaters may have a higher or lower first hour rating than the one in the pic.
You'll want a water heaters "first hour rating" to match your peak hour usage from the chart.
I'm sure you noticed after looking at the chart that its possible to space out your hot water needs and save on your energy bill by purchasing a smaller capacity water heater. After all, it cost much more to heat 50 gallons of water than 30 gallons.
Water heater sizing is not an exact science. Always allow yourself a little lead way.Its better to have a little extra hot water than not enough.