water heaters size

The size of the old unit

If you are replacing a water heater, the size of the old unit can be a good starting point. See its capacity (gallons, liters…) by looking at its Energy Guide yellow sticker, or its nameplate.

If the older unit meets your needs, replacing it with one of the same size is probably a good bet. If you need a different capacity – because of a remodel project associated to more occupants or to the installation of new appliances or for any other reason – taking into account the capacity of your existing unit is still a good starting point.

Below, we list other means and some details on sizing.

Storage tank heaters/ heat pump hot water heaters  sizing

The capacity of gas storage water heaters (including heat pump water heaters) is based on the number of gallons/liters that fit in their tank.

A common rule of thumb to assess it, points to a 30 to 40 gallon gas tank heater for a 2-person household, a 40 to 50 gallon tank for a 3/4-person household and a 50 to 80 gallon model for households with higher needs.

Estimating your peak hour demand

But the water heater tank capacity isn’t all. Another different and more accurate estimation of your needs will take into account the amount of water the model can heat in one hour (First Hour Rating, or Peak Hour Demand: PHD) - something that you can find in the Energy Guide sticker (gas heaters have a greater PHD than electric units: a reason why they demand smaller tanks…)

In other words: compare the Peak Hour Demand (PHD) in the Energy Guide sticker of the water heater your intend to buy, estimate your own PHD, and buy accordingly...

Estimating your Peak Hour Demand for tank water heaters

The US Department of Energy estimates the following average number of gallons of hot water for different tasks:

Washing machine: 32 gallons;
Shower: 20 gallons;
Shaving 2 gallons;
Shampoo: 4 gallons;
Hand and face wash: 4 gallons;
Dishwasher: 14 gallons (dish wash by hand: 4 gallons);
Food preparation: 5 gallons.

You may consider the figures above to get your Peak Hour Demand: just multiply them by the amount of times they occur in the peak hour.

Un example: suppose a 3-people household with all its 3 occupants taking morning showers; suppose they run the dishwasher and have food preparation at that same hour... In that case we will get a  grand total of... 3 x 20 +14 +5, that is, a grand total around 80 gallons. This is the Peak Hour Demand of the household, and the approximate size required for their storage tank water heater.

Tankless water heaters

Tankless models capacity can’t obviously be based on the number of gallons/liters that fit in a tank, since the tank doesn't exist. So we have to consider another method: one based in the number of Gallons/liters of hot water they Produce per Minute, or GPM...

Supposing that your maximum simultaneous needs correspond to 2 showers, and to a washing machine and a dishwasher going at once, than you may estimate the GPM that corresponds to that situation as: 2 x 2.5 + 3.3 + 1.3 = 9.6 GPM.

To make such estimates you can take into account some average figures (also from Energy Star): Shower/Bathtub: 2.5 GPM; Washing machine: 3.3 GPM; Kitchen/Bathroom Sink: 2.2 GPM; Dishwasher; 1.3 GPM

Other ways

Contractors, retail salespersons or even manufacturers can help you decide about the size of your hot water heater. The web sites of some manufacturers also provide information about water heaters sizing, linking it with the number of bathrooms and the number of occupants… or other information.