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Sizing A Solar Water Heater

Solar water heaters are basically conventional water heaters with solar panels. So sizing of solar panels is the same as any normal water heater.

STEP 1: Get an idea of how much water your family uses
Regardless of which type of water heater is used, you should start with having a look at your family's lifestyle to get an idea of your average water usage.
- Baths: How many bathrooms are in the house?
- Showers: How many showers are in the house and how many showerheads, body sprays and side sprays are in each shower? How much water do they use? Standard showerheads have a flow rate of 2.5 gallons per minute, although new water-efficient showerheads have a lower flow rates. Most people are comfortable showering in water temperatures around 102°F to 106°F.
- Schedules: What is the typical bathing and bathroom use schedule in the home? How many occupants are likely to be bathing simultaneously? Is there a "shower rush hour" in the morning or night?
- Bathtubs: How many bathtubs and whirlpools are in the house? How many gallons are needed to fill each to capacity? Note: While small tubs are usually about 40 gallons, deep soaking tubs can hold up to 140 gallons. As with showering, remember, most people bathe at temperatures between 102°F to 106°F.
- Other hot water appliances: Are any other hot water appliances in use at the same time? If so, these need to be calculated also, e.g., dishwasher, hot-water laundry, kitchen use, etc. Most Americans are accustomed to staggering hot water use, so it is atypical to find a home where multiple hot water appliances are needed at the same time.
- How much hot water is needed to deliver the experience you want in their bathroom remodel? For example, is there enough hot water to fill a deep soaking whirlpool or to operate a vertical spa-type shower for any length of time?

Establish peak demand, measured in gallons per hour (gph). Then evaluate tank water heaters on the same gph basis to determine how many gallons of storage are needed to meet this demand.
While tankless water heaters do not run out of hot water, if not sized correctly, the flow rate of that water can be adversely impacted. The temperature of the shower will remain the same, but flow could slow to a trickle. So the first step in sizing tankless water heaters is to add up all the flow rates of showerheads, faucets and appliances that are likely to be in use at the same time.

The peak hour demand capacity or First Hour Rating is more important. This information can be found on the Energy Guide label or brochure. First Hour Rating is an indicator of how much hot water, one unit can provide. For natural gas, for example, first hour ratings range is from 40 to over 100 gallons. It is shown in gallons and is the result of the combination of the amount of usable hot water stored in the tank and how many gallons of usable hot water, the heater can generate in one hour of operation. Example: If 40-gallon natural gas heater stores 31 gallons of usable hot water and it delivers 41 gallons of usable hot water in the first hour, its First Hour Rating is 72 gallons. High First Hour Rating allows you to buy a smaller model as it will result in good performance even during the peak time and high demand while minimizing the cycle and standby losses.

The most used tank size (based on the common family size of four to six) and general rule for water heater sizing are:

30 gallons WH - enough for one bath residential homes or apartments.
40 gallons WH - for two bath residential homes or two applications.
50 gallons WH - for three bath residential homes or three applications.

The above is the general guideline but if want to use the electrical heater for example, for family of four, and you live in a home with two full bathrooms, and using washing machine and dishwasher you should be within 50 to 80 gallons tank capacity range and around 5.5 kW of power produced by heating elements. For the same requirements, and if using a gas, recommendation is to use a 50-gallon tank with 40,000 BTU/hr gas input.

Here are the assumptions you should also consider for water heater sizing:

Electric heater is considered to have close to the 100% recovery efficiency due to its immersion installation.
Gas heaters are considered to have 75% recovery efficiency, which means that 75% of the total heat produced by the burner is absorbed by the water in the tank. 25% is energy waste.
Keep also in mind that around 70% of the hot water is drawn before dilution by the incoming cold water. For example, 50 gallon tank will deliver 35 gallons of usable hot water.

Determining if a water heater is large enough for a bathtub is actually pretty simple; the water heater tank should be about 2/3 the size of the bathtub.  For example, if you have a 40 gallon water heater, it would be just large enough for a 60 gallon bathtub.   A very small bathtub might hold 40 gallons, while a larger single person bathtub could easily hold 100 gallons or more.

Average gallons of hot water per use:

hand dishwashing------------6
Automatic Dishwasher-------15
Shower or Bath--------------20
Clothes Washer--------------30