For starters you could use the EPA tag on the water heater as an estimate. A lot depends on how many people shower, and how often you use dishwasher and washing machine also.

Thanks. It's not easy to find good information about this topic. From the information I gathered from multiple websites the typical usage seems to be somewhere between 2-4kWh/day for around 3 people. Obviously, there are a lot of factors incl. how many people (in the mentioned case it's for 3), the temperature of the water coming into the heater, how often you wash dishes and cloths, how long you take showers, etc. Also, the amount of energy used for heating water is somewhere around 15-20% of the total energy consumption. Of course, that again depends on a lot of factors. I guess the easiest way to find out would be to install a water flow meter as the hot water comes out of the water heater or measure the gas used by the water heater.

Measuring the amount of gas used is a good approach, if you can
do it. That was how this system was estimated. The gas can be
converted to kWh, multiply that by the gas furnace efficiency which
might be in the 80 to 95% range. Electric heat is virtually 100%
efficient, but a heat pump Coefficient Of Performance of around 2
could divide the electric energy needed in half. Bruce Roe

Where you are located has a lot to do with it. In the winter, there is no energy savings if you are heating the house. Heat pumps only transfer heat, you still have to make it. Summer is a bi bonus for cooling and dehumidifier. I have a HPWH in a cold basement and I don't live there in the summer. I do supplement my HPWH with some PV going to resistive element.

Shower heads in our house (zip 92026) have a flowrate of close to 1.25 G.P.M. my wife takes something like 10 minute showers. The H2O exiting H2O temp. for her shower head is ~ 113 F. The hot H2O temp. is mixed (tempered) at the tank outlet to 122F. The cold water mains temp. varies between ~ 50 F and 60 F. winter/summer.
Such conditions mean a shower requires ~ 6,500 and about 7,500 BTU.
The total water heating requirement at my house for 2 people is ~ 25,000 BTU/day including all hot H2O usage and tank standby and line losses.
That equals about 7.33 kWh of energy expressed in S.I. units.
If a heat pump were to be used, divide that load by the C.O.P. of the heat pump.
Of that 25,000 BTU/day, the tank standby and line losses amount to ~ 12,500 BTU/day or about half the total hot water energy demand.
That 50% standby loss figure is higher than a lot of ballpark estimates will tell you for the average home because while our standby losses are quite low, our usage is very low relative to the average household.

In terms of total H2O, we use about 70 gal/day of H2O.
Our average daily hot water usage is ~ 32 gal./day between showers. laundry, dishwasher and misc. uses.
That 7.33 kWh/day or ~ 25,000 BTU/day total water heating load is about 93% supplied by a solar thermal water heater.
If it's any help to you in estimating your usage, we seem to use about 1/3 as much H2O as our neighbors in the HOA and also about 1/3 as much total electricity.

Our total average daily electricity usage after being here for 18 years is 18.89 kWh/day, or 6899/365 days.
All of that load including the hot water load not met by the solar thermal flat plate collectors is supplied by PV.


Your results will vary.

In the case of a ground or air source heat pump the heat is already there so you do not have to make heat. Some air source heat pumps can transfer heat in temperatures below freezing.

We did that! We converted from an oil burning hot water tank that was failing over to an electric heat-pump hot water tank. We did it at a time just before our solar came online and I did not notice any increase in the grid power usage. We also upped the tank size from the conventional 30-gal oil burner to a 50 gal. I have also kept the new tank in "Eco" mode, meaning that it ONLY uses the heat pump for heating the water; not drawing from the grid for direct tank heating. The new tank, in HP-only mode is drawing less than 1kW when in operation.

After installing a grid tied solar system in 2011, I replaced my atmospheric vented gas WH with a Power Vent gas model. Power vent has a small motor attached to the venting system so that the vent can be installed thru a nearby wall. Since the WH will not operate unless it has electrical power I was able to install a timer to control when the WH operates. I then installed a stand alone heat pump and plumbed that device into the WH. I put that device on a timer also.

I wanted to maximize the usage of solar energy to heat my water so the heat pump only operates during day time, 9 to 5, but I also wanted to heat water in the early morning and late into the evening. So the power vent only operates from 5am to 9am and 5pm to 10pm. Neither device operates at the same time. At night neither device is powered and the house stays quiet although neither device make much noise. The advantage of separate devices means the failure of one does not negate the system.

IMO heating water with an electric WH does not make much sense if you have solar and gas available.

At my summer home I have a very minimal PV system and I heat domestic hot water with excess PV into a resistive heater. I have two small tanks. Tanks usually shut off by 10:30 am and wife can use the dishwasher. I have a third 40 gallon tank just for the laundry. That heats up only when every other need is fulfilled. I feed only hot water into the washer and all cycles use hot water. Clothes come out steaming. Resistive gets a bad rap. Solar community just throws away so much power.

Yes, I feed only hot water into my washer also after finding out that the new washers severely limit the amount of hot water into the mix. Plumbed the both the cold and hot inlets to hot water pipes only. Don't tell the government.....I'm sure I'm breaking some rule somewhere. lol

I have two LG washers. The one at the camp with all hot water has a soap dispenser that is spotless. The one at home plumbed with cold water has mold buildup in the dispenser feed tube. I have to clean it out every couple years.

Well, that's not good! I have an LG upright washing machine and run it in Tub Clean mode once a month (with bleach, soap and softener in the dispenser), and once a year, I inspect the filters where the water goes into the unit. Never had an issue.

Go into many homes now and find the door to the washer standing open when not in use. Why?......to keep the smell down because the water is not hot enough to kill the smell producing organisms. It's a very good example of unintended consequences of government mandates and regulations.

That, too. We never close the door when the wash is done. Let's whatever water is in the drum evaporate. Does not take long since the heat-pump HW tank is in the same room and removes the humidity.