I have an instantaneous gas water heater (around 15 years old) that I use for both hot water (drinking, showers, etc.) and underfloor heating. I’m currently working on creating a water dashboard to track various aspects of my water usage (well level, rainwater vs. city water usage, valve status, etc.). I’m also curious if it’s possible to estimate the energy (electricity and gas) used specifically for heating the water.
As an example, here are some screenshots showing shower usage around 5 PM and 7 PM, along with overall gas usage and electricity consumption for the heater.
The equation tells you how much energy is required to heat water. Like your topic title asks for.
You get the energy, E, used to heat the water, from the volume or mass of water and the temperature rise of that volume or mass. That is all that is required. It’s high school physics.
Forget the efficiency. That was a mistake. It is not relevant here.