The "combi boiler" covers a host of units, my late mothers gas boiler was stored hot water, but you could turn the store off and have instant hot water if you pressed the "Eco" button on the boiler, and the amount of water stored would just about fill a bowl.
But with oil boilers, they tend to be floor mounted, and the boiler can't modulate, at least not enough to have instant hot water, so there is always a water store built into the boiler.
Electric showers were fitted when we moved in, they would not have been my selection, but to be fair they work very well, I find I use the electric in a different way to the hot/cold water thermostatic mixer valve shower. I knew the mixer shower took time to get hot, and stabilise the temperature, so I would turn on, and step back and wait for it to stabilise, we had rather a large shower rose, as if the flow was reduced too much it would go cold, I would get wet, step back and soap up, then step under it again to wash it off, and we likely used a good few gallons of water.
But the electric shower takes seconds to warm up, it has a smaller rose so uses less water with same pressure as the thermostatic valve type, and I turn it off to soap up, as the shower cubical is much smaller that the wet room we used before, then I turn it back on to wash off the soap, and the shower head can be hand held, if I had tried doing that in the wet room would have a snaking shower head as that much extra water was used. So likely cheaper using electric as so little water used compared with the one in the wet room. And electric far faster, likely used as much water as the electric just waiting for temperature to stabilise.
I have used a mixer tap shower where there was a circulation pump, so no wait for hot water to arrive, yes very good, but the delay getting hot water in my three story house is a bit of a problem, would I return to a mixer shower, yes I would, they are better showers, but would I pay out a fortune to do it, no, electric works fine.
But I was rather surprised when I had solar fitted, and with it an iboost+ that heats the DHW with an immersion heater using excess solar. The surprise was how much energy used to heat DHW, the iboost+ records how much power as it says "saved" today, yesterday, and last 7 days, since was using oil boiler and in summer it needed to run 4 times a week half an hour a time, know we used around 25 kWh of power to keep DHW warm, in fact a bit too cool, but using electric was using less than 3 kWh a week, remember not running showers, only hand washing. So it would have been cheaper even before I had solar to have used electric.
My electric shower is not very big, I note on the solar software draws around 3.5 kW from the DNO the rest from solar or battery. So likely only around 8 kW, we do have two electric showers, and a 60 amp DNO supply, never blown the DNO fuse, and looking at solar software unlikely it ever would.
The smart meter is a bit useless showing usage, bar graph on phone app, but solar shows far better history. Last time shower used had run out of solar and battery so this
shows the load and power used with an electric shower. And my daughter travels to us to use our shower some 40 mile return trip, we hope to also see us, but clearly the shower she feels is better than her bath.
