Electric water heater: 45 kWh a week, measured by one user
A user measured 45 kWh a week from his electric water heater in winter, with a bimonthly bill of 280 euros. The debate: program it or switch back to gas.
English · Original discussion in Spanish · Published
The electric water heater that sends the bill soaring: 45 kWh a week
A four-person flat, two adults and two children, all-electric, with a gas water heater that broke down. The decision seemed logical: cancel the gas supply and install an 80-litre digital programmable electric water heater. A year later, according to the user who started the thread, the bimonthly electricity bill in winter comes to around 280 euros, and the consumption meter he installed on the socket shows a figure that stings: 45 kWh in a single winter week. In summer, the same appliance drops to 7 kWh.
The figure is not an impression. It is a measurement taken with a dedicated meter, the kind you plug in between the socket and the heater, which also lets you enter the price per kilowatt-hour. The thermostat setting makes all the difference: 70 degrees in winter, 40 in summer. And a five-minute shower, without turning off the tap while lathering up, takes the tank from 70 to 40 degrees. That is where the problem begins.
How much does an electric water heater use on standby?
The first instinct was to switch it off. The second was to find out that switching it off costs more. With the heater on and at rest, the consumption measured over seven night-time hours was 0.7 kWh. With the water at 65 degrees, the temperature drops about 3 or 4 degrees per hour, and the appliance switches on every time it falls five degrees below the setting. That night, the meter read 0.6 kWh. With the setting at 50 degrees, the same period came to 0.3 kWh.
Physics allows no spin. Heating water is proportional to the quantity and the temperature rise: bringing mains water up to 65 degrees costs around 50% more than heating it to 45. Keeping it hot is proportional to the difference with the ambient temperature. That is why lowering the setting from 70 to 60 degrees, or programming the heater only for shower hours, is the most commonly repeated lever. A calculation circulating in the discussion itself points out that the real saving lies in not heating water that will not be used.
Programming the water heater: off or at low temperature?
The accumulated experience points to a specific routine. In the evening, from 19:00 to 22:00, water at 65 degrees so the children's showers and the couple's last. In the morning, a short window at 55 degrees for one shower. The rest of the day, low setting or off. With that configuration, night-time consumption plummets: from 0.64 kWh with the heater fixed at 65 degrees to 0.02 kWh with it programmed at 40 degrees. The difference is not in switching it off, it is in not keeping water hot that nobody is going to use.
The problem appears when the tank runs dry. One long shower, a hair wash, and the 80 litres are not enough. Recovering the temperature from scratch costs: 0.67 kWh to get back to 40 degrees after heavy use. The conclusion that prevails is that the water heater is not expensive because it is on, but because it heats water that then cools down unused.
The thermostatic valve and other solutions
The mixing valve at the heater outlet is the most frequently repeated technical recommendation. It costs between 30 and 50 euros, is mechanical, has two inlets and one outlet, and delivers water already mixed at the desired temperature. The logic is simple: heat losses in the pipework are proportional to the water temperature. If the heater is at 65 degrees and the valve delivers at 38, less heat is lost along the way than if the water leaves at 65 and is mixed at the tap. It is not a thermostatic tap: it is a fixed piece at the tank outlet.
Some have made the leap to a heat pump. A water heater with an integrated heat pump, 80 litres, heats the water to a maximum of 55 degrees and consumes a fraction of what a resistance element uses. The difference in consumption between modes is significant, and two daily programmes are enough for two showers. The noise of the compressor, a slight hum, is the price to pay.
Butane, natural gas or back to square one
Returning to gas is the way out several have taken. One participant says an automatic 11-litre butane water heater has cut the bimonthly bill by 50 euros compared with the same period last year. With natural gas, another user says the bimonthly cost of hot water stays at 34 euros for three people. There are even gas water heaters with a tank, compatible with butane, propane or natural gas, that take up 10 centimetres and are room-sealed.
The comparison is uncomfortable. The electric water heater, with all its insulation and programming, cannot compete with gas in cost per useful kilowatt-hour. Electricity remains the most expensive energy for heating water, and the only way to offset it is to reduce consumption to a minimum: less temperature, fewer hours, fewer litres of hot water left to go cold.
The full calculation, with the night-by-night measurements, the thermostat settings and the recovery consumption, leaves a conclusion that does not quite add up: the water heater uses little at rest and a lot when it works. The question is whether it is worth keeping it hot to save a few cents, or leaving it cold and paying for the recovery. Nobody has yet measured the answer precisely.
A four-person flat, two adults and two children, all-electric, with a gas water heater that broke down. The decision seemed logical: cancel the gas supply and install an 80-litre digital programmable electric water heater. A year later, according to the user who started the thread, the bimonthly electricity bill in winter comes to around 280 euros, and the consumption meter he installed on the socket shows a figure that stings: 45 kWh in a single winter week. In summer, the same appliance drops to 7 kWh.
The figure is not an impression. It is a measurement taken with a dedicated meter, the kind you plug in between the socket and the heater, which also lets you enter the price per kilowatt-hour. The thermostat setting makes all the difference: 70 degrees in winter, 40 in summer. And a five-minute shower, without turning off the tap while lathering up, takes the tank from 70 to 40 degrees. That is where the problem begins.
How much does an electric water heater use on standby?
The first instinct was to switch it off. The second was to find out that switching it off costs more. With the heater on and at rest, the consumption measured over seven night-time hours was 0.7 kWh. With the water at 65 degrees, the temperature drops about 3 or 4 degrees per hour, and the appliance switches on every time it falls five degrees below the setting. That night, the meter read 0.6 kWh. With the setting at 50 degrees, the same period came to 0.3 kWh.
Physics allows no spin. Heating water is proportional to the quantity and the temperature rise: bringing mains water up to 65 degrees costs around 50% more than heating it to 45. Keeping it hot is proportional to the difference with the ambient temperature. That is why lowering the setting from 70 to 60 degrees, or programming the heater only for shower hours, is the most commonly repeated lever. A calculation circulating in the discussion itself points out that the real saving lies in not heating water that will not be used.
Programming the water heater: off or at low temperature?
The accumulated experience points to a specific routine. In the evening, from 19:00 to 22:00, water at 65 degrees so the children's showers and the couple's last. In the morning, a short window at 55 degrees for one shower. The rest of the day, low setting or off. With that configuration, night-time consumption plummets: from 0.64 kWh with the heater fixed at 65 degrees to 0.02 kWh with it programmed at 40 degrees. The difference is not in switching it off, it is in not keeping water hot that nobody is going to use.
The problem appears when the tank runs dry. One long shower, a hair wash, and the 80 litres are not enough. Recovering the temperature from scratch costs: 0.67 kWh to get back to 40 degrees after heavy use. The conclusion that prevails is that the water heater is not expensive because it is on, but because it heats water that then cools down unused.
The thermostatic valve and other solutions
The mixing valve at the heater outlet is the most frequently repeated technical recommendation. It costs between 30 and 50 euros, is mechanical, has two inlets and one outlet, and delivers water already mixed at the desired temperature. The logic is simple: heat losses in the pipework are proportional to the water temperature. If the heater is at 65 degrees and the valve delivers at 38, less heat is lost along the way than if the water leaves at 65 and is mixed at the tap. It is not a thermostatic tap: it is a fixed piece at the tank outlet.
Some have made the leap to a heat pump. A water heater with an integrated heat pump, 80 litres, heats the water to a maximum of 55 degrees and consumes a fraction of what a resistance element uses. The difference in consumption between modes is significant, and two daily programmes are enough for two showers. The noise of the compressor, a slight hum, is the price to pay.
Butane, natural gas or back to square one
Returning to gas is the way out several have taken. One participant says an automatic 11-litre butane water heater has cut the bimonthly bill by 50 euros compared with the same period last year. With natural gas, another user says the bimonthly cost of hot water stays at 34 euros for three people. There are even gas water heaters with a tank, compatible with butane, propane or natural gas, that take up 10 centimetres and are room-sealed.
The comparison is uncomfortable. The electric water heater, with all its insulation and programming, cannot compete with gas in cost per useful kilowatt-hour. Electricity remains the most expensive energy for heating water, and the only way to offset it is to reduce consumption to a minimum: less temperature, fewer hours, fewer litres of hot water left to go cold.
The full calculation, with the night-by-night measurements, the thermostat settings and the recovery consumption, leaves a conclusion that does not quite add up: the water heater uses little at rest and a lot when it works. The question is whether it is worth keeping it hot to save a few cents, or leaving it cold and paying for the recovery. Nobody has yet measured the answer precisely.
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Summary of a discussion on Burbuja.info - Foro de economía, actualidad y política., translated from Spanish and reviewed before publication. Read the full discussion (37 replies).
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