Per and Kari: what hot water really costs
July 3, 2026
Per switches off and drains the hot water tank before every cabin trip, just as recommended. Even so, the hot water costs him over 30 kWh per trip, before anyone has had a shower. Kari has switched to a COAX water heater and avoids the whole calculation.
Per does everything by the book
Per wants to save electricity. Before the family heads off to the cabin, he switches off the power to the hot water tank at home and drains it. That is the right thing to do: a tank that is switched off should also be drained, because standing, lukewarm water is an excellent breeding ground for .
The bill arrives when he lets himself back in on Sunday evening. A typical tank holds 200 litres and has a 2000 watt heating element. Heating 200 litres of cold water up to tank temperature takes around 7 hours:
7 hours x 2 kW = approx. 14 kWhCoax, own calculation
Fourteen kilowatt hours, before anyone has drawn a single drop of hot water. On top of that, the family waits for hours for the water to get hot.
At the cabin it costs more
There is a similar tank at the cabin. It was switched off when they last left, but not drained. In that case it is not enough simply to heat the water up again: since the water has been standing still and lukewarm, the tank should be brought up to a high temperature (around 80 degrees) and held there for a few hours before anyone showers, so that the entire volume is heated through.
- Heating 200 litres: approx. 14 kWh
- Extra heating and heating through before showering: 6–8 kWh
The cabin alone therefore costs around 20 kWh, and with the tank at home on top of that the family is over 30 kWh per trip. And again: waiting time. First 7 hours of heating, then several more hours of heating through before the shower is ready.
If Per instead leaves the tanks switched on while they are away, he avoids the waiting time, but pays 1–2 kWh per day per tank in instead. Whatever he chooses, it costs.
Kari has done away with the tank
Kari has installed a COAX water heater, both at home and at the cabin. It does not store hot water, it makes hot water the moment the tap is opened.
That makes Per's whole calculation disappear:
- Nothing to switch off, drain or heat up again: 0 kWh
- No bacteria risk from standing water, the water is always fresh
- A shower the moment she arrives, with no waiting
- Never runs out of hot water
The only thing Kari pays for is the water she actually uses. A normal shower at 7 litres per minute at 40 degrees uses roughly 1 kWh per 4 minutes of shower time.
The figures side by side
| Per with a hot water tank | Kari with COAX | |
|---|---|---|
| Electricity before the first shower at the cabin | approx. 20 kWh | 0 kWh |
| Electricity at home after the trip | approx. 14 kWh | 0 kWh |
| Waiting time before showering | many hours | none |
| Standby consumption between trips | 1–2 kWh per day if the tank is left on | 0 kWh |
| 4 minute shower | included, until the tank is empty | approx. 1 kWh |
The calculation is simplified: it assumes a 200 litre tank with a 2 kW element and cold water at around 8 degrees. The point holds regardless: a tank uses electricity to store hot water. A COAX uses electricity only when you draw it.
What does that mean for you?
If you have a cabin or holiday home you travel to and from, the gain is greatest: no heating on arrival, no bacteria risk from standing water and a shower from the first minute. See the models or find the right size with the choose a model guide. If you want to work out your own consumption, try the consumption calculator.