heater
Just moved in a new house. All heaters have a plank (same width and depth as the heater) 10cm above them. Please see the picture below:
I conjecture it's for preventing some of the heat to go directly up and stay a bit in the room at middle height.
Is that correct?
Almost all wire you can buy will be rated at 600 volts, so you won't have to worry about voltage when sizing wires. What really matters is current. Your wires will be sized to carry enough current for the load, and your breaker will be sized based on the wire used.
When installing electrical devices, it's always a good idea to check the manufacturers installation instructions (PDF). In this case, it has this to say...
As the table shows, you can connect up to 24 amperes worth of heaters to a single circuit. However, if you do this, you'll have to use 10 AWG conductors and a 30 amp breaker. If you only want to use 14 AWg, you can only put 12 amperes worth of heaters on a circuit.
Since these are 240 volt heaters, you'll only need two conductors (plus a grounding conductor). If you're going to use nonmetallic sheathed cable (Type NM), you'll need AWG/2 with ground, not AWG/3 with ground cable.
You can get a 2000w electric heater for like $50. If storage heaters make no sense right now, don't get one, and buy a plain electric heater instead -- it's not like you're making a huge capital investment.
If you want to make a capital investment, and storage isn't a requirement for you, you can consider heat pumps. They are much more efficient than thermal heaters, since they are only moving heat, not creating it. However, they shut down in very cold weather, so heat pumps need auxiliary heaters, usually electric. If that is a millivolt gas system, the house will heat without electricity.
The usual objection is people don't want to ruin their ceiling with bulky ducts. The answer to that is the "mini-split" (surely there must be a noun here), which is a heat pump with one outside unit and several "registers" which can be controlled separately.
This uses two variations on common heat pumps. The idea is that if the heat is being interchanged with media that is closer to the desired temperature, the heat pump is vastly more efficient.
First, some heat pumps interchange not with air, but with a fluid of some kind. Ground-sourced heat pumps interchange their heat with underground coolant loops cooled/warmed with earth deep underground, which tends to be of moderate temperature. This can also be done with well water. Large facility heat pumps interchange heat with facility service water, which is pumped around the facility. The service water is cooled or warmed at a central boiler room.
With a storage heat pump system, you have a storage tank of a good thermal storage fluid (like water) which you preheat or pre-chill using cheap evening power. Then by day, your heat pump draws efficiently from this tank.
These systems are not common, or not cheap.
Best Answer
Our old apartment had this. I asked the landlord why, and he simply stated that if it wasn't there, all the vertical space above the heater would be wasted. Now there's a shelf there. I found it handy to place hats and gloves there during the winter to dry them off faster.