power is in watts.
watts are volts time amps.
so, with 120 volts and 10 amps, you get 1,200 watts.
with 220 volts and 5 amps, you get ... well, 1,200 watts
if you think of voltage as speed and amps as the size, then lots of fast ones will fill the bucket at the same rate as large slow ones.
now the 'thermostat' is really just a switch or a relay. however you want to call it.
the part that makes it a thermostat is that it has a bi-metal spring. one metal heat at one rate, and will bend the bi-metal part. this bending will push on the parts that close or open the switch part.
a relay is the same thing, only the activator is an electromagnetic coil.
a wall switch is the same thing, only you are the activator.
in each case the parts that allow power to pass are isolated from the part that does the actual actuation.
about those parts that pass the power. well, first off, you have the contacts. these are special bits of metal that are designed to undergo massive electrical sparking. every open and every close has sparking that occurs. with copper, it would heat the copper and then the copper would liquefy and cool, making a perfect welded joint.
those contacts are connected to larger bits of copper to give you a place to connect your wires.
more about those contact.....
if you turn an AC line off and on, like a light, a hair dryer or such, the load causes some electrical effec.
the light is a simple resistance heater. you close the circuit, the power passed, the heating element gets hot and glows.
your hair dryer has a motor. a motor has coils, the coils have to create a magnetic field. what happens is the coils could be thought of as a vacuum. the suck the ever living daylights out of the wire and cause a HUGE surge in power. so much that lights on the same circuit will dim. this is called in-rush current and if easily 5 times the current of the running motor. but, this is expected and is called a transient load. we only need to know it exists.
and lastly there is a DC load, car headlights or such. DC acts differently than AC on the contacts. it welds them more efficiently
why did I tell you this ? because ALL .relays are rated for a resistive load, and an inductive (coil) load and a DC load.
in your case, the H.P. stands for motor horsepower. this is the inductive load.
but, all that is moot and have no bearing on your ACTUAL question.
your un-asked question should have been :
" I have a baseboard heater in which the nameplate of that exact heater says, 1,200 watts, 120vac, 240vac"
"how to I select a relay"
more information that should be listed is the part number and a VERY simple wiring diagram.
how many of these do you have connected ?
we would then ignore everything I told you about MECHANICAL relays and...
....tell you to get a 30 amp SSR, or sold state relay. make sure it has a signal line that can use 5 volts. most have a rating something like 3-32VDC and can take pretty much any form of DC signal to activate..
the activation is an opto-isolator. it uses a simple LED to light and a light detector to know the light is on. this is the isolation as you cannot pass power through light.
the other side, the isolated power side, will be rated in EITHER AC or DC, NEVER BOTH. you MUST make sure you get the correct type of voltage. SSR's are not interchangeable.with voltage.AC or DC.
here is an example from e-bay :
you can see that the power side is between 24 and 380 VAC, so would work.
the current rating is 40, or 40 amps
at about $6 it is a pretty safe bet and it will work with an Arduino
the trigger current is 7.5ma