Yes I normally keep my house at 21 Celsius.
What´s are your wanted temperature? In liter and theory - your aquarium is around 480 L - by experiences it means thar the actual volume of water is around 380 L - maybe lower.
Let us say that you have 26 C as wanted temperature. It means that your heater need to manage to keep the temperature at 26 C when your room temperature is 21 degree C.
The general rule here with a un-isolated and open aquarium is that around 0.5 W per L of water in order to compensate for heat losses when the temperature difference is 5 degree C.
The most losses is caused by evaporation and if you have a frame on the top of the aquarium, you reduce the air exchange over the surface, which creates a layer of very high humidity over the surface. This reduces evaporation considerably.
If you turn this around and instead let a fan blow over the surface, this can lower the temperature in the aquarium by several degrees below room temperature. You must also count with heat losses from the skimmer that rise the evaporation a lot.
On the other hand - your pumps and illumination will add heat into the aquarium. In my case - all my pumps add around 60-120 W (3 circulation and 5 streamers) and the light - I do not know. This result in a usage of around 40 % (in running time) of my 200 W heater in my 300 L of water in similar situations as you - maintain 5 degrees C temperature difference. (I use a frame and around 3/4 of the surface is covered by lighting fixture)
I´m pretty sure that an IM Helio of 2*100 W would be enough for you but an 1*350 W is $62 cheaper and a 2*200 W is only $16 more expensive (
prices from IM homepage) - therefore my choice would be either 1*350 or 2*200 W sets
Sincerely Lasse