A 1500 watt heater will use 1500 watts of electricity power while turned on Max or High Heat.
On the other hand, it will use less power on lower heat settings. On many occasions, the medium heat setting will use 900 watts of electrical power.
However, if you choose the fan only mode, you may be consuming less than 100 watts.
In terms of money per month I will take you through a simple calculation. I hope you can enjoy the mathematics!
Let us say you use the heater every night for 6hrs. It could be more or less, it just depends on your situation.
There are some owing to the medical condition or that of their partners, have to use the heater throughout the night. In the contrary, for others, like myself, you just need the heater to warm up the room.
Further, let us also assume the month has 30 days and you will need the heater every night. This translates to 1500W * 6h * 30 days = 270,000Wh.
To turn the above to KWh, we only need to divide by 1000. Therefore, we expect to use 270KW in a month.
Now, electricity charges differ from place to place. In the United states for instance, we can assume a charge of 12 cents/KWh.
Consequently, this means that for 270KWh, the cost will be $32.4 per month (270KWh * $0.12/KWh).
For this little comfort, your electricity bill in a month is higher by $32.4 per month.
Other Factors to Keep in Mind
If the heater is put on for 6 hours, it is not actively using the electricity for the full 6 hours. This is because, most heaters use a thermostat to regulate temperatures.
Once the set temperatures have been achieved, they will stop heating and only come on when temperatures drop below the permitted range.
Due to this factor alone, your bill will be lower than the $32.4 per month.
In addition, if you are not utilizing the full capacity of the heater, lets say you have set it at 900 watts and not 1500 watts, the utility company will demand from you less than the $32.4.
The last factor to consider of course, is how well your house or room is insulated. Spaces with good insulation implies that the room can maintain temperatures longer than one with poor insulation.
What this does, it increases the period your heater goes off thus effectively lowering the KWh used. At the end, you get significantly less bills.
I sincerely hope this helps to answer your question.
You can also have a look at the best electric space heaters.