My refurbished spark needs compass calibration as it drifted a bit when taking off (indoors). Can I calibrate it at home or it needs to be done outdoor far from electromagnetic interference like TV and metal objects?

Best to do it outdoors in an open area away from any electromagnetic interference like your home appliances...     

Indoors, Spark is in Atti mode, no GPS, and don't need the compass too, the reason it drifts is because the downward sensors can't track any texture in the floor, so it will drift a bit, thats normal. in order to hold its position indoors, it needs some pattern in the floor.

Thank you for reaching DJI forum. I would suggest to do the compass calibration outside or in an open wide area to make sure there is no interference. Here are some few tips:
1. When the app prompts that the compass needs to be calibrated, the drone’s compass has suffered from interference as the aircraft was turned on in a place where there is magnetic interference. Make sure to do the compass calibration and fly aircraft in an open area free of interference.
2. When powering on the drone, ensure that the aircraft is at a standstill. If there is interference or the aircraft is powered on during movement, the compass’s orientation may be altered, and then recalibration is needed.
3. To ensure the flight stability, the app will prompt you to calibrate the compass when there is a need. Please follow the app and calibrate it.
May I know if the drifting issue is still the same when flying the drone outside?

Artur Macau Posted at 2018-8-15 00:16
Indoors, Spark is in Atti mode, no GPS, and don't need the compass too, the reason it drifts is because the downward sensors can't track any texture in the floor, so it will drift a bit, thats normal. in order to hold its position indoors, it needs some pattern in the floor.

Plus enough brightness, to "see" any pattern on the floor