I am trying to convert from my broken boiler furnace to an electric furnace. I have central air and all the vents from that. Can I just buy a heating element and put it into the air handler? If I can, how do I know what size heating element to buy? The search on the internet gave 10 and 15 KW elements. My house is approximately 1000 sq feet.
Thanks for the input.

Actually it is not smart to do that. There are commercial strip heaters available but I would only recommend those to a true professional. To many items to consider. Duct length and any tapers and takeoffs, Running a very heavy electric cable from your circuit breaker panel to the heater and installing at least one safety circuit to prevent a fire just in case the motor stops blowing when the electric heater is running, your circuit breaker panel is probably going to have to be upgraded to carry the increased load, and the giant electric bill that you will get when you are using that heater. Sorry but I do not see it being very possible from here.

You can change your outside unit to a heat pump unit

you can change your outside unit to a heat pump unit

Yes he can if he wants to spend 5000.00 to do it. Air handler inside coil and outside condenser/heat pump would all have to be replaced. The new 13 SEER rules are in place and his old coil will not match up to the new outside HP. Also some of the suction lines have gone from 5/8 OD to 3/4 OD so that would also have to be replaced.