I used two 1500W ceramic heaters in parallel:
http://www.diyelectriccar.com/forums/showpost.php?p=203777&postcount=599
I measured a bit over 3100W output at high blower speed. I can verify it keeps the (small) car warm at 15 F outside temperature. In fact I usually turn it off after around 15-20 minutes in mid- 20's temps as the car interior gets too warm. Then cycle it on off if necessary. Some things to consider:
1) The higher the wattage heater, the larger the hit on vehicle range when it is operating. My car uses about 200 Wh/mile, and the heater uses about 1kw/20 minutes. That's 5 miles reduction in range each 20 minutes. Prior to this one I ran a single 1500 W heater, but it only put out about 1200W and didn't heat the car much.
2) A big advantage of going to two side by side is there is half the resistance to air flow. The larger air flow makes a big difference in heating. Don't stack one on top the other, since that will double the resistance to air flow, resulting in poor flow.
3) If your pack is less than 200V and you use a hot water heater element, you will be limited to 1500 W for shorter elements, 2000W for one something like a foot long, so you would likely want to use two. You can get 240V ones that are 4 or 5 kW. It will likely be mounted under the hood in outside cold temperature with cold wind blowing around it while driving, so will have to be well insulated, and will still loose heat. There is also the thermal inertia the Wayland article mentions. Takes a while to heat the water and metal. A ceramic heater is inside the car, so all the heat goes there, and they warm in 20-30 seconds.
4) It is generally considerably more difficult to mount a ceramic heater. I had to pull the dash and the heater out of the car. It was about an 8 hour job to pull them, mount the heater, re-install (more time to make the teflon holder for the two heaters - you need to ensure air can only go through the heaters, not around them, and make sure the material you use can take the temperature of the heater element touching it - test it before you mount it in the car. I can tell you polycarbonate melts).
5) I wouldn't trust an ssr to turn a heater on/off, since it could fail shorted on. I use a DPDT P&B relay from KTA-ev. It is rated 120V/20A, so I am a bit over the spec, but it is working well so far. One with magnetic blowouts to reduce arcing of the contacts would be better, a contactor even better, especially at higher pack voltages. And of course fuse the leads before they enter the vehicle interior since they are at pack voltage and could deliver very high current before they burned up. The relay or contactor should be outside the vehicle cabin too, so there is no voltage in the cabin with the heater off.
6) Eliminate all air leaks into the vehicle cabin, and insulate where you can. That will make a very big difference, reducing required heater power. If tight, and well insulated, you might get by with one 1500W heater. I did not do as I preach here, but another ev owner claims he did this and less than 1kW heats his truck cab in Montana. He insulated inside the doors, the floors, and head liner.