For smaller pools a 2.8kw electric heater from Bestway or Intex will certainly take the chill off of your water.  These are inexpensive and easy to hook up to your filter pump and plug in with the 3pin plug.  For pools over around 14ft, or even the very deep 12/13ft pools, these heaters won’t make much difference and you need to invest in a more powerful heater.  We offer Elecro heaters on our website in many different sizes.  They are excellent heaters and their 3kw packs a far more powerful punch than the 2.8kw versions from the pool manufacturers.  Our product pages will tell you the best size Elecro heater for the capacity of water in your pool and the 3kw Elecro heaters can still be plugged in with the 3pin plug.  Anything larger though (they go up to 18kw!), does need to be hard-wired in to your electricity supply by a qualified electrician (much like an oven would).  Remember, if you’ve gone to the effort and cost of heating your pool, use a solar cover to slow down the evaporation of that heat.