Apologies to Andy Hi Fi and tvtek who both answered this query yesterday, but I gave out the wrong model of tv. My set takes about 5 minutes to produce a picture, which is then as good as ever. Sound comes...
This fault sounds very much like the CRT (Cathode Ray Tube) is approaching the end of its life. If you can find a TV engineer in your area - preferably one from "the old school" - he may well have a "Tube booster". This sometimes works to "rejuvenate" old CRT's but can also unfortunately, bring an abrupt end to the tube, because it involves passing a higher than normal current through the tube heaters.
Environmentally, it all depends on the age of your current TV and the size of the screen. If it's a fairly large screen, (32") it will probably consume around 250 to 300 Watts of electricity. Modern LCD TV's are more efficient, however gas plasma TV's tend to use slightly more power than LCD models.
As you will know, TV's don't use that much power in comparison to say, a cooker, electric kettle, fan heater or washing machine heater so you can expect to use around 200Watts for your new TV if it has a 32" screen. Power consumption goes up of course, the larger the screen size you have, so if you bought a 40" or 50" screen you would not save anything in power compared with your old CRT type TV.
If your current TV has lasted for 20 years or more then you have done reasonably well. That's not to say that many TV's with conventional CRT's often last MUCH longer than this.
Tandberg CRT TV's have been known to still be working perfectly well after more than 30 years! Much the same with Panasonic.