I agree with Fino, galvanic is too slow to be cost effective on the back and upper arms if you have more than a little hair there. You would do better with blend, and would have the fastest first clearance time in Thermolysis.
You will find that the newer machines are more comfortable for treatments, and make pain management either a thing of the past, or less needed during treatments. In fact, in beta testing the upcoming product from Apilus, I had hair removed from my lower lip line (where you girls apply lip liner pencil) without so much as a numbing cream and it felt like a quick drop of hot water. Of course, everyone is different, but there is no two ways about it, the newer the technology in the epilator, the more comfortable the treatments are, assuming the practitioner is good at choosing settings.
For best comfort I like the Silhouet Tone VMC and the Apilus SX-500 (and the soon to be released Apilus epilator whose name I can not give you till the end of October)
The reason you avoid the sun and try to stay out of hot and humid conditions for at least 24 to 48 hours after electrolysis is because sweating would introduce salt to your broken skin, and cause scabbing where it would not have occurred. Sun exposure tends to cause freckling that could take 3 months to fade.
You want to keep the treated area clean and dry to the best of your ability.
Electrolysis works, and in order to have the fastest results, you need to get as quickly as you can to the place where you are cleared of all the hair each and every time you have a treatment. Once you get there, it is only a matter of time before you are having a 15 minute nit-picking appointment where you are getting a few stray hairs no one notices but you, anyway.