I lowered the temperature that my heater is set at and i woke up this morning at it was at 80.4-80.6 with my lights on. for my 20 gallon tall my light is the below.
Do please consider what you are measuring temperature with. The vast majority of cheaper thermometers are reasonably precise, but not very accurate. In other words, they can tell you if the temperature goes up or down a degree, but not actually what degree it is. I went through this with meat thermometers, and tested 3 we had around the house, and for the same temperature water, would get readings varying by 20 degrees (that's about medium rare vs. well done).
If the thermometer has adequate range, one way to test is take relatively pure water (most tap water is OK) and get it at a furious boil, and test in the water. Then do the same with lots of ice melting in a container of water. This should be close to 32 and 212 F at sea level (numerous tables will let you correct if you are at high altitudes).
And/or test against multiple thermometers. I say this because you are quoting fractional degrees, and (no disrespect intended, we are all on a budget) concerned over the price of a heater, and I would suggest it is at least as likely your thermometer is off vs. heater.
And/or don't worry too much about it. Very few fish/inverts are going to be concerned over the difference in 78 vs. 81, for example -- what they don't want is flip flop between those two. So it is whether it will stabalize at a temperature and hold it you should mostly be concerned about.
1 Marineland Double Bright LED that has stats of 450 lumens (can someone translate that into watts for me)
It depends on the light, I have a 30W that puts out 2000 lumens so probably somewhere in the 8W range. A guess. How much of that goes into heat is a whole different matter of course.
If it's anywhere in the 77-79 range i'm leaving it alone. If it is in that range tomorrow should i hold off on getting a new heater? I wouldn't be able to purchase it til friday at the earliest regardless.
In my opinion look for stability more than a specific temperature, if it can keep it at a temperature and not be running up and down, you're golden. If the heater turns on at (say) 78 and doesn't turn off until (say) 81 -- that's a problem, throw it away, as it will drive your tank in a yo-yo. Be cautious where you are taking readings, however -- that the water is mixed well, and not right up near the heater (one reason thermometers inside of the heaters are somewhat questionable).