Heater Watt Consumption

It should make no difference. A lower wattage heater will be on for longer, theoretically at least a 200 watt heater should be on 1.5 hours for every hour a 300 watt heater is on for. Your usage is watts x hours and so it all comes down to the same thing in the end.

The best way to save electricity is to have your temperature set to the lowest which is compatible with your fish. None of mine is above 23 degrees. I have chosen fish to match this.

For example, if your room temperature  is 20 on average, setting the tank temp to 26 degrees requires raising it by 6 degrees above room temperature, whereas setting it to 23 only raises it by 3, i.e. half as much. That means you will use twice as much electricity keeping it at 26 compared to 23. 

Don't forget the lights, they will also use a significant amount of power.

during the summer months i found that my heaters was very rarely on in actual fact i had the opposite problem in the summer keeping the tank cool enough , but now my heaters cut in a bit more with the colder weather my tank is running about 24 to 25 degrees my tank is about 260 liters also and i run two 150 watt heaters so 300 watts in total and i would guess that my heaters are probably running some were in a average of 15 mins in every hour roughly (have never actualy timed it but i remember when i was doing my cycle and the temp was up to 30 degrees  the heaters was easily on for more than double those times so i think that once your cycle is over and your temp comes down to more suitable for fish you will notice a saving in your electric.

i read a article some were or maybe was on the forum here cant remember but it advised one watt of heater per liter of water and the advised a little extra so that the heater isnt to its limit so i think would be best to stay with the 300 watt or maybe go for one 150 watt heater and one heater 100 watt giveing you 250 watt

Yes likewise, i only heat the rooms i use, and its off from early evening, so best stick with 300w for now, but i do have a spare 100w so buying a 150w might be a good idea, oh and can anyone tell me, the second hand tank came with a 200w Boyu heater, it seemed to be working ok but i noticed the very end of the heater was glowing red as well as at the top, is this normal for some heaters?

The substrate, rocks and bogwood will also be heated to the same temperature as the water and so you shouldn't subtract them from your total volume.

A heater is a heater is a heater and it's heating capacity will depend on its wattage, whatever the make.

Why do you want to use two smaller heaters? I thought you already had a 300w and 200w?

If you have two heaters, they need to be set to the same temperature. Otherwise one will switch off at the lower temp and the other will just stay on longer to complete the heating. I can't see the logic behind having the two set to different temps.

I never believe what manufacturers claim for their products and I really doubt if Eheim heaters are actually significantly more efficient than others. If I were you I would keep the 300w and add a new 200w. Both would combine up to the lower temp, then the one set higher will just finish the job.

If you had 100 and 125w and the 125w failed, I doubt if the 100w would be able to maintain the temperature of a tank that size, particularly if the room was a lot cooler than the tank. If you had 200w and 300w either could maintain the tank temperature if the other failed

Remember, you won't save electricity by using lower power heaters as they will just be on for longer.

With one set to 24 and one to 25 they will both be on up to 24, then one will switch off and the other (set to 25) will stay on until the temp reaches 25. I'd keep them within a degree or so of each other.

As Congo tetras are happy down to 22 according to this site, I'd be tempted to set the temp to 23 and add a rainbowfish which is also more tolerant e.g Red, Lake Kutubu or Lake Tebera rather than the "truly tropical" ones that need higher temps. You could add fish such as danios and a corydoras such as peppered which would all be very comfortable at the lower temp. That would save quite a bit of electricity over a year, more than you might think.