According to Apple, the new Core i7 iMacs are significantly lower power than the older Core i7 iMacs.
https://support.apple.com/en-ca/HT201918
iMac Late 2014 27" Core i7 4 GHz Radeon M295X
Idle: 70W
CPU Max: 288W
iMac Late 2015 27" Core i7 4 GHz Radeon M390
Idle: 63W
CPU Max: 240W
iMac Mid 2017 27" Core i7 4.2 GHz Radeon 580
Idle: 71W
CPU Max: 217W
1. Power consumption data (watts) is measured from the wall power source and includes all power supply and system losses. Additional correction isn't needed.
2. CPU Max is defined as running a compute-intensive test application that maximizes processor usage and therefore power consumption.
3. These numbers reflect a 23°C (73.4°F) ambient running environment. Increased ambient temperatures requires faster fan speeds which increases power consumption. At 35°C (95°F), 50W should be added to reflect increased power consumption.
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These numbers are somewhat difficult to compare as I suspect some of the increased power usage in the older models is likely related to the GPU differences, but nonetheless it would seem the Core i7 iMac in 2017 may be cooler and overall (including GPU changes), the 2017 iMacs are likely quieter than the Core i7 iMac in 2014. According to Apple, the overall system idle power for your 2014 and the current i7 is the same, but at CPU Max, your 2014 uses 70 Watts more power. However, if you want to be absolutely sure a new iMac is very quiet at all times, you might want to consider the i5-7600 non-K.
BTW, take a look at this one:
iMac Mid 2010 27" 3.6 GHz Core i5 Radeon 5670
Idle: 145W
CPU Max: 365W
Ouch, that's a lot of power, even at idle.
That's a dual-core though. They don't list my 2010 quad core i7.
Thanks! What would the numbers be for the 3.8 Ghz i7 Mid 2017 with Radeon 580?