0xc0ffee you are asking the right questions. here is where I am coming from.
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My math comes up that the 1820 will dissipate 27.3W at 700mA versus 24.7W for the 3070. The 3070 makes 3450lumens and the 1820 makes 2793 lumens. So 139lm/W vs 102lm/W = 37% less buds/watt
So if your CXA3070 input power is 600W it would cost about $34/mnth and lets say yielded .4gr/watt in 4 weeks. That is 240gr @ $12 = $2880 worth of bud/mnth.
For the CXA1820 input power is 600W it would cost $34/mnth and yield would be .25gr/watt in 4 weeks. That is 150gr @12 = 1800 worth of bud/mnth.
So lets say we built a large enough CXA1820 lamp that it would match the yield of the 3070. It would have to be 960W and would cost $54/mnth in electric. It would require 40% more space, heatsinks, drivers, wiring, and hanging gear. It would weigh 40% more. It might create enough heat that you would need to kick on the AC and certainly you would need more airflow from circulation and exhaust fans which means more noise and more carbon required and quite a bit more electric cost lets call it 70/mnth.
As far as parts cost, the 600W CXA3070 would be ($40X20 COBs) $800 + ($12X20 drivers) $240 + ($42.5X10 heatsinks) $425 + $100 wiring/chain/shrink tubing/solder/flux/paste etc = $1565
For parts cost 960W of CXA1820, 800W($15X30) $450 + ($12X30 drivers) $360 + (42.50X16 heatsinks) $680 + 140 wiring/chain/shrink tubing/solder/flux/paste etc = $1630
So using my design, it actually costs more to build an equivalent output CXA1820 lamp, would require much more work and space and would be less stealthy. It would also cost 2X/month in terms of electricity. I calculated based on 15cents/kWh because you have to add delivery and supply cost. For those who are in areas of very expensive electricity this becomes even more important (hawaii, NYC, AUS, europe etc) Hawaii cost 40cents/KWh.
Considering these lamps could yield 240gr/mnth, the parts cost for the entire build is paid in less than a month and the monthly bill is $34/mnth after that. The lamp will never diminish in brightness or burn out like HPS.
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I agree the CXA1820 should not be run harder than 550mA in which case it is 34.2% efficient versus 31.2% @ 700mA. This would mean an increase cost up front but it is a positive economy IMO.