Bro if I am not wrong they mean 21 watt light output equaivalent.
LED light output is about double the spec.
Meaning a 3 watt led has a light output about 5 to 6 watt of any normal light
To add on
1. ELN-60-48 run the most 13 leds if you want the best result. You can run more led then like bro James show the forward voltage also drop and hence your led is not running at its best.More led less forward voltage.Less bright.Best to run your led at sepc forward voltage
2. Meaning you can run minimum 6 leds or maximum 11 leds
3. As long as your total led forward voltage is not more then 43 vdc
Cheers
If your SPS is still brown at the top of your tank then you need to lower your light set.
But make sure you let them have time to color up first .At least 3 weeks .
But if they start to turn white be careful.
Then you need to move them further down the tank or aways from the light