3 watt LED vs 15 watt LED in light bars

As above you need to look at the actual power output of the LEDS, just about all sellers will be quoting the "theorectical" output power, not the actual output power.

For example a bar might be advertised as 120W and has 40, 3W, LED's.

The actaul specs show the bar draws 7.5A@9V or 67.5W this is well short of the 120W theoretical output power.

Generally most manufacures will derate the LED power output to maximise LED life, but some using poorer quailty LED's are well underated.

Some sellers will show the input current, if it works out to be around the same level as the advertised power output, ie 120W bar shows 12A@10V then they have probably just used the theoretical power again to calculate the current drawn.

Ask the seller to provide an independent test certificate showing the actaul output power and lumens etc, a reputable manufacture should be able to supply these.

Love the suggestion HKB , but not so sure we'd end up with a genuine test certificate.

All else being equal its better to go with 3w than 15w over the long term.

Heat is the killer of leds and some go downhill very fast.

Its easier to get rid of heat from 5 3w sources than a single 15w source .