Solar Barrage is currently underperforming compared to other AoE DoTs. Keep in mind, Solar Barrage is in fact a DoT--it pulses 3 times over 6 seconds and does NOT benefit from the pulsing Empower. Also, even though Solar Barrage is listed as a 6-second DoT, it has a 1.1 second cast time, so for numerical breakdowns I calculate the actual duration of Solar Barrage as 7.1 seconds (bear in mind also that there is a significant opportunity cost to using Solar Barrage; though it might be ~1 second shorter than the DoTs I compare it to below, the cast time is time lost where you could be casting other abilities).

Here are some comparisons, same set up with Lightning staff equipped and Destro ability slotted for +8% AoE damage:

Blazing Spear
Cost 2690
Initial Hit: 5007
DoT: 1285/s
Duration: 8s
Radius: 8m
Misc: Instant Cast/Offers Synergy
DoT total damage = 10,280
DoT + Initial hit = 15,287
Actual DPS = 1,910

Blockade of Storms
Cost 2690
Initial Hit: N/A
DoT: 2142/s
Duration: 8s
Radius: 18x12
Misc.: Sets Concussed enemies off balance for 4 seconds
DoT total damage = 17,136
Actual DPS = 2,142

Solar Barrage
2690
Initial Hit: N/A
DoT: 5304 / 2s
Duration: 7.1s
Radius: 8m
Misc.: Empowers between each pulse (3x)
DoT total damage = 15,912
Actual DPS = 2,241

As you can see, Solar Barrage does less total damage per cast than Blockade of Storms, and just slightly (700 damage) more damage per cast than Blazing Spear. Solar Barrage also has the worst/least beneficial utility buff of the three: Only a handful of Magplar abilities benefit from the Empower buff (Empowering Sweep, Aurora Javelin, Toppling Charge, and the initial hit of Vampire's Bane), compared to Blazing Spear which returns resources to Allies and Blockade of Storms which provides offbalance.

In terms of actual damage per second, Solar Barrage actually pulls slightly ahead of Blockade (101 more DPS) because its DPS is calculated over 7.1 seconds rather than 8 seconds. However, Solar Barrage has an enormous opportunity cost to using the ability since 1.1 seconds of its duration are a channel, meaning you aren't doing anything else excepting casting Solar Barrage. Compared to Blockade which is instantly and allows for you to immediately start casting other abilities. This results in an enormous drop to theoretical DPS when comparing the two abilities.

Solution:
The overall damage of Solar Barrage is fine; the cast time is the problem. If Solar Barrage were made an instant cast 6s DoT and lost the ability to Empower, it would still provide less overall damage than a single cast of Blockade of Storms, but Solar Barrage's actual DPS would go from 2,241 to 2,652 making it an attractive AoE DoT option.