I was just wondering if anyone knows what the best type of solar panels are best for output of energy per panel. I am going to use it to power machines. In addition, what's cheaper to make for power?
Technically there are only two solar panels in Tekkit, and they're not interchangeable: there's the Industrial Craft2 solar panel and the Redpower solar panel. What you're probably mixing in with those are the various IC2 solar arrays, which combine several IC2 solar panels into one block for increased power output and smaller footprint.
If you're powering Redpower machines, only the Redpower solar panel will provide the type of power that your machines need. In that case, there are no alternatives to compare against.
If you're powering IC2 machines, then the amount of power output is not a simple issue: putting too high a voltage into your machines will destroy them explosively.
When you factor in "cheapness" as an issue in which is best, then "best" stops being possible. The thing is, the IC2 solar panels with higher power output require ridiculous amounts of resources to build—if you have to ask the price, you can't afford to use solar power! If resource cost is a concern at all, you are far, far better off stuffing charcoal from a tree farm into one or more Generators that output 10 EU/t. It's cheaper, safer, produces more EU/t sooner, and is easier to build. Add automation to your tree farm and automate your charcoal production, and you'll still have put in fewer resources per EU/t than if you invested in solar power.
If cost is no object though (such as if you have a solid Equivalent Exchange diamond generator running), then you can start thinking about the benefits and trade-offs of the various compact solar arrays. The advantage of the higher-output arrays is more raw EU/t; the disadvantage of those arrays is that most machines can't use that EU/t without stepping it down with transformer blocks or transformer upgrades, and you will needs some high-power energy storage blocks in order to capture all that EU because your machines won't consume it as quickly as it's produced.
Looking at the construction of solar panels and arrays shows that the cost increases geometrically while the output increases linearly, making the value proposition poor unless you have resources to throw away:
A single solar panel generates 1 EU/t and requires:
A single LV Solar Array generates 32 EU/t and requires:
A single MV Solar Array generates 128 EU/t and requires:
A single HV Solar Array generates 512 EU/t and requires:
The grand total cost of a single HV Solar Array block that produces only 512 EU/t is (rounding fractions up):
And that doesn't include the power storage blocks, connecting cabling, and transformer blocks/upgrades necessary to use the higher power outputs. Solar is a great way to get a trickle of power at the beginning for unattended machinery in the early game, but serious solar power usage is really only reasonable in the late-game of Tekkit.