Glass with a LowE coating will always outperform clear glass. As Ericjeeper pointed out, in order to qualify for the tax credit you need both U-factor and SHGC need to be below .30. This is not possible unless you have LowE windows. There are LowE coatings designed to allow solar heat into your home while blocking the heat in your home from going outside. There are also coatings that block solar gain as well.
Thermal or heat transfer thru a window is going to be both radiant and conductive. LowE coatings are designed to limit radiant heat transfer thru the glass while argon or krypton are intended to limit conductive heat transfer.
In the everyday world, we directly experience ultraviolet, visible, and infrared light. Different LowE coatings will block different portions of each "type" of light depending on the type of coating.
While UV will fade your carpeting, give you a tan (UVA), cause a "sunburn" (UVB), or may even lead to skin cancer (UVA again), it really doesn't add any appreciable heat to your home although sometimes people will talk about LowE coatings blocking UV as part of "heat" entering your home.
Visible light can also fade your carpeting and it does add heat to your home, but a window that blocks 100% of visible light wouldn't be of much value, so some folks will suggest tinted glass which does help to keep a home cooler by limiting the amount of light into the home. While LowE coatings perform a similar function, they just do it a lot better than tinted.
Infrared is the primary contributor to heat gain or heat loss thru a window. Infrared is divided into near, mid-range, and far (and it has a number of subdivisions depending on whether you want to change the channel on your TV using a remote, or if you want to spy on an enemy at night, or if you want to study the spectral distribution of a star...and more).
Anyway, direct solar heat gain is "near infrared". High solar heat gain LowE coatings will pass near infrared or direct solar heat gain allowing the solar heat into a home, while low and moderate solar heat gain coatings block percentages of near infrared or direct solar gain.
When you are out in the sun and you get that nice warm feeling on a bright sunny day (appreciated in winter, less appreciated in summer unless you are lying on a beach somewhere), you are experiencing radiant heat gain in the near infrared.
On the other hand, if you are standing next to a concrete wall and you can feel the sun's heat being "reflected" off the wall, you are now experiencing far infrared. It is still heat, and still radiant, and it is going to warm you, but the wavelength (frequency) of the radiation has changed from near infrared to far infrared when the near infrared was absorbed and then reradiated by the wall.
All LowE coatings block far infrared, but as mentioned different coatings allow different levels of near infrared to pass thru them.
If a homeowner wants direct solar gain, they can get a high solar heat gain coating that would allow near infrared to pass thru the window, and that near infrared heat will warm the furniture, the floor, the walls, anything that it penetrates. The near infrared will be absorbed and then reradiated in the far infrared spectrum – which is blocked by the coating and results in solar heat gain in – and heat stays in.
When the temperature outside is 0 and the inside temperature is 70, a clear glass dual pane window will allow about 21 btu/hr/ft² of radiant energy to escape to the outside thru the glass (assuming the window isn’t in direct sunlight). Adding a high solar heat gain coating lowers that number to 5 btu/hr/ft².
A moderate solar heat gain coating will lower radiant heat loss thru the glass to 2 btu/hr/ft² and a low solar gain coating will come in at 1 btu/hr/ft².
Given that these are ideal conditions and that there are slight variations in coatings depending on manufacturer.
On the other hand, if you are back on that beach and you are walking barefoot in the sand and you are just short of tears because your feet are being slowly roasted, you are experiencing conductive heat gain. In an IG window, this is where the advantage of adding an inert gas in the airspace since the gas limits the passage of conductive heat transfer thru the IG unit.
Conductive is direct contact thermal transfer. Touch that warm wall from the earlier example and you will experience conductive heat thru your fingertips. And to get a little more technical, the heat that you feel from that wall even when not touching it is going to be a combination of radiative and conductive. Conduction because the air molecules near the wall are transferring heat between them. If you are close enough to the wall you will experience that as well.
In the IG unit, air molecules will transfer heat energy from the warmer glass pane to the colder glass pane – conductive transfer. Argon or krypton or xenon or whatever else they come up with will affect conductive heat transfer thus improving the energy performance of the unit.
Given the same 70F inside and 0 outside that I mentioned earlier, conductive heat loss thru the clear glass dual pane IGU is going to be about 13 btu/hr/ft² but add a LowE coating and conductive heat loss will increase to about 15-16 btu/hr/ft² (depending on coating). Seems like a LowE coating might actually be a bad idea in sme conditions???
Nope, since in the given scenario the coating lowers long wave infrared radiant heat loss from 21 btu/hr/ft² down to 5 btu/hr/ft² or better (again depending on the coating), thus the overall heat loss thru the IGU is improved significantly when using a LowE coating.
In the real world, a dual pane clear is going to lose in the neighborhood of 34 btu/hr/ft², but a dual pane with a low solar gain coating in the same conditions cuts that loss in half – down to 17 btu/hr/ft². A dual pane IGU with a high solar heat gain coating will cut total heat loss thru the IGU over 41%.
There are LowE coatings that are designed for folks who want winter solar heat gain, but unless people know to ask for a high solar heat gain coating then they are almost always going to end up with either a moderate or low solar gain coating which is more of an industry standard.
But again, in all conditions a window with LowE will outperform a window that doesn't have the coating.