Energy Costs by Source over Time I'm looking for a comparison I can make between "traditional" energy sources and solar energy. 

Ideally, I'd like to show how much solar energy has yet to come down in price before it is cheaper than, say, gas or coal. I would like to know the historical rate at which this has been currently happening. 

The question is then what metric is good for this. Levelized cost of electricity by source in time would be my preference, however, I cannot find data for this over time and by source (e.g solar, coal &amp; gas).. 

Any advice would be appreciated.
PV is already cheaper than gas or coal in most of the world, on a like-for-like basis. So that sort of answers your main underlying question.

It's estimated that new PV will be cheaper than existing coal and gas within the next 2-5 years, depending on where in the world we're talking about.

LCoE is a hypothetical construct resting on a bunch of assumptions that often don't hold, so isn't wildly useful.

For real data, look at actual auction results for new generation that's priced per unit electricity supplied to the grid. That's pretty easy for PV, but you may struggle to find recent comparable auction data for dirty old generators such as gas and coal, because those are rarely procured as auctions priced per unit electricty.

You then need to adjust those costs for the factors not represented in the auction prices.

For PV, that's the cost of integration into the grid (balancing intermittency, reserve capacity, reactive power, and so on). Those costs are pretty tiny - well below USD 10/MWh - but will vary, depending on the particular grid. Look out for additional subsidies such as the CELs (tradeable clean energy certificates) in Mexican auctions, tax-breaks, free land, state-subsidised loans, etc.

For dirty generators, you'll need to add in the unpriced social costs, including the insurance risks of accidents, and the waste costs: greenhouse-gas marginal social costs are around USD 100 /tCO2e (see peer-reviewed work by Chris Hope &amp; others).