As the old saying goes, it's difficult to make predictions, especially about the future. And hindsight shows forecasts about the solar market are tricky.

Editor’s note: This is a guest post written by Zachary Shahan, editor of CleanTechnica and Planetsave. The views expressed in this post do not necessarily reflect or represent the views of ABB or its employees.

Frost & Sullivan, a large market research firm, recently put out a report on the global solar PV market from 2014 to 2020 that lined up almost eerily with a late-2013 report from Navigant Research, a cleantech market research firm.

The Frost & Sullivan report predicts that the global solar PV market will more than double in size between 2013 and 2020 to become a $137 billion market; Navigant Research says it will be worth $134 billion by 2020.

With such similar forecasts, one might think the market is fairly easy to predict. That’s hardly the case. Accurately anticipating the scale of a fast-growing, disruptive technology market even a few years out is notoriously difficult.

Take a look at the chart above of solar cell prices between 1977 and 2013. In 2003 (look closely), market researchers were anticipating that prices would continue their steady decline. Instead, they stabilized from 2005 through 2008 and then all of a sudden collapsed, and continued to drop over the next few years. Of course, during those later years, solar PV growth shot through the roof.

In hindsight, everything is clear: an undersupply of polysilicon kept solar cell prices fairly stable for some years before a massive oversupply of solar cells and solar modules sent prices into free fall. But no-one expected this back in 2003.

The market research reports from Frost & Sullivan and Navigant Research take a multitude of factors into account, such as government policy, the price of solar PV and electricity prices, as well as when solar will reach “grid parity.”

However, since these factors are themselves inherently unpredictable – who knows what government policy will be in five years, let alone what public attitudes towards solar will be – these forecasts need to be taken with a large pinch of salt. Will the global solar PV market double in size between 2013 and 2020? Not if it follows the S curve growth of disruptive technologies.

New and disruptive technologies take time to pick up steam in the market. But once they reach a certain critical mass, the ensuing exponential growth surprises people time and time again. If you start with a penny and double it every day for 30 days, you end up with $5,368,709.12. Even when we know this formula, the scale of growth still surprises us. The question with solar is how fast the growth will be once people realize how cost competitive it actually is.

In six years’ time, solar PV won’t only be cheaper, but an exponentially larger number of people will have friends, family members, or neighbors who have made the transition to solar. This social dynamic is self-perpetuating; once a certain percentage of your peer group has panels on their roofs, you’ll install them, too. The same goes for corporations and even utilities.

Another factor to keep in mind is that high-penetration solar markets typically reduce utilities’ profits. They respond by increasing electricity prices, which in turn encourages consumers to install solar panels. Utilities in Germany, Australia, and the US have been trying to find ways around this problem, but even if they somehow get to raise prices only on solar producers, that still pushes people toward off-grid setups and microgrids. Ultimately, the business model of the power industry will have to change to accommodate renewables.

I’m not going to venture a prediction about the size of the solar PV market in 2020, but I’ll be very surprised if it isn’t a lot bigger than $137 billion.