Running Some Numbers On Rooftop Solar Energy in 2018

Solar power systems installed in the areas def...
Solar power systems covering the areas defined by the dark disks could provide more than the world total primary energy demand in 2006 (assuming a conversion efficiency of 8%). Image from Wikipedia.

I recently asked physicist Richard Muller whether he thought the price-performance of solar electricity generation would follow a Moore’s Law-type curve. He said that this would not occur due to improving the efficiency of solar collection, as the current levels of efficiency – 20-40% – are reasonably high. However, he added

“I do expect the price to drop by a factor of 10, so we will have lots of solar.”

Well, in the nature of things, there’s definitely a limit to how much energy a solar PV collector can get from a square meter of sunlight. (There’s about 1kw of energy in a square meter – as I learned in Physics For Future Presidents, by Professor Muller – so we can expect to get 400 watts or less.) The amount of this energy per square meter we can collect will go up, but asymptotically approach (at best) the physical limits.

On the other hand, I’d argue that the cost of collecting it can go down a nearly unlimited amount – certainly multiple orders of magnitude. So what will solar PV look like in 2018 – ten years from now?

There’s no physical reason that a solar collector roof would cost more than an increment above a conventional roof – the increment having to do with interconnecting the roof to the house’s power system. That is, if the conventional roofing material includes solar collection capabilities. (For example, check out Solar Century’s C21 solar PV roofing tiles.)

Just to run the numbers out:

  • One square (100 sq feet, or ~9 sq meters) of metal roofing costs about $2,000 installed ($1,110 for the material, $750 for the installation). Metal roofing is not the cheapest type, but it’s convenient for these calculations.
  • One square (or 9 square meters) represents about two kilowatts of solar PV at 20% efficiency. This is about the electricity used by a normal single family household.
  • Today 2kw of PV costs at least $12,000 installed, or six times as much as the regular roof. The payback for installing 2kw at that cost is typically 8-10 years.
  • Assuming one quarter of the solar PV cost is installation (this is a high estimate), then a factor of ten reduction in the cost/kw for the collector takes the cost for this 2kw system down to $3,900.
  • If the solar PV panels are structural roofing as well, then I’m paying a $1,900 premium for each 2kw of solar PV. That premium represents just about a one year payback.
  • In fact, it’s so cost effective, I would probably choose to put 4kw on my roof – twice what I need. The excess can be used to fill my not-very-efficient storage devices such as a fuel cell or batteries for my plug-in hybrid SUV.
  • By using integrative design to solve two problems at once with my solar PV collector system – energy and roofing (and I could also include insulation and possibly other benefits depending on the design) – and projecting the factor of 10 reduction in price/kw, it becomes cost effective to over-generate, meaning I can then also replace my trips to the gas or hydrogen station

So, when I talk about solar PV having a Moore’s Law-like improvement in price-performance, this type of integrative design is what I mean. Technology improvements in the collection itself (efficiency and manufacturing of the collectors) get us the first factor of ten price reduction. Then other types of innovations – such as roofing materials becoming solar collectors – get us another factor of two-to-five.

Moore’s Law about the price-performance of silicon chips was and is based on a constant reduction in feature size. Smaller feature size drives price-performance of chips because of several factors, but primarily due to the fact that electrons have to travel around, and the shorter the distance they have to travel, the faster they get there. A similar “feature size”-based price-performance reduction is happening in hard disks as well; the engineers keep making the magnetic domains smaller and closer together, so the heads have less distance to travel to get to data and more data can be packed into the same space.

However, in contrast, the price-performance revolutions in solar PV are not going to be based on feature size – the feature size is already fixed at 1kw per square meter. And in fact feature size, especially for residential solar PV, is not a big problem – most of our roofs have much more space than needed for our energy needs. Instead the revolution is going to come about on the cost side, via a combination of:

  • More efficient and cheaper manufacturing approaches, ranging all the way down to nano-assembly eventually
  • New PV materials (e.g., CIGS and organic PVs) and improvements in existing materials
  • New form factors, such as integrating PV into roofing, windows, and siding, that embed the cost of the PV incrementally into existing costs
    • Note that because roofs and other housing surfaces are so large, PV cell efficiency is much less important than cost per KW, as long as the cost is incremental to the existing material
  • Other new approaches we can’t even think of right now

Just as the semiconductor and disk drive industries have had to come up with innovation upon innovation to maintain the pace of Moore’s Law, so will the solar industry – and it will.

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