By Eric Martinot
Imagine a time when every rooftop in the world has a solar panel on it. Looking at Google Earth, all the rooftops you see would have panels. Panels would provide power or heat, or both. I am calling this vision "no rooftop left behind." That phrase may ring familiar to Americans who recall the "no child left behind" legislation in 2001, which mandated educational standards for children—everyone is included, no one left out. Let's think the same way about rooftops. Let’s adopt this phrase into our common lexicon. Let’s make this a rallying call for the future of our planet. 
If you use "no rooftop left behind" in posts, publications, or campaigns, you might link back to this page. And if you care to share real-world experience or achievements towards "no rooftop left behind," whether big or small, there are two discussion pages, a Facebook page, and a LinkedIn group for people to post experiences, cases, and stories, including projects, new businesses and finance models, examples of local policies, community energy systems, micro-utilities, etc. Also to comment on this article. 
No Rooftop Left Behind
Many are coming to the conclusion that putting solar panels on virtually all rooftops worldwide is entirely possible. As just one example, UBS Bank, in a 2013 research note, wrote: "Purely based on economics, we believe almost every family home and every commercial rooftop in Germany, Italy, and Spain should be equipped with a solar system by the end of this decade." 
Recently, NRG Chief Executive David Crane was quoted by Reuters as saying, “What really gets me excited in the morning is that there are 50 million American buildings that should have solar PV on them.” 
By one estimate, there are already at least 15-20 million rooftops worldwide bearing solar power (PV) panels, primarily residential and commercial buildings. There are also more than 90 million rooftops worldwide with solar hot water/heating systems installed, the vast majority in China. 
There must be well over 1 billion rooftops worldwide, so there is still a long way to go. 
Global solar power (PV) capacity stood at about 140 gigawatts (GW) in 2013. (For comparison, all the nuclear power plants in the world total about 370 GW.) Scenario projections for solar PV capacity by 2030 by a variety of organizations range up to 1800 GW or more. (See GFR Chapter 6.) Even if the annual market were to stay constant at 2013 levels, the world would reach about 800 GW by 2030. These numbers would represent a 6-fold to 12-fold increase from 2013, with likely a similar scale increase in the number of solar PV rooftops. (Also depends on the future balance of rooftop vs. “utility-scale” ground-mounted systems, and the balance between commercial and residential rooftops. Some believe that utility-scale systems will take a growing market share.) 
And a doubling or tripling of rooftops with solar hot water/heating by 2030 would not be inconceivable given China’s ambitious future targets for solar hot water and other global market trends. 
Roughly 40 GW of solar PV was added in 2013, a new market record. That translates into perhaps 3-4 million new rooftops, something like 400 rooftops per hour. In 2012, about half of all global investment in renewable energy went to solar PV. 
Most don’t realize how quickly the market for solar PV has grown in recent years. Ten years ago, when the annual market for grid-tied solar PV was only 1 GW, no one imagined a market 40 times larger so quickly. China, Japan, and the U.S. are now the leaders, usurping the positions of former leaders Germany and Italy. The Chinese market was 7-9 GW in 2013, with 12 GW planned for 2014. Japan was close behind at 6 GW. The U.S. was above 4 GW. 
The growth of solar PV has been driven by a variety of policies around the world over the past twenty years. Historically, most policy support has been in the form of capital subsidies, guaranteed prices from feed-in tariffs, or net metering to allow consumers to offset consumption. One of the first subsidy policies was Japan's "Sunshine" program of 1993. Later, Germany’s feed-in tariff applied to solar PV. (Feed-in tariffs now exist in over 80 countries around the world.) The U.S. has had a variety of state-level subsidies, and since 2005, a national investment tax credit. China came late to the game, but policies of recent years, including a new 2013 feed-in tariff, have brought its domestic market from practically zero to being the global leader. 
These policies of the past twenty years have been instrumental in bringing about major economies of scale in manufacturing, along with technology improvements, that together have lowered solar power costs enormously. In recent years, the cost of a solar PV module has fallen below one dollar (US) per watt, which back in the 1980s was the considered the "holy grail." Experts used to say that once solar PV falls below one dollar, “the game is over, solar will be everywhere." Well, that time has arrived. (And accounting for inflation, today’s cost is actually well below one 1990 dollar.)
City governments around the world have also been supporting rooftop solar for more than a decade with a variety of policies, subsidies, and targets. There are now hundreds of such cities and local communities. As one example, the city of Iida in Japan targets 40% of all rooftops with solar PV by 2030, up from the current 7%, which itself is a doubling from 3.5% in 2011. Several cities in China target high shares of buildings (50% or more) with rooftop solar hot water. Some cities in Europe have policies or mandates for solar PV on new construction. 
The Competitiveness of Solar PV
Many talk of "grid parity" for solar PV. Generally, “grid parity” is accepted by most to mean equivalence of solar PV generation costs with retail electricity prices. However, this concept can be misleading or distorted due to subsidies and a variety of electricity-market practices and rules (i.e., differential prices across customer classes, seasonal pricing, and net metering rules). If customers face time-of-use pricing, or prices based dynamically on grid conditions, then grid parity may well exist at some times but not others. Furthermore, financial experts point out that “cost of electricity” metrics, including grid parity, are not as important to investors as rate-of-return financial metrics. (For more on grid parity, see "Solar PV" in Chapter 6 of the GFR and associated endnotes.)
The International Energy Agency, in its 2013 World Energy Outlook, points out that grid parity may not mean economic competitiveness because solar generators must still pay their share of fixed grid costs, even if most of their power is self-generated. However, that is only true under a policy model in which "stranded assets"—the generation, transmission and/or distribution infrastructure rendered unneeded by the growth of distributed solar power—must still be paid for by all consumers equally. Other policy models may allocate costs differently, resulting in different winners and losers. In countries with fast growing power demand, the issue may be mute, as solar PV slows the need for more grid investment—part of the “value” of distributed generation. 
So questions arise like: is it socially equitable for a declining base of customers to pay increasing rates to cover fixed infrastructure costs? Should self-generators bear the full grid costs? And how should electricity markets price and value the attributes of "flexibility," "reliability," "back-up," and "capacity,” rather than just “kilowatt hours produced”?
Still, there is no dispute that in many jurisdictions today, in places with high electricity prices and good solar resources, grid parity is already being reached, even without subsidies. Spain and Hawaii are good examples. Many experts envision expanding "waves" of grid parity reaching a growing number of jurisdictions around the world in the coming 5-10 years. Rooftop solar will become competitive without subsidies in a growing number of locations around the world, provided that the policy challenges ahead are addressed.
Innovations and Challenges Ahead
The main innovations and challenges ahead for rooftop solar relate not to the technologies themselves, but to innovations in business and finance, policy, and integration.
1. New business and finance models. Historically, rooftop solar meant outright purchases by building owners, requiring large capital outlays up-front. However, over the past several years, a variety of other models have emerged, such as leasing, fee-for-service, and pre-paid, which allow customers to pay nominal monthly amounts. Some models even allow renters rather than owners to choose solar, and many of these new innovations have been gaining mainstream acceptance by commercial financiers.
A growing variety of new "energy service company" models promise new forms of financial viability, from third-party building-level energy managers, to micro-utilities, district-energy providers, community-owned systems, cooperatives, and special-purpose entities. (For more on business and finance models, see Chapter 3 of the GFR.)
2. A new generation of power-sector policies and market rules. Policies must now shift from purely cost- or price-based support for renewables to defining new rules and market structures for electricity. In some jurisdictions, there is already some urgency, as established power companies suffer large financial loses on generation and grid assets due to the influx of renewable power. Those loses are leading to growing resistance to renewables by some of these companies, not primarily for technical reasons, but for financial ones. The required transitions implied by such financial loses must be addressed, either by the companies themselves, or through policy. (The GFR discusses the question in Chapter 3: “Will utilities lead, follow, push-back, or perish?”)
Policies must maintain grid access and pricing rules for solar, but there are many variations possible. “Net metering” policies are spreading, but differ widely. Net metering policies essentially allow for outflows of power to the grid—“grid access”—and set one buying price for incoming power and one selling price for outgoing power. Depending on jurisdiction, the selling price may be the same or lower than the buying price. Solar power production can offset or exceed local consumption, but only within established market rules.
Some net metering regimes give full retail price for outflows of solar power, others only wholesale prices. Some cap generation at the level of consumption. Other regimes allow surplus generation beyond the level of consumption, and thus the possibility of earning a monthly profit from the utility.
Feed-in tariffs have played the same function as net metering, providing guaranteed prices and access. Some see a transition underway from feed-in tariffs to net metering, although some solar advocates are loath to suggest the end of feed-in tariffs. In any event, policy evolution, not termination, will be necessary. In jurisdictions without either a feed-in tariff or net metering, access to the grid is prevented, hindering solar growth even in the face of clear grid parity. Spain in recent years has been a classic case, after its feed-in tariffs expired.
Policy must also support new business models, for example "peer-to-peer" energy sales among neighbors, based on nominal “wheeling” charges for use of local wires. London and Sydney, for example, are considering streamlined rules and procedures, and modest wire fees, to support peer-to-peer sales.
3. Materials and systems integration.
In 2012, only about 1% of the global solar PV market was for so-called “building-integrated” PV. However, the integration of solar PV into building materials, such as roofing materials and glass facades, represents an important innovation trend. Another trend is for energy-service companies or solar installers to offer systems with batteries packaged together with solar. Such systems are becoming common in China and India, with growing demand for them elsewhere. 
(For more on integration with buildings, see “Buildings” and “Building Integrated PV” sections of Chapter 2 of the GFR.)
Objections and Myths
Finally, there are a few common misconceptions about solar that should be addressed, for the naysayers who argue we'll never get close to "no rooftop left behind."
1. “The lights will go out.” Contrary to the conventional thinking of past decades, there are several viable options for balancing high shares of renewable energy on power grids. And all of these options are already being used somewhere around the world today to manage the variability of wind and solar power.
Demand flexibility, or “demand response,” is one such important option. Large numbers of customers, especially industrial and commercial, agree to allow some of their load to be modulated using smart controls, over periods ranging from minutes to hours, in return for lucrative monthly payments. Such "aggregation" of many such customers allows for a large capacity of demand to be varied in real time in response to variations in renewable power supply. Already, almost 10% of the entire capacity of the U.S. electric grid is harnessed as demand response, creating the potential for enormous flexibility.
Another option is natural gas peaking turbines. These are very cheap in terms of their capital costs, several times cheaper than a coal power plant or a wind farm, so the costs of having them sit unused are modest. Their primary cost comes from the cost of the natural gas fuel itself, which is only incurred when the turbines are needed.
Conventional power plants like coal and nuclear can also be adjusted to enable them to “ramp up/down" and "cycle” their power output to follow variable renewable generation, although not without additional costs and accelerated maintenance. Ramping can be scheduled based on sophisticated day-ahead weather forecasting models that can predict future renewable power output, which is already being done in several jurisdictions. Renewables are variable but they are not unpredictable.
Several countries or states are already facing the “grid-integration and balancing” challenge as shares of variable renewables rise to levels above 30%, and even higher at peak moments. Germany is the best example. In Germany in 2013, solar and wind power provided up to 60% of the country’s (instantaneous) peak power demand on some days, a new record. (And 36% averaged over a whole day.) A 2013 Greentech Media article covering this fact was titled: “Germany Hits 59% Renewable Peak, Grid Does Not Explode.” 
(For more on options for managing variability, see "Electric Power Grids" in Chapter 2 of the GFR. And for links to dozens of articles and references on this subject, see my new Power-Grid Integration page
on the information part of this web site.)
2. “Not enough raw materials.” This is an important issue but I won’t dispel it here, because it could ultimately prove true. But that would also spell the end of our industrial civilization as we know it. Given that there are more than a billion cars in the world today, a billion rooftop solar panels pales in comparison with the materials needed for the transport sector alone. The hope may come in advanced recycling techniques, but this is a problem not unique to renewables or solar.
3. “No power at night.” The fact that solar only produces power during daylight is mitigated by several factors. First, power grids experience high "peak" loads during the daytime, which can be met by solar, while nighttime loads are much lower and can be met by other generation. For example, air conditioning loads are a significant source of peak load in summer in warm climates, and correspond exactly with the time of day that solar power is producing the most power. Second, solar power can be stored during the daytime and used in the evening, on a daily cycle using pumped hydro storage or other forms of energy storage. (Pumped hydro capacity already exists in many countries. And concentrating solar thermal power (CSP) plants can store their daytime power in the form of heat, and use the heat in the evenings to continue to generate power for several hours.)
4. "Cloudy days.” First, solar does produce electricity even on cloudy days, just not as much. So it’s really a question of economics and nothing else. Either less energy is produced or more panels are needed to make up for the losses due to clouds. With the costs of solar panels falling precipitously these past years, solar profitability even in cloudy climates will improve. Rooftop area may become a limiting factor, but eventually it should become profitable to install panels on the sides of buildings, given cheap panels and high-volume adoption of building-integrated construction materials and practices (like solar glass for building sides). Second, cloud-induced variation can be predicted in advance and managed (see #1 above on keeping the lights on). Third, over a wider geographical region interconnected through transmission grids, solar balances out as passing clouds or storms reduce power only in selected spots, while the average output across the whole grid remains constant.
Let me clear that I'm not saying that solar power is a panacea. There are many other renewable energy technologies that are equally important to the future. Biomass power and heat, especially in northern latitudes during wintertime, is an essential part of the future. Northern European countries already make strong use of biomass for energy. Wind, geothermal, hydro, and concentrating solar thermal power (CSP) all have an important role in power grids of the future, including in balancing and complementing solar. Many believe that we can approach 100% of our electricity from renewable energy in the future using these sources, coupled with appropriate power-grid balancing and management options. (See Chapters 1, 2 and 6 of the GFR.)
Still, solar presents a singular opportunity for distributed energy, for autonomy, for communities, and for entire cities. Most would allow that future energy systems will be a combination of centralized and decentralized, along with intermediate levels like district energy systems. More scenarios and visions are painting the decentralized part of this picture. For example, in the book by Amory Lovins/Rocky Mountain Institute “Reinventing Fire” (2011), one scenario shows a U.S. electricity system in 2050 that gets 80% of its power from renewables, and half of those renewables are integrated into an interlinked network of micro-grids with distributed energy resources.
And solar presents the opportunity for visions like "no rooftop left behind." Many people actually envision solar on all available surfaces, like parking areas, highways, and other public structures. Looking from Google Earth, all the rooftops we see would have panels for electricity and/or heating. Indeed, 20 years from now, a rooftop without solar on it would seem strange, almost "naked," to the sensibilities of the day. Let’s make "no rooftop left behind" a rallying call for the future of our planet.
 As of 3/6/14, Googling "no rooftop left behind" yielded only six hits. Four of these are from a site that refers to rooftop gardens. The other two hits are from a 2009 symposium report published by a Danish group on the subject of energy and food security, in which Janet Larsen of the Earth Policy Institute writes: "I like to envision [a] “no rooftop left behind” campaign, where every building will sport solar panels, solar water heaters, and/or rooftop gardens." So the first use in an energy context could be credited to Larsen. However, since nothing appears on the web since then, I feel justified in hereby coining the phrase for global use. I'll keep track of the growth of Google hits in the future.
(I do notice, that "no roof left behind" yielded 74,000 hits, including an organization called noroofleftbehind.com, but that the first of these hits were oriented towards roofing construction, and "roof" as a meaning for "house" or "household" or "family" rather than the actual top of the roof. And home builders/roofers that also install solar use the phrase as well.)
Report: "Energy, Climate, and Global Food Security," proceedings of a symposium arranged by the Danish National Group et al., 2009, page 14. http://www.fredsakademiet.dk/library/food.pdf.
 I plan to use the phrase in presentations and possibly as a book title. Although book titles cannot be copyrighted, and someone might well beat me to it, I am going on public record here!
 Financial Times, "Renewables: A rising power," Aug. 8, 2013, by Pilita Clark.
 There are no published estimates that I've found, so the 20 million rooftops is my own back-of-the-envelope conservative estimate. Further comments and data are sought to improve the estimate. The European Photovoltaic Industry Association (EPIA) in its Global Market Outlook 2013, shows that about 70% of global added capacity in 2012 (22 GW) were rooftop, and 30% were utility-scale systems. Since most of the existing stock has been added in recent years (Of the 140 GW existing as of 2013, see Note , 100 GW of that was added just in 2011-2013.), one might assume that 70% of the existing grid-tied stock (140 GW minus less than 1 GW for all off-grid stock) is rooftop. Taking broad but conservative liberties, assuming that 40% of this rooftop capacity is for residential systems at 3-kW each, and 60% of this capacity is for commercial/industrial systems at 100-kW each (if lower average capacity, then more rooftops would result), the total number of grid-tied rooftops existing at end-2013 would be about 14 million. The actual number could be considerably higher than this, given that many commercial/industrial systems fall into the range of 10-100 kW. There may be better numbers that I just haven't seen yet.
Then add off-grid solar home systems in rural areas of developing countries, which I estimate at 5-7 million given past numbers in Chapter 5 on rural renewable energy of annual editions of the REN21 Renewables Global Status Report. There used to be global estimates for solar home systems, but the last such estimate was 2.5 million in the 2007 edition. Since then, hundreds of thousands of systems have been going in annually. The 2013 edition reports over 2 million solar home systems just in Bangladesh alone. The 2011 edition reports 600,000 in India as of 2010, but there has been rapid market growth in the past few years. China had several hundreds of thousands, if not millions.
So together, 14 million grid-tied, conservatively, and 5-7 million off-grid, yields "at least 20 million" but could also be considerably higher.
As for 90 million rooftops with solar hot water systems, this is based on past methodologies I used in writing the REN21 Renewables Global Status Report, in which number of solar hot water rooftops has been published over the years. The 2013 edition gives 255 GWth of glazed solar heat capacity in 2012, or an equivalent of 360 million m2. Assuming 3 m2 per residential rooftop, and 80% of the surface area for residential buildings, both of which are conservative considering most systems in China are smaller (closer to 2.0-2.5 m2/system), that leads to at least 90 million rooftops in 2012. (The 2010 edition of the GSR gives 70 million rooftops; see Endnote 132 of the 2010 edition for similar estimation details.)
 There have been various estimates of the total roof surface area worldwide, such as 20 m2/capita, but apparently not in terms of number of roofs. (See California Photon, Can Rooftop Solar Satisfy Most of Humanity's Energy Needs? http://californiaphoton.com/energy/world/rooftop.html.)
This needs more work too. Consider two data points:
(a) One paper estimates the total number of buildings in Germany at 38 million in 2004, for a population of 82 million, or about one building for every two people. ("Estimating the number of buildings in Germany," Martin Behnisch and Alfred Ultsch, http://www.mathematik.uni-marburg.de/~databionics/papers/09BehnischUltschGfklHamburg.pdf)
(b) The U.S. EPA estimated 128 million residential housing units in 2007 and 5 million commercial buildings in 2003 in the U.S., not counting other types of buildings. Given the preponderance of single-family homes, there might be 80-90 million residential buildings. So there are likely over 100 million buildings for 300 million people, or one building for every three people. (US EPA, 2009, "Buildings and their Impact on the Environment: A Statistical Summary," http://www.epa.gov/greenbuilding/pubs/gbstats.pdf.)
Given that developing countries will have fewer commercial buildings per capita, and larger household sizes, there must be fewer buildings in developing countries than the above ratios suggest. If one building for every four people, then there would be 1.8 billion rooftops worldwide. If one rooftop for every five people, there would be 1.4 billion. Probably one of the biggest unknowns is the ratio of multi-family residential buildings to single-family buildings.
 Some argue that "ground-mounted" or "utility-scale" solar power will become dominant in the future, rather than rooftop systems, based on lower per-unit costs for ground-mount systems today. For example, the European Photovoltaic Industry Association, in its 2013 Global Market Outlook, projected that utility-scale systems will take a growing share of market growth in the coming years. There is no question that these types of systems will have a role in the future. But the verdict is still out on how non-rooftop vs. rooftop systems share the future market. It also depends on land-use patterns and the amounts of unproductive unused land, for example in arid areas, available in different regions.
One argument against utility-scale is that the power more often feeds into transmission networks directly, and can only be sold at much lower wholesale rates, rather than higher retail rates. Or that it suffers from "merit-order" disadvantages (see the "PV Parity Final Report", www.pvparity.eu). Distributed rooftop power places the power right where it is consumed, reducing transmission and distribution costs. It is not my intention here to convey the full arguments on both sides, many others are doing that.
The REN21 Renewables Global Status Report 2013 gave 100 GW existing at the end of 2012. The European Photovoltaic Industry Association Global Market Outlook (2013) gave 102 GW for end-2012. A preliminary estimate by Bloomberg New Energy Finance in January 2014 gave 39 GW added in 2013.
For some scale of comparison for capacity numbers in GW, the global power grid, counting all forms of generation, including fossil and nuclear, amounts to about 5700 GW. (*) And there is about 370 GW of nuclear power capacity existing worldwide. (**) If current solar PV market volume continues without further acceleration, i.e., 40 GW per year, and if all nuclear power plants under construction are completed before 2021, then solar power capacity will exceed nuclear power capacity by around 2021. (***)
(*) The 2012 Global Status Report gives 5400 GW as of end-2011. By end-2013, another 250-300 GW was added (no source yet), for a total of 5650-5700 GW.
(**) The Nuclear Energy Institute (nei.org) gives 372 GW of existing capacity, and 70 GW of capacity under construction.
(***) Solar PV would reach 460 GW by 2021 if 40 GW per year were added from 2014-2021. Assumes any new nuclear plants that start construction in 2014 or later would not be finished by 2020.
 China is targeting 280 GWth by 2015, or 400 million m2, up from 150 GWth in 2011, almost a doubling in just four years. Several years ago, China's target was 210 GWth by 2020. (REN21 Renewables Global Status Report 2013.)
 See note . For investment, see the investment trends chapter of the REN21 Renewables Global Status Report.
 Historic data see Martinot (2004), Renewable Energy World, vol. 7, no. 5. China data from Solar Server, "China became the world's largest solar PV market in Q3; nation is poised to additional multi-GW deployment," http://www.solarserver.com/solar-magazine/solar-report/solar-report/china-became-the-worlds-largest-solar-pv-market-in-q3-nation-is-poised-to-additional-multi-gw-deployment.html. Japan data from Institute for Sustainable Energy Policy estimates. US data from Solar Buzz, Jan. 8, 2014, "Record 2013 Solar PV Installations Promotes U.S. to Strongest Market Outside Asia-Pacific," http://www.solarbuzz.com/news/recent-findings/record-2013-solar-pv-installations-promotes-us-strongest-market-outside-asia-pa
 For policy status, see the current edition of the REN21 Renewables Global Status Report. For policy history, see past editions. For China policy, see Solar Server in Note  and a variety of information at www.frankhaugwitz.info.
 For city information, see the local policies section of the policies chapter of the REN21 Renewables Global Status Report. Also see REN21/ISEP/ICLEI, "Global Status Report on Local Renewable Energy Policies" (2011), at www.martinot.info/cities.htm
 Reference is to sidebar in IEA World Energy Outlook 2013, Chapter 6, p. 219.
 Market share of BIPV from Renewables Global Status Report 2013, page 42.
 Greentechmedia.com, October 30, 2013, "Germany Hits 59% Renewable Peak, Grid Does Not Explode," http://www.greentechmedia.com/articles/read/Germany-Hits-59-Renewable-Peak-Grid-Does-Not-Explode