One of the constraints of solar power is that it is not always available: it is dependent on daylight hours and clear skies. In order to fill these gaps, a storage solution or a backup infrastructure of fossil fuel power plants is required — a factor that is often ignored when scientists investigate the sustainability of PV systems.
Whether or not to include storage is no longer just an academic question. Driven by better battery technology and the disincentivization of grid-connected solar panels, off-grid solar is about to make a comeback. How sustainable is a solar PV system if energy storage is taken into account?
In the previous article, we have seen that many life cycle analyses (LCAs) of solar PV systems have a positive bias. Most LCAs base their studies on the manufacturing of solar cells in Europe or the USA. However, most panels are now produced in China, where the electric grid is about twice as carbon-intensive and about 50% less energy efficient.  Likewise, most LCAs investigate solar PV systems in regions with a solar insolation typical of the Mediterranean region, while the majority of solar panels have been installed in places with only half as much sunshine.
As a consequence, the embodied greenhouse gas emissions of a kWh of electricity generated by solar PV is two to four times higher than most LCAs indicate. Instead of the oft-cited 30-50 grams of CO2-equivalents per kilowatt-hour of generated electricity (gCO2e/kWh), we calculated that the typical solar PV system installed between 2008 and 2014 produces close to 120 gCO2e/kWh. This makes solar PV only four times less carbon-intensive than conventional grid electricity in most western countries.
However, even this result is overly optimistic. In the previous article, we didn’t take into account “one of the potentially largest missing components”  of the usual life cycle analysis of PV systems: the embodied energy of the infrastructure that deals with the intermittency of solar power. Solar insolation varies throughout the day and throughout the season, and of course solar energy is not available after sunset.
Until the end of the 1990s, most solar installations were off-grid systems. Excess power during the day was stored in an on-site bank of lead-acid batteries for use during the night and on cloudy days. Today, almost all solar systems are grid-connected. These installations use the grid as if it was a battery, “storing” excess energy during the day for use at night and on cloudy days.
Obviously, this strategy requires a backup of fossil fuel or nuclear power plants that step in when the supply of solar energy is low or nonexistent. To make a fair comparison with conventional grid electricity, including electricity generated by biomass, this “hidden” part of the solar PV system should also be taken into account. However, every single life cycle analyse of a solar PV ignores it. [3, 2].
Until now, whether or not to include backup power or storage systems was mainly an academic question. This might change soon, because off-grid solar is about to make a comeback. Several manufacturers have presented storage systems based on lithium-ion batteries, the technology that also powers our gadgets and electric cars. [4, 5, 6, 7] Lithium-ion batteries are a superior technology compared to the lead-acid batteries commonly used in off-grid solar PV systems: they last longer, are more compact, more efficient, easier to maintain, and comparatively more sustainable.
Lithium-ion batteries are more expensive than lead-acid batteries, but Morgan Stanley’s 2014 report on solar energy predicts that the price of storage will come down to $125-$150 per kWh by 2020.  According to the report, this would make solar PV plus battery storage commercially viable in some European countries (Germany, Italy, Portugal, Spain) and across most of the United States. Morgan Stanley expects a lot from electric vehicle manufacturer Tesla, who announced a home storage system for solar power a few days ago (costing $350 per kWh).  Tesla is building a factory in Arizona that will produce as many lithium-ion batteries as there are currently produced by all manufacturers in the world, introducing economies of scale that can push costs further down.
Other factors also come into play when it comes to home storage for PV power. Solar panels have become so much cheaper in recent years that government subsidies and tax credits for grid-connected systems have come under pressure. In many countries, owners of a grid-connected solar PV system have received a fixed price for the surplus electricity they provide to the grid, without having to pay fixed grid rates. These so-called “net metering rules” or “feed-in rates” were recently abolished in several European countries, and are now under pressure in some US states. In its report, Morgan Stanley predicts that, in the coming years, net metering rules and solar tax credits will disappear altogether. 
Utility companies are fighting the incentivisation of PV power succesfully with the argument that solar customers make use of the grid but don’t pay for it, raising the costs for non-solar customers.  The irony is that the disincentivization of grid-connected solar panels makes off-grid systems more attractive, and that utilities might be chasing away their customers. If a grid-connected solar customer has to pay fixed grid fees and doesn’t receive a good price for his or her excess power, it might become more financially savvy to install a bank of batteries. The more customers do this, the higher the costs will become for the remaining consumers, encouraging more people to adopt off-grid systems.