Solar energy is undoubtedly renewable. We must make sure it is also as sustainable as possible.

So far, 2014 has been a good year for solar power. In February, the world's largest solar thermal power project opened about 65 km outside Las Vegas in the Mojave Desert (http://go.nature.com/P4cvjv). Long, sunny days in June meant record solar power generation in numerous European countries (http://go.nature.com/WjcbQm), and in July the price of electricity dropped to zero in the middle of the day in Queensland, Australia, driven largely by the use of rooftop solar panels (http://go.nature.com/Och7Hg).

Soaring solar power use is due partly to a huge drop in costs and a rise in production of the photovoltaic solar panels that convert sunlight to electricity. For the past 30 years, the cost of these solar panels has fallen by 10% per year, while production has risen by 30% per year (Nature 507, 300–302; 2014). The surge in production comes mostly from China, now the world's biggest producer of solar panels. In China, the panels are produced for a much lower price compared with Europe and the US, but this monetary saving comes at a cost.

Less-energy-efficient production methods (Nature Geosci. 6, 507–509; 2013) mean that the carbon footprint of a solar panel made in China and used in Europe has been estimated to be almost twice that of a panel both made and used in Europe (Solar Energy 105, 669–678; 2014). In addition to the carbon footprint, environmental protection can be a concern (Nature 509, 563; 2014).

Further unintended consequences of solar panels may include shading the land, as well as modifying wind flow and the distribution of rainfall, and thereby soil moisture and nutrient cycles. The impact of giant parks of solar panels on local levels of carbon exchange between soil and the atmosphere is not yet understood. However, this must be considered in any carbon accounting of solar energy production.

If solar power is to play an important role in a sustainable future, the carbon footprints and environmental impact of the production, trade and use of solar panels must all be minimized. We need to look beyond maximum energy output at a minimum price tag.

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