Lockdowns have been a controversial aspect of the COVID-19 pandemic. Yes, they undoubtedly save lives, but they also cause major ongoing economic issues – disrupting industries, causing job losses and associated financial pain.

But another thing lockdowns have done all over the world is decrease air pollution, and new research shows an interesting flow-on effect of this.

The new study has looked at solar power in Delhi – one of the most polluted cities in the world – and has found that the reduction in air pollution has allowed significantly more sunlight to get through to solar panels in the city, increasing their output.

"The increase that we saw is equivalent to the difference between what a photovoltaic (PV) installation in Houston would produce compared with one in Toronto," says first author Ian Marius Peters of Helmholtz-Institut Erlangen-Nürnberg for Renewable Energies in Germany.

"I expected to see some difference, but I was surprised by how clearly the effect was visible."

Delhi went into a strict lockdown on the 24 March 2020. The team looked at the PM2.5 particle concentration, a measure of fine particles less than 2.5 micrometres in diameter, in the air at the US embassy in Delhi before and after the lockdown – and compared them to the last few years at the same time of the year.

They also looked at the clear sky irradiance – which is how much sunlight reaches Earth's surface without being scattered or absorbed by particles and gases in the air.

The team found that, overall, the amount of sunlight reaching solar panels in Delhi increased by around 8 percent in late March 2020, and 6 percent in April 2020 compared to similar dates in earlier years.

"PV installations in March and April received more than 6 percent more light in total than in previous years and will continue to generate record amounts of electricity as long as air pollution levels stay low," the team write in their paper.

"We expect the same to be true for urban PV installations in other cities with high air pollution levels and COVID-19 related restrictions. Examples for such cities are Kolkata, Wuhan, Mumbai, Dhaka, Los Angeles or London."

Although it's very unlikely these levels will stay low and the air will stay clean forever, it is a great reminder that humans can and do drastically change the world around us, and pollution doesn't need to be a permanent part of the landscape.

It's also a timely reminder that as we slowly return to what will become 'normal' after this pandemic, it's an opportunity to rebuild a more sustainable future – with cleaner air, healthier kids, and a more liveable planet.

"The pandemic has been a dramatic event in so many ways, and the world will emerge different than how it was before," Peters says.

"We've gotten a glimpse of what a world with better air looks like and see that there may be an opportunity to 'flatten the climate curve'. I believe solar panels can play an important role, and that going forward having more PV installations could help drive a positive feedback loop that will result in clearer and cleaner skies."

This research has been published in Joule.