A giant bottle made up of dozens of smaller ones carrying messages about climate change is at the the Senedd as Earth Hour is marked.
The installation from environmental charity WWF-Cymru holds hundreds of messages from people all over Wales.
The National Library of Wales, Caernarfon Castle and Swansea's Waterfront museum will also switch their lights off on Saturday evening.
Musicians will perform next to the bottle during the switch-off.
WWF-Cymru head Anne Meikle said: "We are thrilled by the public response to this year's Earth Hour Wales. It is the biggest yet, and we're delighted that, for the second year, all 22 local authorities have signed up, together with businesses and over 300 schools.
"This includes Coastlands Primary School, whose pupil, Rebecca, won the UK-wide Earth Hour poetry competition."
Assembly presiding officer Elin Jones AM said: "We strive to improve our environmental performance; not just by supporting Earth Hour each year, but making improvements like a 40% reduction in our energy footprint since 2008."