Power station workers in Gwynedd are being balloted on strike action over a pay offer.
Unite the union said staff at First Hydro Company pumped storage plants at Dinorwig and Ffestiniog in Snowdonia have "bent over backwards" in the pandemic.
A pay offer of 1.4% and a £1,000 lump sum for this year followed an RPI linked raise (to a maximum of 4%) for the next two years was rejected.
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Unite said: "First Hydro Company (FHC) are making vast profits and can afford to do much better than 1.4%. FHC have returned tens of millions of pounds to shareholders in recent years.
"The workforce have worked flexibly and professionally through the pandemic. There is still time for FHC to make a fair pay offer.
"To date, First Hydro have failed to make an acceptable pay offer, we urge the company to return to negotiations before any action takes place."
First Hydro Company have been asked to comment.
The ballot closes on October 20.
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