SWALEC"s parent company, Scottish and Southern Energy Group, is celebrating the signing-up of its six-millionth customer.
A number of staff initiatives have included a Six Nations" fun day challenge which involved dressing up in the national costumes of favoured teams.
The group has gained almost 1.5 million new customers since 2002 in the fiercely contested utilities market.
SWALEC claims to have contributed an extra 120,000 customers to the growth.
It now supplies 1.2 million customers in Wales with electricity and gas.
SWALEC operations manager Roger Pettit said: "Our combination of best-in-class customer service and the value of our brand means we will continue to gain customers."
The SWALEC workforce has grown to 650 at its Cardiff Gate offices.
More jobs are being created due to its move back to electrical contracting.
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