Almost 90% of the power utility’s debt is in foreign currency, meaning that a 21% appreciation of the shilling against the dollar this year helped Kenya Power to book 7.88 billion shillings ($61 million) of unrealized foreign-exchange gains in the year ended June 30 after loan revaluations.

“Unlike last year, the impact of the finance costs was a bit gentle,” Chief Executive Officer Joseph Siror told investors on Tuesday after the company reported a 30 billion-shilling profit, the highest in its history. “We’re only dealing with about 680 million shillings,” he said.

Revenue jumped 21%, driven chiefly by commercial and industrial customers. The company added almost 450,000 new users to its grid in the year.

While Kenya Power’s electricity bills are in shillings, its purchase contracts are predominantly in foreign currency and the company has asked authorities for approval to collect payments in dollars.

The shares surged as much as 71% to a six-year high of 6 shillings. The stock’s relative strength index exceeded 70, indicating it may be overbought.