About 200 TrustPower customers have abandoned the company in favour of cheaper electricity since an advertising war broke out among three companies.
Energy Online and Powershop each launched advertising campaigns in Tauranga a month ago, which claimed TrustPower customers would be better off with them, despite the annual TECT cheque rebate.
TrustPower has since launched its own campaign in response and is preparing to lodge a formal complaint with the Commerce Commission and Advertising Standards Authority.
TrustPower community relations manager Graeme Purches said a total of about 200 of the Tauranga company's 59,000 customers had switched to other providers since the campaigns were launched.
"The churn has been really, really small," he said.
About 41 TrustPower customers switched to other providers last week, he said.
"If you look at it as a percentage it's nothing," Mr Purches said. "There's been a lot of money spent for not much reward."
TrustPower was based in Tauranga and had a reputation for supporting the community, he said.
"We're quite open that we're never going to be the cheapest in town but we don't aim to be.
"We want to be sustainable."
Powershop chief executive Ari Sargent said the company had gained almost 400 Tauranga customers since they began targeting the area two months ago.
Most of those were former TrustPower customers but some had switched from other providers, he said.
The rate of change was about what he expected but said it was likely to increase as they established a solid customer base during the next few months.
"It's similar to what we've found when we've gone into other areas. It take a while to get people's confidence up."
Mr Sargent said he was surprised to see Tauranga residents who had been with the same company since 1996 wanting to swap to Powershop.
"It's clear that quite a number of people can definitely save money," he said.
Energy Online spokesperson Richard Gordon would not reveal how many customers the company had gained through the campaign because of "commercial sensitivity" but said it was on target.
"We're relatively early in the campaign. It's something that will continue for some time."
Advertising would tail off during the summer holidays but would pick up again next year, he said.