The benefits of solar power panels need to be carefully weighed against the costs, EECA says.

Timaru man Stephen Malone thought he'd make major savings on his power bill after installing solar power panels.

But it has not worked out that way. After spending more than $11,000, he has seen a small decline in his bills but nothing like he expected.

He estimates he's saving on average about $20 a month. He still pays a monthly fixed charge. And he's even found himself in the odd position of sending more power back to the grid than he is using.

DOUG FIELD/STUFF Stephen Malone is not impressed with his energy savings.

According to his last bill, "I'm exporting 272 [KWh per hour] when I'm using 246."

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"I've had more from their prompt payment discount."

ROSS GIBLIN/STUFF In Wellington, Craig Burt is taking part in a Contact Energy experiment in "distributed energy," turning a cluster of neighbouring homes into a self-sufficent mini power grid. Power companies are exploring new ways of expanding the grid through self-generation.

Malone's disenchantment with solar power was made worse by the fact that his salesman promised him savings of 60 to 75 per cent of his electricity bills. It wasn't written into his contract but he has it in writing on a back-of-the-envelope quote.

That promise, according to the Energy Efficiency and Conservation Authority EECA, would be "very difficult to achieve".

But when Malone went to the company, he was told the salesman no longer worked there.

Christian Hoerning​, a senior technical advisor from EECA, says Malone may well have been over-promised and could have grounds to take a complaint under the Fair Trading Act.

And even though solar power is destined to become a big part of our future, the authority is concerned that people understand that it's not cost effective for everyone.

To help people do the maths, it has put a solar calculator on its website.

"I think people that are interested in solar are confronted with a lot of conflicting and confusing information," Hoerning said.

Solar power stacks up best for people who use much of their electricity when it's being generated – during the day and especially in summer.

"If you use a considerable amount of electricity during the day or if you can change the way you use electricity ... do your washing in the morning, have your shower in the morning so that the hot water gets regenerated while the sun is shining and generating electricity...

"If you can make these changes because people are at home during the day, then a solar system would be more likely to make financial sense."

Malone's smallish household uses most of its power at night. So he decided to feed his excess energy back to the grid "because we're north-facing and get a lot of sunshine here".

He is paid at about 8c/KWh for that energy, but he is buying back his power at night at 22c/KWh, a situation that's fairly common from utilities across the country. Hoerning said this was an equation that caught many people out.

Solar power also stacks up where connecting to the grid is expensive, when there's a battery to store excess energy, or where power prices are high.

And it is important not to get too big a system for one's needs, so the owner uses all the electricity he or she is generating.

The litmus test is how long it takes for a system to pay itself off. Malone was told by his salesman that he could expect to make a return within six years.

But Hoerning said that was unlikely – typically a pay-off was at least 15 years away.

"In most cases, it will take many, many years to break even."

The great hope is that rapid advances in technology will one day make solar panels cost-effective for everyone. At the moment, batteries still cost tens of thousands of dollars.

"They're not economic at this stage," Hoerning said. "However, prices have been dropping over the years and it may be that we get a point in the future where the economics for these systems look more favourable than they currently do."

Consumer NZ's Jessica Wilson said her organisation had not fielded many complaints over solar power systems not delivering on promises, but it was often approached by people wanting to know more.

"Consumer interest in solar is growing. In our last electricity survey, 9 per cent of consumers said they'd installed a solar power system, up from 5 per cent the previous year.

"About 45 per cent of homeowners also said they'd thought about installing solar."

However, industry group the Sustainable Electricity Association (SEANZ) says savings of the magnitude promised to Malone are "quite possible," if certain tips and tools are used.

Diverting unused energy to power hot water or electric vehicles, and setting day-time timers on appliances could make a big difference, spokesman Kristin Gillies said.

"If you put a pile of solar panels on your roof and then you're not home during the day and don't do anything else, then they're going to take forever to pay themselves off."

Unfortunately for Malone, the company who installed his system, New Zealand Solar Systems, went into liquidation late last year.

No complaints about the company had been received by the Ministry for Business, Innovation and Employment's consumer protection team.

But the Commerce Commission recently criticised another company, NZ Homes Services, for financial claims it was making about solar panels.

Gillies said he did not think the industry was worse than any other but as solar energy took off, there were some companies coming over from overseas and boasts being made.

"This is why SEANZ exists, because we want to keep the cowboys out of the market."

SEANZ also has a tool on its website to help people optimise their solar use.