Solar power systems can save households money, but don't expect them to eliminate your power bill.
The systems have become cheaper in recent years, making them more accessible to homeowners. Panel prices are now about a fifth of what they once were.
The Green Party last week promised that if it got into Government, it would offer low-interest loans for those borrowing up to $15,000 to install panels to reduce their power bills.
But solar power expert Phil Rumble, of Rumble Electrical and Solar, said people should not think of it as a magic solution.
The amount of money you could save with a solar power system tied to the residential power grid would depend on many things, such as how much sun your house got, how much power you used and when you used it, he said.
A standard installation for an easy-access site would be about $8500, for a 3kW system, which would deliver 3kW/h. If a house got five hours of sun a day, the system would generate 15kW daily.
Most grid-tie systems cannot store any excess power. So if it was not used as it was generated, it would be sold back to the grid system. Depending on your power company, that can paid for at a lower rate than the tariff the company charges.
Most power companies pay between 3.5c and 15c per kW generated but charge up to 28c, so panels must create at least twice as much power to sell during the day to offset the cost of bought power at night.
For that reason, Rumble said, solar power systems were more effective for people who were home during the day to use the power.
A 3kW system would cut power bills by anything from 30 to 75 per cent, depending on the variables, he said. Few people would be able to eliminate power bills.
"If the bill is $300 and you can save $200, that's worthwhile. To get that extra $100 might double the cost of the installation."
More openness and honesty was needed in the solar industry, he said. Too many operators tried to sell people systems with elaborate promises they would not live up to.
"It's getting better. There's less bad quality stuff around."
Systems must be installed by qualified electricians. Lines companies usually charge about $300 to hook systems up to the grid.
Waitakere woman Linda Williams has had a solar power system on her roof for a couple of years. She paid about $9000 for the eight panels and said her power bills had fallen about a third.
She works from home but her last three power bills have been $70-$80. She said it worked well because she was below Meridian's cut-off at which it paid less for power put back into the grid.
There are no Government subsidies for the installation of solar power or hot water systems. KiwiBank offers loans for homeowners to pay for a solar, wind, small-scale hydro or geothermal power system in their home loan. The bank contributes $2000 over four years towards the cost.
Westpac and Meridian have a Solar Shed loan for farmers, which gives 100 per cent finance for a 10kW system, repaid over three years.
What power companies pay
*Contact will pay 17.285c per kWh to residential customers who have distributed generation systems including solar with a peak load of less than 10 kilowatts.
*Meridian offers 25c per kWh for the first 5kWh of excess energy each day, and 10c per kWh for anything above that.
*It charges $85 to install an import-export meter.
*Genesis' HomeGen product is for customers wanting to export power back from their generating equipment, which could be solar, wind or hydro. Customers need an export meter. The current export price is about 6c a kWh depending on which network the customer is on. Genesis said that was roughly equivalent to what it would be paid for its own generation.
*Mercury pays 3.5c a kW. A spokeswoman said that rate was low but reflected the availability of competing renewable energy in the wholesale market. "This rate is currently under review."