Davis Canning says his power bills have dropped by $100 or $150 a month since he decided to switch to solar power for his house.
The Havelock North man signed up for Solarcity's solarZero plan about two years ago.
Instead of buying a solar power system outright, solarZero provides a system in return for a monthly fee paid over 20 years. Solarcity owns and manage the components.
Canning says their bill has always been less than it would have been without the system, and in some months of summer, he and his wife had not had a power bill at all.
Solar power systems have dropped in price significantly over recent years. But homeowners are being warned that it is still not always a one-way bet.
Here are some things to consider before you invest in a system.
Check your expectations
Solar power does not usually mean no more power bills.
Solarcity chief executive Andrew Booth said people could expect on average to cover the capital costs of an installation, if paid for outright, in nine or 10 years.
From that point forward you'd have free daytime power and roughly 30 per cent of your power bill should disappear."
Those who took the "solar as a service" option that his firm offered should see savings immediately, he said. The average customer in Auckland saved about 20 per cent, he said.
Get the right size, and type of system for your house
Booth said it was common for people to believe that bigger was better but it was important that a system was the right size for a house.
"People believe the economics work better the larger the system you install but there's a danger you end up installing a lot of silicone on the roof and you don't save the money you believe it will."
He said the system should be sized to meet the households' energy requirements during the day.
Most New Zealand houses use grid-tied systems, where any power that is generated by the solar panels on the roof, and not used immediately, is sold back into the grid. Batteries to store that power are a more expensive option, although their price is dropping.
If you live in a house with a lot of shade, you may not get enough sunshine to make it worthwhile.
Trade Me has begun offering Solarcity's Sunny artificial intelligence chatbot which helps homeowners decide if their house is suitable, including data from Google Earth, GIS data from across city councils and NIWA data on local weather patterns.
Consider your power use
Simon Coates, of Concept Consulting, said solar power systems (for example: GOAL ZERO NOMAD 100 ) did not always generate the power savings that people expected because of how they used power.
People with grid-tied systems used to be able to sell back their power at almost the same rate that power companies charge consumers.
But now most companies only pay what they would pay other wholesale power generators. This means that you get less money for the power you generate than you pay for the power you use. You'll only really save money if you can use the power as it is being generated.
"The big savings are when consumers can reduce their power consumption for which they are paying 22c a kWh hour," Coates said. "If they're exporting the power, they get paid what it is worth to a retailer, which is about 9c or 10c a kWh."
Look at how your household uses power and whether you could adjust that to make the most of your own supply.
Canning said solar power worked well for his household because he and his wife are both retired, so someone was usually home during the day. They set their dishwasher to go while solar power is being generated and try to shower when the sun is up. They set their washing machine and dryer to go one at a time during the day. "That's possible when someone is at home," Canning said.
Coates said it was possible that the power tariff system could change in future, to better reflect the cost of delivery and keep up with the changing market.
That would mean power would become most expensive at the times when there was most demand, such as the evening peaks. Those are the same times that solar systems would not deliver.
Buy quality components
There is a growing number of cheap solar power parts available, but it is important to get a quality system.
Because it will take many years just to cover the cost of the installation, the parts need to last at least that long. Look for long warranty periods and qualified installers.
Coates said people should consider their motivations for installing solar. He said, if it was purely to lower their carbon footprint, an electric vehicle would have a bigger impact. There was a risk that increasing solar use would just displace wind power options that would otherwise be built, he said.
"If you want to do your bit for the environment, buy an electric vehicle. Two-thirds of household emissions are from driving your vehicle, on average, not from consuming electricity."
