In summer, a 2kW system could be generating around 12 kWh per day. In winter the system could be generating around 9kWh. It's reasonable to use five hours of sunlight per day as an average.
In a perfect world, with panels aligned due north, on a 30 degree slope, you'd get 10kWh a day. It's more likely, therefore, to be 8kWh per day, or 2920kWh a year. At 0.27c per kWh (about what the power companies charge you) this is $788 worth of power.
Now to calculate the return on investment (ROI) on a system. If you use all the power your array produces in your home or business, you can divide the total cost of your system by your savings per year to get your ROI. In this example, a 2kW system costing $7500 will take 9.5 years to pay off ($7500 divided by $788), without taking into account our ever-rising power prices. For a system that produces electricity for at least 25 years, that's not bad going.
For a home which uses the New Zealand average of 25 kWh each day, it's not hard to imagine using up to half of that power during sunlight hours, particularly if people are in the building during the day. If they aren't, it's possible to put a bunch of appliances on timers - towel rails, dishwasher, washing machine, dryer - even water heating - while the sun is shining.
Here's where it gets interesting. Now triple that solar array to 6kW. You're now producing 24kWh each day on average. You're still using some of that power but the rest is being exported to the grid. Your power company pays you well for some of it, but as soon as you get over a certain threshold, the price you get for your panels' electricity is very small indeed - as little as 3.5 cents per kWh. In this case your return on investment isn't so rosy, and payback times may balloon out to 15 years or more. Therefore it's important not to go too big.
A large number of homes are now wired up to the grid using smart meters, which let you know your home's electricity usage to the half hour, extremely useful for choosing solar array size. You can see exactly how much power you use during daylight hours, and buy a solar array to suit. Element's recommendation is to buy an array that produces 5kWh more than your use during sunshine hours - those 5kWh hours can still be sold back to the retailers for a decent rate, while at the same time you're covered for any changes in your lifestyle.
What electricity companies pay you
When you use your own generated electricity, you save buying at the retail price. Here's what companies can offer if you produce more than you use and export to the grid:
• Contact: 17.285c per kWh for a peak load of less than 10kW
• Genesis: 5.25c - 5.95c per kWh depending on location
• Mercury: One-for-one offset of export and import provided exports are less than imports and 3.5c per kWh over that
• Meridian: 25c per kWh for the first 5kWh (per billing day) and 10c per kWh after that
• Nova Energy: Buyback offer under review.
• Trustpower: 7c per kWh
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