How much solar PV do you need?
You’ve decided to join the solar revolution and have a crack at turning the sun’s energy into electricity to power your home or office – great stuff! Now it’s time to figure out – how big?
The solar equation is a slippery thing – be it on a global or local scale. Projected global solar booms have been forecast before, but this one looks like it’s for real – thanks mainly to the dropping price of solar panels over the past five years.
So, now you’ve committed to getting solar panels on your home or business, some maths is required to ensure the time it takes to pay off your system is as short as possible – ensuring your investment is a good one.
Some power companies pay you for the excess power you generate, some don’t. Of those that do (see sidebar), rates can vary, and power companies don’t particularly enjoy handing you a cheque at the end of the month, rather than the other way around – although this is possible.
The average home in New Zealand uses around 25kWh (kilowatt hours) per day.
A 2Kw (kilowatt) solar PV system will generate 2 kilowatt hours (kWh) of power for every hour of sunlight.
NIWA statistics show that in summer, in Auckland, the average hours of sunshine a day is 6.5 (October to March) and the average daily winter sunshine hours is 4.6.
In the summer a 2Kw system could be generating around 12 kWh per day. In the 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.