I'm trying to work out the maths for my girlfriend's dad. He's had a couple of quotes from installers, and they're quoting 5-8 year payback times, but as soon as you ask them for details on the figures they get all vague, or come up with something that sounds really dodgy. They're recommending 2-3kW systems so I don't think they're trying to oversell it, but we'd like to know if it's going to actually make financial sense.
Quoted prices are $6900 for 2 kW or $8900 for 3 kW, incl everything.
I'm pretty sure I've got a good idea of how much the system would generate - a 3 kW setup in Auckland should apparently put out about 3750 kWh over the year. At a variable rate of 27c (incl GST) per kWh, that'd be just over $1,000 if it was all used. Which would seem to be a good investment over a 20-year lifetime.
But if only half was used and half exported (at say 8c), he'd only save $656, which isn't as clear-cut - only 7% simple return on a depreciating asset. Then you have to make assumptions about future power prices, cost of finance, inflation etc etc which gets pretty messy over 20 years.
I've tried to estimate it based on looking at his usage graphs and thinking about what use could be shifted to the daytime, but it's getting quite complicated. I don't really need to know what it'll be exactly, but do need a ballpark of what's realistic.
So my question for people with solar installed already is - how much of it do you manage to use? Is it realistic to assume nearly 100% use or is it likely to be more like 50%?
Thanks!
Trusted
