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# 177470 3-Aug-2015 16:18
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Hoping to get some help from people who've installed solar PV

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!

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  # 1357970 3-Aug-2015 16:36
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I think the payback there is pretty optimistic, but if you take power price rises into consideration, it may make it better (cost of power from your retailer is never going to go down is it?)

My parents are getting  a system installed, they know it isn't likely to pay them back for quite some time, but they like the idea of 'doing their bit' and they are in a position of being able to afford the outlay.

I'm sure having smaller power bills wont hurt either.

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  # 1357999 3-Aug-2015 17:00
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Friend of mine has solar, with a diverter thingy so pretty much all of it goes into heating hot water - he has two tanks plus a spa. He's saving less on his power bill than me (30%), and all I did was switch to Flick Electric.

Problem with solar is without storage you have to match load to generation, and the only thing you really need when the sun shines is air conditioning and maybe water heating. Even a small amount of storage could make it much more practical.

 
 
 
 


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  # 1358003 3-Aug-2015 17:07
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fortydayweekend: 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!

 

 

Thats the 64K question, and it depends alot on the household,

 

How many people?, Are they at home or work during the day?, Can their appliances run delayed? (so they can run during Solar generation time)



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  # 1358022 3-Aug-2015 17:15
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trig42: I think the payback there is pretty optimistic, but if you take power price rises into consideration, it may make it better (cost of power from your retailer is never going to go down is it?)


Murphy's Law says the price will go up, right after we install solar :)

timmmay: Problem with solar is without storage you have to match load to generation, and the only thing you really need when the sun shines is air conditioning and maybe water heating. Even a small amount of storage could make it much more practical.


Yeah, I looked at that! Turns out you'd need a garage full of Powerwalls to go completely off-grid during the winter, and it'd cost tens of $1000's. Maybe something for the replacement build in 20 years :)

wellygary: How many people?, Are they at home or work during the day?, Can their appliances run delayed? (so they can run during Solar generation time)


It's complicated - sometimes people at home during the day, but not all the time. Kids home in the afternoons. Appliances can be put on timers, hot water is electric. No pool or spa.

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  # 1358073 3-Aug-2015 18:10
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you really need to work out the base load and then go from there

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  # 1358085 3-Aug-2015 18:17
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Powerwalls are expensive, SLA batteries are probably the way to go right now (bit of a guess). They're big, heavy, and cheapish. 

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  # 1358089 3-Aug-2015 18:28
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timmmay: Powerwalls are expensive, SLA batteries are probably the way to go right now (bit of a guess). They're big, heavy, and cheapish. 

... and need frequent maintenance, or their lifetime is shortened considerably.  Even then, you are lucky to get 10 years out of them, and replacement cost is pretty steep -- typically $5k or more for a decent-sized system.  An off-grid power system I priced up a few years back had a battery bank costing $15k, and unlike solar panels, batteries haven't come down in price significantly due to the amount of lead used in their manufacture.

SLA batteries also need a secure enclosure, vented to the outside to get rid of hydrogen build up, and acidic fumes.  They are only suitable for enthusiasts, or for large installations that are maintained by a contractor.  It's early days yet for the Powerwall, but this battery bank, and succeeding technologies are the way of the future.  Once the manufacturing volume increases with exploding sales of electric vehicles, the economies of scale will kick in, and it will be a different ball game.

Right now, unless you are off-grid, PV arrays, inverters and Powerwalls are a bold experiment for early adopters only.  The economics usually don't stack up unless you have some special circumstances in your household.  Look for that to turn around in the next couple of years.





 
 
 
 


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  # 1358142 3-Aug-2015 19:47
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Seems no-one should follow my advice on solar eh? ;)

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  # 1358149 3-Aug-2015 19:56
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Do not rely on selling power back to the grid to justify a solar installation.
In other countries, where there is no legal requirement to buy back the power, the power companies are opting to stop offering buy-back deals. If you dont use the power, they wont buy it.

So if you can sell some power now, thats great, but as solar becomes more ubiquitous, the novelty will wear off and the power companies will just go back to buying from the cheapest source.




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  # 1358691 4-Aug-2015 14:01
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raytaylor: Do not rely on selling power back to the grid to justify a solar installation.
In other countries, where there is no legal requirement to buy back the power, the power companies are opting to stop offering buy-back deals. If you dont use the power, they wont buy it.

So if you can sell some power now, thats great, but as solar becomes more ubiquitous, the novelty will wear off and the power companies will just go back to buying from the cheapest source.


Thanks, I guess over a 20-year investment timeframe it pays to be pessimistic.

Still not sure how much of the generated power is likely to be used, but thanks to everyone for your help!

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  # 1358720 4-Aug-2015 14:53
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+1 on not bothering to factor in sell back. Most NZ power companies now offer 8c/KwH (was around 16c/KwH a year ago), which is probably about a third to a quarter of what they will charge you for their power.

Another thing to be aware of is as soon as you tell your power company that you have solar and want a 2-way meter installed the power plan options available to you reduce dramatically and your buy-in power will probably be at a higher per KwH than what you pay now. Think Hotel rack rates rather than expedia special rates, i.e. say goodbye to any special deals.

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  # 1358759 4-Aug-2015 15:23
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Case by case really.  Assuming you're not at home during the day, you know when the sun is at it's peak, then your demand doesn't match the generation.  Maybe having longer start delays on washing / dryers / hot water heating (via time clock control) / dish washers / slow cookers will come but presently most delays are for two hours last time I checked.

If you are a commercial building with cooling loads during summer, then the load profile is strongly aligned to your generation, and the numbers make it worth investigating.

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  # 1358781 4-Aug-2015 16:02
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I think solar PV is only a good idea if you've got instant gas, wetback fireplace, or solar water heating installed first.

Water heating accounts for approx. 1/3rd of you power bill, so try to remove that from he equation first.
Its definitely a waste to use your solar power to electrically heat your water.

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  # 1358793 4-Aug-2015 16:24
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Solar water heating is awesome.  We put it in our last house and it slashed our summer power bills and reduced our winter ones too.  We get 2,000 + sunshine hours though.




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  # 1403217 9-Oct-2015 19:24
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Dairyxox: I think solar PV is only a good idea if you've got instant gas, wetback fireplace, or solar water heating installed first.

Water heating accounts for approx. 1/3rd of you power bill, so try to remove that from he equation first.
Its definitely a waste to use your solar power to electrically heat your water.


I have has solar water for 6 years and solar PV the last 2 years so I have some real world experience to share on this subject.  My experience with solar water was great, lowered power bills in summer and plenty of hot water, sometimes too much.  It was not so effective in mid winter however and I needed to boost it regularly.   My 3kW PV was also great for the first 18 months and then the retailers cut the buyback from $0.25 to $0.08 so I set about finding a way of  using excess power generated rather than exporting it for a miserly $0.08 per kWh.  I fitted a diverter that sends any excess kWh to the HWC and over winter I immediately noticed we now had plenty of hot water, just like in summer using the solar water panels.  In fact I am now planning to on-sell the solar water panels as they are no longer needed IMO, because in summer I will have too much capacity.  I observe that solar PV are more effective in scattered sunshine and overcast days at providing energy than solar water panels.  You get instant power when there is a break in the clouds with PV whereas with solar water it takes some minutes for the panel to warm sufficiently for the pump to activate and heat the HWC.  Moving as much household power use to sunshine hours as possible helps the PV decision, dishwasher, washing machine, even vacuuming.  LED lights, wood burners, heat pumps all help lower your night time power consumption without serious lifestyle disruption.

So I have had a different experience to Dairyxox, I think using solar PV to heat water is brilliant.  Regarding the size of solar PV my recommendation is to buy a monitoring unit and measure your power use so you understand exactly what your daytime power use is.  If you have a 300W base load as I do then I recommend no more than 2kW of PV panels and send any excess to your HWC.  In fact 1.5kW would probably be enough.  It all depends on your daytime power usage, if you have a  show room running a bunch of fluros during the day PV would be a great option. (as would LED fluros)

My experience with power plans is that even with an import/export arrangement I am not restricted to my choice of power plans, I am on the low user rate.  Winter power bills are around $70 to $170 with liberal heat pump use.

Anyone want to buy 2 x solar water panels, pump and controller? 



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