Sustainable Energy NZ #6 – our place in the sun – doing the math on solar power

Welcome to the sixth post in the Sustainable Energy without the Hot Air – A New Zealand Perspective series. Today we’re crunching the numbers on solar potential in New Zealand. For the background to the work please our introductory post here. Also check out our earlier posts on the potential of hydro power,  geothermal and wind, and yesterday’s summary. Note: the units are in kWh/day/person – ie. if you ran a 40W lightbulb for 24 hours, it’d take ~1 kWh over the space of a day. We then divide it by person to give you a sense of the scale of the resource proportionate to the size of the population. Be sure to check out the methodology. For reference – we’re looking to replace around 55 kWh/d/p of energy currently generated by fossil fuels. 

So, solar! We’ve got a lot of it, or do we? Our lower latitude means that New Zealand’s solar potential is certainly rather better than that of the UK and the current world leaders Germany. A roof inclined at the optimal angle in NZ gets on average 181W/m2 in Northland, 178 in Auckland, 195 in central Otago, 185 in Canterbury. (This is based on averaging all available NIWA hourly radiation data at suitable measurement sites). This is impressive compared to the UK average of 110W/m2 and 130W/m in Germany.

There are 4 ways to harness solar energy:

  1. Solar hot water – panels that directly heat water.
  2. Photovoltaic (PV) – panels that convert the sun’s energy directly to electricity.
  3. Concentrated Solar Power (CSP): Actually a range of technologies that use reflectors to concentrate solar energy either into heat engines or onto very high efficiency PV.
  4. Biofuel – photosynthesis; this is considered separately.

Installing 10m2 of north-facing, solar hot water heating panels could deliver 8kWh/d/p of hot water per person per household (average household being 2.6 people). While this amount of energy is more than we require, sadly we currently are unable to store it for colder, gloomy winter days. Photovoltaic PV panels on 20m2/p of north-facing roof (3kW system) would deliver 4kWh/d/p per household, which is more than enough to cover the baseload energy use of a NZ house during sunshine hours, and has the benefit of being able to feed extra power generated into the grid for use elsewhere.

What about having a solar farm instead of using everyone’s house? Let’s consider, for example, covering all of Central Otago (which has pretty decent irradiation levels) with concentrating solar power station installations with efficiencies of 15W/m2. We could halve the area to allow for skifields, dwellings, shaded slopes, mountain tops, etc. This gives us a huge 330kWh/d/p! However, the environmental and fiscal costs would also be huge, and probably unacceptable. If we confine these solar farms to an area the size of the Maniototo (40,000 hectares, which is a block 20km x 20km, OR for you North Islanders, is roughly equivalent to the size of the Auckland urban area) we could still provide 30kWh/d/p if completely covered. While such a scheme would provoke outrage, it should be pointed out that these concentrating solar farms deliver 5-8 times as much power per square meter as wind, so the overall impact footprint on the New Zealand landscape would be a lot lower. The cost is currently 2-3 times hydro, geothermal and wind but is likely to come down in the near-future. Further, the price of PV panels is now about 2/5ths of what it was in 2009 and keeps dropping.

On an individual house basis, installing, say, 5kWh/d/p of solar hot water heating makes good economic sense especially if you use a lot of hot water (or have teenagers!). Larger scale investments will have to wait for relative costs to improve, but a potential for 9kWh/d/p of house-based PV plus 34kWh/d/p of large scale solar production for a total of 48kWh/d/p seems reasonable.

A small note on the Parliamentary Commissioner for the Environment report on solar hot water heating: The commissions findings that solar hot water heating is not particularly beneficial for reducing emissions (as they did little to reduce peak demand, which is overall the most polluting form of generation) is very valid. We still believe there are other benefits to be had from solar hot water heating such as insulation against rising energy costs and hot water in the event of a blackout. That said, the Commissioner’s recommendations about shifting hot water heating to a night cycle is something everyone should do: it saves money, and helps the country by consuming non-peak power to heat water.

Conclusion: New Zealand has great irradiation scores compared to other countries that are actively promoting solar, and the costs are rapidly declining to the point that it’s likely to be cost-competitive without subsidies soon. Like wind, the energy generated is variable (generated during the day, though bigger solar plants can generate at night) but again can be balanced by using hydro. In short: Solar has large potential and is the way of the future.

Further Reading:

There is a heap of material on solar out there, but a good place to begin is with Do the Math (can you tell we really like Tom Murphy’s work?). The UCSD professor breaks down the potential for solar at a global level. We should note that this is the same article as was linked in the wind post. If any readers have any further sources that are worth reading, please feel free to suggest them in the comments section. 

34 thoughts on “Sustainable Energy NZ #6 – our place in the sun – doing the math on solar power”

  1. Hi Oliver and Phil,

    Do the 5 and 9 kWh/d/p assume that every household has both solar hot water and photovoltaic panels installed? It’s not quite clear to me how you get to those figures.


  2. It depends on how much north-facing roof you have available. The solar hot water heating is done the same way in MacKay, assuming 50% efficiency and 10m2 of roof covered. The PV calculation is for 20m2 of roof covered. If you have 30m2 roof available, you could do both. The calculations can found in MacKay here except that we are using NZ figures for insolation, average house occupancy etc.

  3. Mackays formula is brilliant for getting a handle on scale and physical possibilities for energy, but if doesn’t touch price, or matching supply to demand. Electricity demand in New Zealand has a high daytime plateau, with a peak around 8-9 am and a higher one around 8 pm. There is also a strong seasonal pattern, with the maximum drawdown on cold winter evenings. That is when our hydro reserves are lowest, since South Island precipitation is locked up as snow, and river flows are at their lowest. Proponents of solar power claim that in sunny, hot areas midday solar panel output matches high demand for air conditioning, but here that is not so. Wind is also seasonal, but less so, and is still much cheaper than solar. Concentrating solar power with storage, which could match daily, but not seasonal, demand, is far more expensive than PV. I would guess that only off grid solar panel users would buy batteries, and even they would use wood or gas for high load stuff such as cooking or winter hot water. Occasionally you will see a headline that one day wind made half Spain’s power, or Germany was getting some impressive percentage from renewables, but such peaky supply is just as bad as very peaky demand. Night rate hot water or electric vehicle storage can help shave the peaks and fill the valleys in power use, but widespread PV will not.

    1. Hi John,

      Thanks for your points – absolutely agree that this is one of the limitations of McKay’s approach. The timing tends to be just as important as the scale of the resource.

      As much as I’d like to encourage everyone to just chill out a little bit and change their lifestyles to cater to when energy is available, I’m aware this isn’t a politically acceptable solution, and as a result this power balancing will be really important. There’s also another factor when you have peaky power: industry processes tend to suffer, as they are in Germany.

      I don’t have a solution to this just yet other than to say that everyone is facing this problem, and as a result there is a lot of energy going into finding solutions. Large scale storage will hopefully help (from companies like Ambri) and CSP will improve regarding storage and come down in cost.


  4. just like wind, the solar contribution must be measured on a cumulative basis, not on a peak basis. NZ’s hydro power again serves as a perfect ‘battery’ by avoiding the spillage of water through the turbines when the sum of sun+wind satisfies a significant part of the demand. Sure, we will need peak evening or morning peak load plants but still, a significant amount of the ‘low lake water’ concerns will get helped along with all added power and certainly also wind and solar.

    What makes solar panels on private homes so attractive is the fact that each home produced and consumed kWh is one kWh not purchased at retail prices, not wholesale prices!! At that rate a modest 2KW solar grid tied system, which you can get installed incl GST for under NZ $10,000 these days will return a significantly better return on investment than leaving the $10,000 in the bank, get 4% and then pay tax on that. Do the math. This 2K system would produce some $500 worth of power per year in form of avoided purchase to the owner.
    Private PV systems have the added benefit to unleash private investment into the necessary transformation into a sustainable future and empower people to take pride and ownership of steps towards the solution of our problems.

  5. latitude

    Our lower latitude means that New Zealand’s solar potential is certainly rather better than that of the UK and the current world leaders Germany. A roof inclined at the optimal angle in NZ gets on average 181W/m2 in Northland, 178 in Auckland, 195 in central Otago, 185 in Canterbury.

    Hmm … Northland and Auckland are at a lower latitude than central Otago and Canterbury! Would someone explain those numbers then?


    1. Noel – probably related to sunshine hours. If you live in Northland (like me) you’ll realize it rains an awful lot more than regions such as Canterbury and Central Otago.

      1. I think that needs more qualification. The sunnier parts of Northland (away from the western flanks south of Kaitaia) are not significantly less sunny than some parts of Central Otago (eg Maniototo). Air clarity a factor as well?

        1. For clarification, the data is from NIWA insolution stations. It will obviously depend on which station you use. However, I would definitely expect cloudiness to be a significant factor when comparing northern NZ with Central Otago.

  6. Storage for Winter

    While this amount of energy is more than we require, sadly we currently are unable to store it for colder, gloomy winter days

    Community based projects can be the key to storage proposals short or long term and are also the key to installing PV at lower prices. than currently quoted for individual dwellings.

    Try this for a community project providing heat storage for winter during summer in a 52 house subdivision in Alberta at 51.1°N.


  7. Be interesting to get some figures on the long term storage capacity of the Alberta system, and on how much the back up gas system had to do. Saw a German outfit that was doing much the same with a deep borehole and a heat pump; the ground is a much better heat sink than air, but the costs of installation were very high.
    @ Thomas
    If your solar panels put out enough power to make a difference in winter,( ie if a lot of people are doing it), they will put out far more in summer. But if the energy companies have enough capacity to cover any shortfall, without fossil fuels, in winter, they too will have excess capacity in summer, so the value of your power to the grid will be very low. In Germany the grid operators are obliged to take solar power at mandatory prices above market value, but most of the solar power produced is not from rooftop but from farm scale installations, and wind gives much more watt/hours per Euro than either. Germany can dump excess solar over the border on sunny summer days, but New Zealand will have to restrict output if PV achieves scale. That will lengthen the time your panels take to pay for themselves, and everyone will still have to pay for the big companies’ winter capacity.
    This was Jan Wright’s argument against solar hot water, but it applies much more strongly to PV. Solar hot water might not be much use in winter, but for most of the year you can still have a hot shower in the morning or the evening. If you drive your electric car to work, PV can’t cook your breakfast, charge your car, or cook your supper after you get home. Most of your output will have to be sold to the grid, so you’ll only get wholesale, not retail value from it.

    1. John: I linked to the how page but the home page says:
      “The solar energy system is currently surpassing its 5 year performance goal of 90% solar fraction.”
      I suppose that means the same as the second point below:
      – The largest subdivision of R-2000 single family homes in Canada, each 30% more efficient than conventionally built homes.
      – A first in the world, with over 90% of residential space heating needs being met by solar thermal energy.
      – A reduction of approximately 5 tonnes of greenhouse gas (GHG) emissions per home per year.


  8. The analysis of solar is firstly to see what size resource there is. The major barrier is price. However, if price came down enough to make generation comparable with other sources (and arguably SCP is getting there), then I dont think you can discount storage. If you have excess power in summer then pump water back into hydro lakes. We have, and will continue to have, a mix of generation sources. Managing our hydro storage is one way in which solar could increasingly be part of our system.

  9. I get grumpy at the suggestion that littering the Maniototo with solar panels (or wind turbines) would be politically unacceptable. Try photographing the Southern Alps from anywhere on the Canterbury Plains without damn power pylons intruding in the image, yet everyone who expects the light to come on when they open the frig door for a cold beer accepts them.
    It’s not as if the solar pv panels preclude sheep from grazing beneath them although they may limit tower irrigators, a major intrusion on the landscapes of Central Otago that seems to have been accepted with minimal question.

  10. From a Contact Energy discussion forum on pumped hydro –
    ‘Having gathered more information about the hydrology of the Clutha River, I now accept that the Luggate and Queensberry proposals are not suitable for pumped storage operation. I knew that the lake areas were small. What I have recently gathered is that the operating range of Roxburgh would be a limitation and that the operating ranges of both Luggate and Queensberry would also be relatively small, thus limiting the operating time to a few hours.

    New Zealand’s electricity demand has an overnight trough lasting a number of hours, from roughly mignight to 7am. There may also be a smaller trough in the middle of the afternoon (providing air conditioning isn’t in high demand) which is often when wind generation peaks. A pumped storage system that can absorb 100MW even for just a few hours would have some value, particularly if it could be implemented at little additional cost.

    The detail that makes pumped storage systems impractical on this section of the Clutha river is the large uncontrolled river flow – in this case from Lake Wanaka – which would be filling up the storage lakes nearly as fast as any pumping could – or faster under some conditions – and which could fill the storage lake before the peak power demand occurs (at 6-7pm) anyway.

    Because of these small operating ranges, the generation on the Clutha River is and will be largely run-of-river generation with only limited daily controlled variation and very limited storage from month to month. The additional generation will therefore add to rather than assist with the mismatch between seasonal supply and seasonal demand, and will also increase the periods when intermittent and must-run generation exceeds demand.’
    Energy storage is a difficult area – except for coal, of course !

    1. I am not quite sure what point you are trying make. Do you believe this blog comment on a specific dam rules out the possibility of pumped storage in NZ? Did you look at other papers on pumped storage here?

      1. I posted this link a while back.
        As I said then, I think it’s much better to use ordinary hydro to balance other power sources, than to try to pump water back up; I doubt many current dams could be retrofitted for it , as you need top and bottom reservoirs close together but with good vertical separation; building new pumped hydro means you’re about doubling the cost of your power, since the capital costs are on a par with those of renewables. Plus entropy clips the ticket.
        Using EV’s is good for soaking up unused production, but again I don’t think it will work two ways. Using them as Vehicle to Grid storage brings up a lot of issues with reducing battery life, making demands on the local grid it wasn’t designed for, and inconveniencing the owners. Again, you’d have to pay the EV owner a lot more, to make it worth his while, than he paid to charge it, so the power you got back would be both dearer and less of it.

        1. “you need top and bottom reservoirs close together but with good vertical separation”

          So the crater of a volcano (or equivalent high basin) close by the sea or any large enough body of water would fit requirements, specially if there was a town nearby that decided to go for PV with their own smart grid. I don’t have one such place in mind – yet.

          Of course the government’s plan to tie councils to core services plus the division of communities by denialism would make town projects to mitigate climate change unlikely unless they sold themselves to large exploitive commercial operaters. Our fake ETS scheme renders all mitigation projects unnecessary; 🙁


  11. A couple of comments:

    An advantage of domestic PV is that there is no loss in transmission. I understand that this is significant.

    The storage challenge is also interesting and comes up when considering geothermal. One possibility (that I haven’t looked into the figures) is to use the energy to create a liquid fuel during those times when supply exceeds demand.

  12. Domestic PV though really needs storage as well to be effective. Everyone I know with PV has expensive storage.

    Geothermal is run-always baseload. Solar on large scale does need to deal with the timing/storage because availability does not necessarily match demand.

    1. Phil, the new trend is grid connected solar without private storage. It is a very cost effective way to lower your power bill plus what’s better, every kWh generated by the panels is actually used somewhere.
      With private battery storage which is not grid connected a large amount of potential generation is never used. The batteries of off-grid systems are often full at 11am on a sunny summer morning for a system that needs to cope with winter conditions, while the grid connected system passes power that can not be consumed at home to other consumers in the grid.
      Just as with wind integration, solar in NZ benefits from the hydro storage implied in avoided hydro generation (and waters not spilled) or carbon fuels not burned.
      NZ can install a lot of solar before we would run into grid integration trouble. Plus when you talk to people in the power industry you can hear about a growing summer daytime demand from air conditioning these days which is in perfect sync with solar availability in the same region.
      As far as storage goes I think we might see communal grid storage solutions where the scale of requirements can be met with reasonable technology such as molten salt batteries or flow cell batteries. These are technologies that don’t scale down well for home use but would be ideal for village or community storage.

    2. Whereas no-one I know has storage – ah, not true, one friend living in the boondocks does, but his system is antiquated and that’s really only the exception proving the rule.

      In Australia – where solar’s done very nicely out of the net-feed-in tariff, to the extent of having to pass legislation to stop people opportunistically installing arrays on vacant sheds etc. – a suggestion of battery storage for a PV system would be greeted with a look of utter befuddlement, I suspect…

  13. Hey I have storage for my dinky wee 25 watt panel supporting my water pump and a few LEDs. At present it only needs to work about 15 minutes per day per person. It needs a bit more to do.


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  15. As to pumped storage, there’s more than enough to be had at Lake Onslow, should the dam be raised, new tunnels built, and so on. Though the people with holiday homes in the proposed footprint may object somewhat to being shifted, and it’s a fairly long tunnel needed back to the Clutha.
    Been a while since I read the study, but my recall is if connected to the Manorburn to the north to fill the entire depression it would hold enough power to run the entire country for a year (electric only, if they improved transmission lines to allow). Seasonal variation not even an issue. Merely following demand during the day would be child’s play.

    Great what pumping water up a big hill can do.

    1. Hey Tussock,

      Thanks for your comments. Any chance you can remember more about the study? It’d be great to be able to cite something like this in any future updates of the document.



      1. Dug around a bit. Original study I can find is the MSc thesis of Sara Bear (not online, , followed up on in the media and at various local conferences by her supervisor, associate professor (hydrology) at Waikato, Earl Bardsley. Conference slides …

        I’ve read the figures, but can’t seem to google them up. May no longer be online? You could contact Bardsley anyway, he seems keen to get the word out. Contact Energy and Gerry Brownley have both said it’s not a goer (both being unspecific about it, looks like it breaks the current market structure to me).

        Anyhoo, it’s not going to built for what we have, but it’s sitting there waiting should we need a source of power storage. 12,000 GWh potential, or something like 3,000 kWh/p. Which is only a couple months of your 55 kWh/d/p.

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