What do you think about smart hot water systems, that work similar to batteries, eg automatically switching on and off accepting you excess generation throughout the day as you produce it.
As a technical concept it's a great idea beaten only by network switching of the same loads (in the Australian context only Qld, NSW and a few small isolated systems eg King Island have the infrastructure set up to do that).
There's a lot to love technically about having loads which automatically respond to solar generation. The only thing better is the ability to directly control the loads in response to all system conditions but doing it automatically in response to solar is the next best thing.
As a consumer financial decision I'll sound a big word of caution however and say that the concept won't meet consumers' expectations of lowering bills in many, perhaps most, cases. That's the short answer, what follows below is the detail if you want it.
The concept is to take electricity that would be fed into the grid during daytime and use that to heat water, thus avoiding the cost of some other means of heating the water (eg off-peak electricity).
Now where the problem arises is when you do the maths.
*The energy you divert has a cost of lost income, that being at whatever feed-in tariff (FIT) rate you receive.
*The energy you are replacing will in the vast majority of cases be off-peak electricity if the same water heater is used or alternatively gas if we're talking about a new system replacing something existing.
Now the trouble is that for a lot of Australians their FIT rate will be very similar to what they could buy off-peak electricity for meaning that there is nothing to be saved. That is the first problem.
The second problem is that every commercially available product I've seen which does this requires that either a 24/7 electricity supply is connected and used as the booster, or that if an off-peak supply is connected the booster be manually operated. I've yet to see any "off the shelf" product which is able to automatically boost using a separately metered off-peak supply.
If you have time of use (TOU) metering with everything through the one meter (fairly common in Vic and NSW, exists in other states with the notable exception of SA), or if you live in a place where there's only one flat rate tariff (NT and a large portion of consumers in WA and remote towns not on the main grid on other states) that won't matter in the slightest since you'll incur no cost penalty, compared to how you'd otherwise heat the water, when boosting.
If however you have a separately metered supply for water heating, which is almost always the case in Qld, NSW, ACT, Tas and SA and also for many in Vic, then even a small amount of boosting at peak rates will blow any savings to pieces. There's the big flaw.
As an example, suppose that:
FIT = 20 cents / kWh
Off=peak price = 24 cents / kWh
Continuous tariff price = 42 cents / kWh.
Those are the actual prices, rounded to the nearest cent, charged by a major ASX listed retailer in SA.
Now you're likely to heat somewhere around 70% of your water using the solar in SA's climate. Yes everyone knows that SA gets seriously hot and has lots of sun, it's 1am and outside my house it's 29.5 degrees right now, but there are also quite a few days with no sun at all and those just happen to be mostly in Winter when the incoming water is coldest thus requiring the most heating.
So let's do the maths.
70% at 20 cents and 30% at the continuous tariff of 42 cents = 26.6 cents per kWh on average.
So you'll be paying ~11% more to heat your water than if you just stuck to good ole off-peak from the grid. That also doesn't require purchasing an approximately $1000 device to control the water heater, plus installation by a licensed electrician, or alternatively a "smart" water heater at whatever that costs.
Now you are probably thinking why not just rig it up to boost using the off-peak?
Technically that can be done but I've yet to come across anything "off the shelf" which does it. Whilst you could wire it up quite easily to work that way, any electrician is going to be more than a tad reluctant to be doing so given issues of compliance with Australian Standards, electricity distributor standards and the consequences if anything goes wrong given that the installation contravenes manufacturer's instructions (versus the law which specifically requires electricians to adhere to the manufacturer's instructions). Doing this is commonly referred to in the industry as the MacGuyver approach to heating water (a reference to a well known 1980's TV program for those not familiar) and for good reason. It'll work but it's a lot of bits stuck together.
But let's suppose you do go down that track. Divert the solar and boost with off-peak.
70% of water heated from electricity that you'd otherwise receive the FIT for and 30% heated from the off-peak grid supply = you'll save 11.67% using the prices I quoted from a major retailer.
So, a ~12% saving that'll cost you ~$1000 for equipment plus an electrician's labour charges and some minor materials to install (if you can find anyone willing to set it up that way). Hmm....
Just in case that didn't convince you, it gets even worse if you just ask the electricity retailer for a better price. Sign up with Origin, take all the discounts for direct debit, not receiving paper bills etc and you'll have an off-peak water heating rate that is less than the FIT. In that case diverting solar into the hot water would be completely pointless.
In contrast a decent quality heat pump will actually save energy as such and we're talking a 70- 80% saving there. The good ones will work just fine on off-peak supply also so there's no issue with using less but paying more for it.
At a suitable property a solar water heater will save 50 - 100% of the energy otherwise used and if correctly designed will boost with off-peak electricity or alternatively gas. So no costly peak rate power involved there.
For others gas can still be worth a look. It's a bit out of fashion at the moment but depending on circumstances I wouldn't write it off by any means. Really comes down to the house design and so on - there are situations where gas wins simply because it's physically more practical.
Overall, there are circumstances where the idea of diverting solar to heat water can work. If you're on a flat rate for all electricity consumed and the FIT is considerably lower than the price you pay for power from the grid is one scenario. If you've got a truly massive solar system and will never need to boost the hot water is another. If you've got Time Of Use metering and a low FIT that's another. For the majority though the finances aren't that great.
Alternative options sort-of achieving the same end result:
Tasmania - Tariff 93 is a Time Of Use (TOU) product for all your household electricity including solar feed-in. As part of that you get off-peak rates weekdays 10am - 4pm and 9pm - 7am as well as all weekend. Now just use a simple timer to heat the water 10am - 4pm and your solar will automatically be the priority source with any additional energy always charged at the off-peak rate. Done!
SA - Not available yet but there's a definite thought about doing something which will look awfully like the Tasmanian approach. Times for the cheap rate are likely to include 10am - 3pm and 1am - 6am. Just heat the water 10am - 3pm and you've got your solar being used as priority and backup at the off-peak rate. Nothign fancy required, just any old electric water heater and a simple timer. Note however this doesn't exist yet, it's just at the planning stages.
SA - If you want a battery then AGL still has some heavily subsidised ones available including Tesla Powerwall 2 and LG systems. There are some conditions, like needing to sign up with AGL for your electricity and participate in their Virtual Power Plant program, but they're offering huge discounts on normal pricing so it may interest some. This is an AGL scheme.
SA - The state government has a battery subsidy scheme available, the rate being $500 per kWh for most and $600 per kWh of capacity for concession holders with a maximum payment of $6000. At present it only applies to systems manufactured in SA, which gives you a choice of Sonnen products only, but next year it becomes broader and others will be available.
Vic - I'm not sure of the exact details but the state government is handing out $ for solar in a pretty big way. At present Vic has a considerably lower % of homes with solar as compared to Qld or SA so they could add quite a lot before any real technical issues emerge (but go too far and they sure will).