In February last year I had the first residential Tesla Powerwall in Melbourne installed, along with a brand new solar system. “How much did the battery cost?” is the first question everybody asks, but it’s not necessarily the right one. Yes I paid a bomb but for my own special reasons, I just had to get in on the action.
In any case, I wasn’t very special for very long. New data from SunWiz tells us that just under 7000 grid connected battery systems were installed in Australian homes and businesses over the course of 2016, and since I first announced my Powerwall installation, via One Step Off the Grid.
It’s coming up to 12 months since we switched the system on; here are the vital statistics* and my own conclusions about its performance.
My 5kW solar system has generated 5800kWh over 51 weeks. Bang on what you’d expect for solar in Melbourne with a 13 per cent capacity factor against nameplate. No surprises there. Panels are so cheap these days, we installed as many as we could fit on the roof. While others wash their cars in the driveway on weekends, I get up on the roof with a squeegee.
Of the total amount of solar power generated, I have exported 3000kWh of electricity to the grid, getting paid 8c/kWh, while I have imported 1800kWh from the grid at 17c. Bummer. Time we had a one-for-one feed-in tariff, to eliminate distorting investments and behaviours.
The Powerwall stored and discharged 1600kWh of electricity, or about 33kWh per week. The aim, of course, is to fill and discharge the battery every day, while never importing any electricity. But we are not there yet, given our pattern of electricity use which is mostly at night time. There have been just a few days when we used more than 100 per cent of the capacity of the 6.4kWh Powerwall battery. That is to say, we filled it, discharged some, then filled it again.
Our electricity use is about 14kWh per day. We were always pretty frugal, but we do have our splurges on the hottest and coldest days, with electric heating and cooling.
Just over 50 per cent of my production was exported to the grid – it ‘overflowed’ the battery and it wasn’t used in the house. So I’ve either got an oversized panel system or an undersized battery. Or something. Gee I wish there was an app that would take all my data and tell me my ‘right sized’ storage, and give me an internal rate of return. That’s the big number everyone is speculating about.
One use for my excess electricity (on average 8.5 kWh/day) could be to store it as hot water. At the moment, we have instantaneous gas hot water. Maybe I could switch that to electric heat pump or resistance storage and disconnect the gas entirely? Big savings by eliminating that daily gas connection charge, and the gas itself isn’t getting any cheaper.
And now for something really exciting. I’m signed up with Reposit and Diamond Energy to export electricity to the grid at times of high prices, via software and a meter that they control. On Friday, my iPad flashed me a notification:
“GridCredits Event. Your battery has been requested to discharge for 44 minutes starting at 05:15 PM. You will receive $2.47.”
That’s $1/kWh at a time when the Victorian wholesale market was bidding about $1500 per megawatt hour. I’m competing with the big boys! That’s only happened three times this year, but looking forward, who knows? Fifty thousand more homes and businesses like me and we are on our way to a completely new form of energy market. Exciting times!
* I’m no tech head, so I hope I’ve got this right. I’ll give access to my monitoring portal to any energy boffin who wants to have a crack, as long as they’re prepared to write it up for OneStepOffTheGrid
Greg Barber is leader of the Greens in the Victorian parliament
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thank you Greg
nice to get some real detailed fedback on your experience
interesting about responding into the peak market
Well done Greg. Just telling it like it is. As an early adopter, I paid heaps for my 2.4kw panels in the suburbs of Adelaide. That’s why my “pay back” is likely to be about 15 years, even with an ongoing Feed In Tariff. I also have a couple of wonderful old gum trees on the north side of my house that shade my panels at certain times, especially in winter. With time of day export pricing, I can see how people could move the panels to the western side of their homes and as you say – join the big boys as electricity generators during hot summer afternoons.
“Gee I wish there was an app that would take all my data and tell me my ‘right sized’ storage, and give me an internal rate of return. That’s the big number everyone is speculating about.”
PV Sell does this.
Energy storage in your domestic hot water tank is a good idea. You could even use the hot water via wall mounted radiators for room heating (they do this a lot in the UK). You would need a bigger tank but it doesn’t have to be mains pressure so potentially cheaper. Also it could run up to 80 or 90 C as tempering valves are available for these temperatures and hence store more energy.
I don’t think just 8kwh/day is going to get Greg 80c hot water, he will need 53watts/litre just to get to 60c from a resistive element, even a HP with a COP of 4 wouldn’t do it for 1,000 lt tank.
8kWh will heat a 1000 litres just 6.9 degrees C. To heat 1000 litres from a winter 15 C to 80 C takes about 75 kWh and with a COP of 4, 18.8 kWh. However no heat pump is going to maintain a COP of 4 over such a wide temperature range. The COP will be less than 1.25 so cheaper and simpler to use resistive heating. Evacuated tubes would also be struggling at 65 C above ambient.
I agree it would take 75kwh to heat 1000litres to 80c with a resistive element, at off peak rate of 13cents/kwh= $9.75 daily. However it wouldn’t be quite that bad in practice as the tank temp wouldn’t be exhausted to ambient 15c, more like 30-40c. Your pretty well right on the HP COP depending on the brand.
Now radiators will use 3-5kwh each, heating 4 rooms for 6hrs per night would use 72kwh of heat at the least.
I’ve done the modelling of using evac tubes to heat that amount of water to 80c, as the sun is free, but the cost of the collectors, plumbing, cost of radiators, stainless tank, pumps, controller,labour and materials is cost prohibitive.
It clearly shows that you don’t know enough about evac tube technology as they can heat water to 90c no problem on a good day and enough of them in winter.
Thanks Greg, I believe you could do with extra battery and even pre cool or heat your home before you get home too. You could use a heat pump, but with my experience the money would be better spent on an Edson Evacuated Tube SHW system for the dollar outlay. That’s if you have room left on the roof, however they can be placed on a southern roof on a pitch frame facing North or even West. That will take care of your hot water better as long as it sized correctly and cheaper to run when it’s cold as HP’s loose efficientcy when it is. In Melbourne you should only have to boost an Edson system say 30 days or less of the year but that’s a guess. Mine in NSW only 8 days in 2016. Contact Giles if you would like get more info on it, He knows who I am and he will give you my mobile number, just mention this post.
I second that. SHW and ditch the gas.
A PV diverter might not help much in Winter.
Thanks for agreeing Rod. It’s great to find someone else who knows the truth.
Only from 30 odd years experience with SHW and 17 years with a smallish PV system.
I know the limitations in Winter here in Adelaide.
Are you talking about selling and installing or just as an owner? What type, ET or flat plate? I would like to know about the limitations of an Adelaide winter please.
Didn’t make that very clear did I. Just (!) an owner.
3 x flat plate, thermosyphon in roof 400 ish Ltrs Aqua heat.
I am a fan of this type of HWS as it should last 25 years. 2.4KWp PV
9 Months of the year I can leave the booster off. June and August I monitor it using a cheap indoor/outdoor thermometer temperature sensor at the outlet. I manually turn it on at the board when the temp gets too low. The adjustable thermostat is set pretty low. I try to make sure it boosts every 3 days to avoid legionnaires.
July is the killer. I know we get the odd fine sunny day but not enough to add significant heat to the tank let alone stored heat for the house.
The women in the house insist on morning showers so I boost more often than I would like.
The house uses about 4KWh/day most of the year, excluding the off peak HWS. I’ve included data for July since I installed my 2nd array.
EDIT. I’ll try to post a more readable table.
Period
Imp Per Month
Exp Per Month
J Per Month
Inv/Mth
Jul/2009
199.9
124.3
178.6
214.0
Jul/2010
205.5
131.9
185.8
219.0
Jul/2011
171.2
133.5
159.9
213.0
Jul/2012
65.1
165.7
115.0
216.0
Jul/2013
87.1
151.5
136.2
207.0
Jul/2014
73.3
131.2
135.9
176.0
Jul/2015
79.2
156.2
193.0
211.0
Jul/2016
81.1
122.6
137.7
170.0
Interesting article but some of the numbers don’t appear to stack up. “Our electricity use is about 14kWh per day”. This would be in the vicinity of 5100kwh per annum. “while I have imported 1800kWh from the grid” this would be in the order of 5 KW per day. Perhaps your 14kwh is pre-solar?
“My 5kW solar system has generated 5800kWh over 51 weeks” = 15.89 kwh per day. A 5 kw system should generate a more than this. Rule of thumb. System size x 4 = average daily generation, thus 5KW x 4 = 20kwh per day. Could it be an east west configuration? My own system in NE Vic Nth facing generates 3.95 kwh/kw per day averaged over 6 years. Australian wind and solar “sunnymate” can direct excess energy to hot water storage. Might be worth looking into.
Numbers are slightly off by not enough to be pre-solar. Remember he also exported 3000 kWh … so 5800kWh generated, less 3000 kWh exported, plus 1800 kWh imported, is 4600 kWh used, divided by 357 is 12.9 kWh per day, not far off the 14 kWh per day mentioned.
On the hot water question, I looked at a range of options to maximize our own solar use and settled on a standard electric water tank heater and a $100 switchboard timer to only power the tank between 1-4pm each day. This results in close to zero grid usage and we’ve never run out of hot water at drastically lower prices than alternatives.
Sorry for late reply. Your numbers are saying you need a second battery. This would cut your imports right down.