CHOICE membership

Solar storage batteries: Tesla Powerwall and 17 others put to the test


Hi Mark, some CHOICE lab staff sit on Standards Australia committees as CHOICE nominees. For example, I sit on technical committees CT-002 ( Broadcasting and Related Services ) and TE-001 ( Safety of Electronic Equipment ).


Thanks Gordon,

This has been a truly informative and useful post for those of us thinking about battery storage.

We will most certainly give this idea a miss until the battery issues have been addressed because if we can run our reverse cycle for heating and cooling when needed in a very well insulated 3 year old house and all other appliances during the day and still be in credit then there really isn’t a reason why we should invest in battery storage at all.



What are you doing for hot water at the moment Nick? If you can power that system from PV during the day, either via a heat pump (best) or resistive heating, then that is another battery of sorts - storing the energy as heat, rather than as chemical energy in a battery.
Many people with solar are still letting their electricity providers control their water heating, with a controlled load circuit heating the water at night, but if you have PV and are on a FiT that is less than your controlled load rate per kWh, then that is not the best way to do it.
With a heat pump ideally you would let it run through the middle of the day (or whenever you typically see maximum PV system output if panels not north facing), or with resistive heating you can use a diverter, which diverts energy normally exported into heating the water seamlessly - ie the diverter handles the variations in PV system output automatically. Alternatively you can just use a time switch to do the heating around the time of typical maximum system output.


Hi Gordon,

When the system was first installed we were looking at a device that would allow us to redirect power from the PV to the hot water storage system during the day first then any excess back to the grid.

This device proved unreliable in the real world and was withdrawn from production.

We currently have the hot water on off-peak at 15.5 cents and our FiT is at 20 cents so we are more than covering that cost and have, you could say, 4.5 cents credit left over. For information purposes we are on Origin’s Solarboost 20 plan with only a 1% discount and 20c FiT. A handy spreadsheet I have created compares most of the major players using figures from actual bills over a 12 month period.

We have found that bigger discounts do not mean bigger savings as those that give the big discounts have a much higher base rate to start with and still end up being more expensive.

All panels are true North Facing so we are getting some serious power output for a good part of the day from sunrise to sunset and part of our criteria when choosing a block to build was that it needed to be Northern aspect.

The open plan living area is all North facing with fairly large windows so we get maximum heat from the sun all day without the need to use the aircon until very late in the afternoon.

With the Enphase microinverters we are able, with Enphase managing apps, to see the outputs for each panel and we use those readings as a guide as to when to clean the panels.

We are of course always looking for ways to maximise our investment and are always open to suggestions and ideas to get the most out of our system.

We don’t pay the ‘LazyTax’ for anything including energy, insurances etc. and we are a single income family that looks at making our money go further.



A heat pump or even a bolt-on to your existing HWS could cut your water heating expenditure by up to 3/4, using a CO2 HP such as from Sanden, which still has a high COP at low ambient temperatures.
Payback time would depend on how much you currently spend on water heating- big users will pay it off relatively quickly, but if you don’t use a huge amount of hot water, it may not be worthwhile, as HPs are fairly expensive to buy.
Evacuated tubes are another option, here in the hills SE of Tamworth our 30 tube/250l system is good for about 360 days per year with no boosting, for our 2 person household.


Hi Gordon,

You got me thinking, in a good way of course.

Did some maths and found that our total cost for our hot water for the past 12 months was $400 inclusive of supply charges and GST so based on the 3/4 running cost saving for a heat pump versus the cost of supply and installation we are again looking at a ROI of between 8 - 10 years.

We are a 2 adult and 1 child household and when we did the maths 4 years ago for Solahart hot water the ROI was over 10 years as our hot water costs at the time were $300. Usage is very similar to today so the cost difference is due to energy price increase over that period.

After some quick research, Evacuated tubes appear to be a viable option as an add-on to our current 315lt Everhot Off-Peak storage system and if it works in Tamworth then there’s a good chance it will work in Armidale.

Going by what you have and what you are saying, it appears to be reliable and stable and just for info purposes, what brand of tubes did you install and is the water hot enough?

I really appreciate the information you have provided as this in IMO is more credible than the any marketing information provided on the manufacturer websites.



I believe the latest research is retrofitting batteries (unless it’s quite small) isn’t economically sensible - but getting a larger battery when installing the first panels is much closer to economical. A query I’d like someone to look out for - a risk of electric cars is people coming home from work as the sun goes down and plugging into the mains during peak load on the system; this is addressed in part by having ‘smart chargers’ which switch in after demand has peaked. But what I want to know is, has anyone come up with a way to ‘fast-charge’ a car from a home battery?


Hi Nick, we are getting a bit OT here, however, hot water is a much less expensive and more reliable form of energy storage than batteries :wink: 250l of water at 90C stores as much energy (>14kWh) as useful hot water, as a Tesla 2 battery does as chemical energy.

I’ve got evacuated tubes from Run On Sun - Andrew Butterworth, just near Deepwater, not too far from you. Easy to find online :slight_smile:
Yes the water is plenty hot enough, before I put the tempering valve in, I measured 95C water at the bath tap.
My tubes are ground mounted behind the house - much easier for maintenance than on the roof. For about 9 months of the year, Sept-May I partially cover the tube array, as it gets too hot and can waste water via the P-T valve.


That’s not really a viable option, as car batteries are almost always of significantly larger capacity than domestic storage batteries. Even my off-grid 21kWh (18kWh usable when new) LiFePO4 battery is dwarfed by any of the Tesla car batteries.

I suspect in the future, when the electricity providers finally get with the program on renewables, that controlled load EV charging will replace controlled load water heating as a ‘demand filler’ for periods of lower demand.



As a standard PV Li-ion battery is around 10kW, one would need many just to cover the capacity of a e-vehicle. The PV system would also need to be significant to be able to charge the battery system in preparation for charging the vehicle. The costs would be significant, especially considering battery life and possibly significantly more than the cost of the e-vehicle.


It may be more a case of topping up the home battery from the car and charging the car from sources outside the home eg workplace or shopping centre.


I was actually aware of that. (Size difference) Yes, house batteries are smaller. A Tesla car battery would power several houses. My point is - most commutes are about 60km. 20,000km in a year is 55km per day in the most simplistic averaging (of course, ‘average days’ are rare). Can we top up (not fill up) the bigger battery with a littler, solar-fed battery for tomorrow’s commute? Is this technically difficult? Has anyone done it or is working on it?


Of course that is possible, and plenty of people already do it. It won’t be a fast charge as you originally asked though. I think the largest commonly available home chargers are rated at 7kW, more than enough for a top up to cover a typical daily commute. How much of that comes from the home battery really depends on its capacity. In some months of the year it will be possible to use power directly from the PV system in the mornings and/or afternoons, if you have suitably orientated panels.


One point to consider is in using a home battery to top up a car battery, the benefit of the one lot of energy is being shared across the cycle life of two batteries. It doubles the cost of the energy by reducing the total cycles on the home battery. There are also conversion losses in the cycle with a discharge charge efficiency possibly as low as 80%.

There may be a benefit of being able to swap out batteries, something some EV manufacturers have considered. Two similar sized home batteries, one at home on charge and filling in household gaps with the other in the car.

Alternately the availability of public charging is another way to possibly trade home feedin for remote charging while at work?


As @gordon pointed out there are already people doing it. Tesla manufacture a car charging station as part of their solar powered home vision.

A Qld user made a video for Tesla:

Re charging stations for Tesla:

More re charging including residential:


If you did want to fast charge an EV from home, flywheel energy storage could potentially be used. These units can deliver very high output and are currently used in high power demand environments to cover short grid outages. For example:

I’m not sure if anyone is actually using flywheel energy storage for EV charging at present, but IMHO it is a viable means of charging (and if not, remember you heard it here first! :wink: )


How would that work? I thought car batteries were a bit heavy (200-300kg) to move.


Put it on wheels? Special rack? I guess they would have an answer if they intend to do it…


Unlikely as some propositions might appear.

More recently a different approach which removes the ownership of the battery from the vehicle owner. The vehicle owner uses a mechanised system to exchange a battery due for recharging for a freshly charged one. Similar to refilling the fuel tank. The cost of the energy purchase is minimal, however the driver is on each change paying for the part life of the battery.

Nothing to prevent a similar system for home use, assuming the cost of a second battery is within the budget.


OK so you use modules.

a) Each one is within Worksafe limits that one person can carry, say 20kg, hmm lots of modules. Save on gym subscriptions and pumping iron.

b) Your home charging station has a little mobile crane to move the modules from car to rack and back. For those who are reasonably fit you might make them 50kg each, the crane carries the weight but doesn’t dispense with inertia which still has to be wrangled.

Both are possible I guess but I don’t see it being a solution for everyone.