On the back of the federal rebate, huge batteries are on sale for bargain prices. There’s a catch to many of these deals however: you can’t possibly hope to actually charge these monster batteries to capacity as they are paired with completely inadequate inverters. Here’s what to watch out for.
In a bid to make the rebate useful for businesses, the Cheaper Home Batteries Program discounts as much as 50 kWh of the usable capacity of a battery system – much more than most residential households could need.
This in turn has sparked an arms race of advertisements promoting ever-bigger batteries at ever-smaller prices for homes that simply don’t need them. The cheapness however isn’t just down to the rebate discount – there’s something crucial these batteries are missing.
All Capacity, No Power
My old mate Sam had a Landcruiser “shorty” called Daisy. The previous owner had installed an additional fuel tank, more than doubling the total capacity to 183 litres. With a modest 85kW 4 cylinder using 7.8litres/100km (30mpg) Daisy had a ludicrous range of 2300km. That’s almost Adelaide to Perth, 25½ hours straight.
So if you get a 5kW inverter and a 50kWh battery as we’ve seen advertised in one instance, you’ve got the same combination: modest power output and a novelty oversized tank. The trouble is this doesn’t offer the same advantage that it is does for a car’s range. An average full day of sunshine on 6.6kW of solar will only half fill the battery. Or charging from the grid, sweating its arse off, the inverter will take 10 hours straight.
Once filled, you wouldn’t be able to extract the energy particularly fast either: 6kW to run the toaster, kettle and dishwasher would mean you’re still importing energy from the grid despite a charged battery. And the backup capacity from a 5kW inverter isn’t going to run your whole house in a blackout.
A general rule of thumb, subject to your particular needs, is a 2 or 3-to-1 ratio for battery and inverter size: so a 5kW inverter being appropriate for a 10kWh or 15kWh battery.
Be Aware Of Power and Kilowatt Hours
The key is to understand the difference between kilowatts (peak rating) and kilowatt hours (storage units).
For another car analogy:
- Kilowatts is the grunt under the bonnet. Going fast means you need more peak ‘orsepower under your right foot.
- Moving your car means doing work, using litres of petrol in the tank, measured slowly and accurately into the engine, burnt gradually over time.
- In other words, an aerodynamically efficient car might need 15 kilowatts of power to move it through the air.
- So if you’re consuming 15 kilowatts to keep a steady speed for an hour, that’s 15 kWh of energy out of your tank.
The Worst Example Yet
In what is the worst example I’ve seen to date, a crowd that’s been importing 4WD accessories for all of five years has just set up a brand new ABN for an energy business. They’re doing a “group buy” taking your money in advance so they don’t have to carry the capital cost.
50kWh of battery for just $5100 sounds like an unbelievable deal …
Until you see it is paired with a hopelessly undersized 5kW inverter.
The promise is FIFTY kilowatt hours of storage for $5100 installed.
- Do they realise that as the importer, they’re responsible for warranty under Australian Consumer Law?
– I’m not sure the gravity of that obligation has struck them - Are they approved to parallel import when there’s a local office for the battery?
– That’s worth asking - Have they priced in some after-sales support and warranty stock?
– They’re not transparent on prices, but obviously the federal rebate is paying more than the battery costs - Do they care?
– Apparently they’re out to break the solar industry - Have they designed the system properly?
– No. We can say that categorically.
The qualification I’ve done to be a certified solar and battery electrician goes into great detail. We’re trained to examine the customer’s infrastructure, their electrical loads, their peak demand, their potential solar yield and most importantly, the amount of surplus energy they should have available to fill a battery.
Flogging everyone the same massive battery ignores all of this detail.
Not every designer is going to run every calculation, but being trained means knowing the process.
Can’t Huge Batteries Just Export For Huge Profit?
Worse still, expecting a system to perform properly and claiming you can make wads of cash using a retailer that offers wholesale prices like Amber with 70c/kWh for export is not right.
With 10 times more battery than inverter capacity, it’ll take ten hours to export a full battery for a princely $3.50 per hour. Never mind that you’ll need two days of average solar yield to fill the battery first, as the inverter supplied isn’t actually integrated with Amber smart-shift.
Energy trading relies on being able to respond quickly to 5 minute price fluctuations. So if there’s a “grid event” and the prices go berserk, you need to automatically export a heap of energy during an short window of time.
If the spot price is on fire, a 30kw inverter might occasionally earn a $20 bonus or $200 bonanza in a fleeting half-hour, but with a 5kw inverter, you’ll be fighting a bushfire with a garden hose… and without smart-shift, you’ll have to be on the app to make a manual intervention at just the right time.
Whatever The Question, The Answer Is More Solar
Inverters aren’t the only issue: how much solar you have is also key. I’ve always maintained that the question doesn’t matter, more solar is always the answer, because generating your own energy is cheaper than importing it and “excess” solar isn’t a thing. Even if the networks resort to export charges instead of feed-in tariffs, setting your system for zero export will zero the problem.
You need extra energy, over and above your daytime loads, to charge up a battery. Qualified solar and battery designers can explain how they specify a solar power system to generate “excess” electricity, and how much battery capacity you need to store this surplus.
So you can come at the problem by working out:
- the storage required to cover your dusk till dawn consumption,
- adding some extra capacity to account for degradation,
- then installing enough solar to harvest what’s required to fill the battery
- and enough to satisfy your daytime appetite.
Or you might have a limitation like roof size or budget constraint, in which case it’s a matter of :
- working out your maximum solar yield,
- subtracting the daytime demand,
- arriving at the battery capacity to store the surplus,
- adding some storage if the budget allows,
- charging this extra battery capacity by importing cheap off-peak electricity.
We Should Have A Second Bite
Part of the problem with the oversized battery epidemic is that households can only claim the rebate once, meaning there’s a temptation to go big or go home. This would be addressed if the battery rebate worked more like the solar rebate, which can be had in any number of increments up to 100kW of installed capacity.
Indeed in the early days, many commercial customers started with 10kW, jumped to 30kW the following year and then really bit the bullet for 99.99kW once they were sure the savings were stacking up.
It would be great to see the same logic applied to the battery rebate. People could buy what they can afford without overcapitalising on something out of fear. A lot of quotes I’ve seen mean a lot of battery capacity is going to be installed, but there’s a risk it will be effectively stranded behind a small inverter.
The first 5kWh of battery capacity offers great benefit to the grid by lopping peak demand, but for public infrastructure owners (the electricity network operators) the savings grow smaller as battery sizes increase.
For domestic customers, I expect 10 to 15 kWh will be a sweet spot in terms of value, again with diminishing returns if batteries grow significantly more.
As bad as going too big is, that’s arguably not the worst you can make with this one-shot rebate. That might be going for too small a battery: I’ll cover this in an upcoming article.
For more on battery capacity, read our deep-dive explainer on battery sizing.
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Exactly the process I went through when considering a second battery under the rebate scheme. Sure there are days when I might fill a battery with excess solar (I have about 12kW, the most we could fit on the roof and the distributor would allow), but on the days that happens I also don’t tend to need the excess power, because it generally means it’s warmer. And heating is our biggest draw, apart from an EV, which isn’t at home during the day on work days so isn’t normally charged from solar anyway, it’s charged from cheaper overnight grid rates.
Using OVO’s 3 free hours during the day does help there, I can recharge the existing battery from that and potentially partially fill a second, but I’m not spending $10K+ on the off chance those cheap rates continue (which I’m sure they won’t, as grid-level batteries begin to mop up that daily excess production). Having one Powerwall 2 seems to be the sweet spot for us, and also seems to be the conclusion of your article, for most people.
Without actually having the time to look into the batteries used 5k for 50kwh isn’t bad it’s everything else that is filled with concerns.
Helps its not actually excessively oversized by too much for where i live as outdated calculations were around 30kwh needed at minimum to cover nights. Preferably dc batteries to enable over sizing on panels to something like 200% to charge them better.
as the the practical limit for inverters on my house is 10kw-15kw with the 15 needing a negotiated connection contract as ergon only has standard contracts for 1-10kw and 30-100kw
Seriously, SolarQuotes is one of few reliable sources of info re things solar.
A recent article in The Age/SMH went out of its way to explain the terminology “Kilowatt” and “kilowatt HOUR”, got them A-about-F. Even after numerous comments pointing it out, wasn’t unedited.
So, thanks once again SQ and AB.
Additional to issues mentioned is fact that export rates are capped at 5 kW in many if not all jurisdictions, irrespective of size of inverter and whether exporting to grid solar power directly or discharging from battery.
Also, unless your system is compatible with Amber’s tech, you won’t be taken on as a customer to even manually issue command to discharge battery. Btw, I have been paid over $100 for an evening peak time discharge on a few occasions. But that’s rare. Most days the accrued credit is about $2 after ev charging, pool pump. resistive hws, eyc during middle day usually when FiTs are negative.
I suppose how much of a shonk doing this type of greatly under powered install depends on what you are up to
– perhaps planning on replacing the small inverter with a larger/better inverter later on when you can afford it?. Because as you say, only one bite at the battery cherry.
– perhaps you might be a dodgy landlord installing 5kw solar panels and a small inverter to have your massive battery subsidised on your rental, when you fully intend to remove 3/4 of the battery from and use elsewhere where the subsidy wont apply?
Winter Usage the last 7 days :
34.8kwh with 20.40kwh created
36.0kwh with 19.9kwh created
26.5kwh with 17.79kwh created
34.7kwh with 18.60kwh created
33.8kwh 12.0 kwh created
39.8kwh 10.80kwh created
40.5kwh and only 8.0kwh created
Worse in the last 10 days was we used 42kWh and and only 2.50kwh created all day.
8.5kW inverter. – 12 east string and 10 west facing panels on the other string averaging 7.8kw generated during peak summer
Summer time it’s great – winter we will always suck. Retailer is increasing the service charge from 85c a day to $1.12 and dropping 4c to 1c feed in – its obvious that its time to kick the kids out ! lol
Can’t see how a big battery would help with our usage as we couldnt charge the beast when we need it in winter.
Chris, high winter consumption such as yours seems to require a pile of PV, a thumping great battery, and *ample inverter/chargers*, if grid independence is a goal. Admittedly, even with 27 kW of PV and 46 kWh of battery here, this heavily overcast morning’s miserly 0.8 kW was barely breaking even, but the sun’s out now, yielding 8 kW. It is the wide mouthed frog which gobbles what’s going, and my 24 kW of inverters + 10 kW of MPPTs have charged at up to 15 kW, though not in winter. Double the PV would be nifty in winter, but even 27 kW is overkill in summer. As Tony Seba advises, “Size renewable production for the trough.” The roof’s the limit … unless you add ground mounted PV or a carport.
If grid rules obstruct bigger inverters, then a big 48v battery might appeal, as MPPTs can easily add DC coupled solar which adds no export power. Mind you, my brother has 22 kW inverters, 19 kW PV, and only 10 kW export limit, all grid-kosher. Some states are somewhat backward, I fear.
For us the main use for a larger battery in winter is charging off-peak at night and using the battery for peak times in the afternoon and evening. On a Time of use plan in Sydney the peak rate (Red Energy 2pm-8pm) is about twice that of off-peak (10pm-7am). We do watch total power draw in the evening to try and keep it below the battery/inverter limit.
I guess it saves us roughly $40/month. Not much but it helps.
BTW – the Tesla Powerwall app has always had a “storm watch” function that automatically charges the battery if a severe storm is approaching. Recently the app was updated so that the user can start a “scheduled” charge from the grid for a selected number of hours.
So, for example, if I know there will be large consumption in the evening and there will be outcast skies during the day I can elect to charge the battery at shoulder rates (i.e before 2pm weekdays).
Then there is the power draw you require.
It is good to have an inverter that can produce enough power to power whatever your peak draw is. A small inverter will be unable to output enough power back into your house regardless of how big the battery is. If you have an all-electric house, even if you are not silly enough to try to charge your EV off the battery, the inverter output might not be able to handle the output of the kettle if you have the oven or the induction cooktop running.
So, you also have to consider maximum power draw when designing the system.
Of course, the grid will pick up the excess demand if you do the kettle and oven thing.
Where I live, we have a lot of blackouts, so this is particularly important to factor in.
Ok, 5kW to 50kWh is a bit too big but a ratio of 3 to 1 is great. This gives the option to arbitrage with Amber, and power the house or even refill the EV all in the same 24hr period. My 2020 install of a Tesla pw2 in a 1:1 ratio is ok, but not big enough in an all electric household. Also Tesla pw3 incompatability (with pw2) and price is meaning it’s currently a poor choice.
Good discussion on power generation, and storage. But let’s not forget the other side: consumption.
I used to work for a crusty electrical engineer. When asked for more solar panels, his response was, can’t you instead reduce your consumption?
Some people say they use 30 kWh per day, 40, 50. Maybe they are already super efficient with their energy usage. In that case good on you!
But if not, there’s plenty of energy efficiency tips on this site. So if you think you need more storage and more generation, also remember to try for less energy usage…
On the other hand, assuming the inverter is appropriate for the household, doesn’t a still somewhat oversized battery have a longer lifespan due to a reduced C-rate?