Sonnen or Tesla Powerwall: which should you choose?

Sonnen joins the Aussie battery market, offering to pay bills in return for solar. But how do the providers compare? | GRAPHIC

West Ryde homeowner Geoff Manners with his sonnen home storage battery. Picture: Hollie Adams

If an electricity retailer offers to pay your power bills, do you rush out and join up, or laugh uncontrollably in disbelief?

That’s the question Australian consumers face, with German solar battery provider sonnen announcing the offer to local consumers last night. We know that power price hikes are no laughing matter.

The system works like this. If you install solar panels, and buy a sonnen battery to store excess solar power, sonnen will pay for the supplementary electricity you draw from the grid, within limits. In exchange, you make some of your battery power available to sonnen and the grid when it is needed.

With the sonnenFlat system, sonnen effectively takes on two separate roles. First, it is the manufacturer of solar power storage batteries that you buy through retailers such as Natural Solar, who install the battery and link it to your solar panels and the grid.

Basic day-to-day use is similar to other solar battery systems. You can use energy directly from solar panels when it’s sunny, or store it for later use, and where the battery can’t supply your needs, you can draw power from the grid.

However with the sonnenFlat package, sonnen takes on a second role, as your electricity retailer providing that supplementary grid power. There is no Energy Australia, Origin Energy or any other retailer supplying power to your address, just sonnen.

In return for access to a whole heap of free grid power, you agree to let sonnen occasionally raid your battery storage and upload some of it to the grid, in times of high demand. sonnen sells that storage to the grid, at what is likely to be a high price if there’s short supply.

It’s a formula that’s worked for the company in Germany, where it has 60,000 customers. It seems this is what makes the deal financially viable.

Sonnen told The Australian today it would likely draw on less than 3 per cent of a fully charged battery.

The amount of free power that you get depends on the size of the sonnen battery you buy, and in Australia, you have the choice of 8, 10 or 12 kWh battery systems.

In lieu of the usual retailer monthly or quarterly grid access fee, sonnen charges you a monthly fee of similar size to a mobile phone monthly plan.

For the three systems, the monthly flat fee is $30, $40, and $50. It offers a tempting alternative to the large grid access fees that some retailers reportedly are considering as many households moved to solar.

The free grid power allowances for those three battery sizes is 7500, 10,000, and 12,500 kWh per year.

Sonnen says it will charge you for power if you exceed those quotas but it says those usage charges will be less than through the usual retailers. Sonnen told The Australian today that it is yet to set those tariffs locally, so we can’t say for sure if they will be cheaper.

To be part of this scheme, sonnen says you must be connected to the National Electricity Market, with a minimum 5, 7.5 and 10 kW solar panelling depending on which battery system you sue.

Sonnen is not only supplying grid power, but also is managing the volatile economics around sharing your battery power with the grid.

However, sonnen is not the only company offering intelligent power management. Although it does it differently, Australia’s Evergen seeks to reduce energy costs by charging a home battery with excess solar or cheap power, whichever is more economical.

It uses an energy intelligence system developed by CSIRO.

Sonnen says that sonnenFlat will let householders plan their finances and save thousands of dollars a year in power charges.

It says when it takes power from the grid, it leaves sufficient power for household use. It says the grid upload system is viable because of the ability of lithium iron phosphate (LiFePO4) batteries to charge and discharge up to 3 times a day.

Natural Solar, which also installs Tesla systems, says it is expecting huge demand.

“This technology is a huge game-change for Australian homeowners, and we are expecting unprecedented demand for sonnen flat home battery products coupled with solar systems, as a result of the low set rate for energy,” says Natural Solar CEO Chris Williams.

Sonnen’s managing director of global sales Philipp Schroeder says he expects sonnenFlat will “solve many of the grid stability and energy security problems experienced in part of Australia”.

“Households will embrace battery storage for their homes regardless of what politicians do or say,” he says.

Sonnen says it uses a self-learning algorithm, and data from the weather bureau, to identify the best time to start charging and, even with no one at home, to activate appliances.

So, Tesla or sonnen?

That’s a dilemma likely to face prospective buyers, especially with major supplier Natural Solar carrying both. Of course, there are other battery solutions - such as Australian entrepreneur Simon Hackett’s Redflow system - but how does Elon Musk’s Powerwall system compare with sonnen’s?

Natural Solar’s Chris Williams sees them as different solutions. “One of the key advantages of the Tesla Powerwall is the ability to offer full-house backup. Given the current outages and increasing grid instability, it is a huge driver for households that have been affected,” Mr Williams says.

He sees sonnenFlat appealing to mid-high consumption households and those who cannot fully use home energy storage solution because of the small amount of time spent at home - and hence at ease with supplying battery power to the grid. It might also appeal to households that already have a reasonably large solar system installed.

“SonnenFlat gives homeowners the ability to group systems together in a virtual powerplant allowing homes to work together as a generator, creating additional stability for the network and therefore passing the added value across to the homeowner,” he says.