Unreliable Backup Power: The Mistake Everyone Keeps Making

We’ve seen a big change in the solar industry recently. With batteries becoming standard, everyone is getting backup.

The problem is that poor design and sales misunderstandings are leaving people, especially three-phase customers, with a total oxymoron: unreliable backup power.

The good news is that solar power is being installed in huge numbers, and the attachment rate for batteries has gone through the roof. Some companies are even giving up on solar-only installations; if the customer doesn’t want a battery, the installers don’t want the job.

However, a few installers and customers alike have been caught out with an embarrassing lack of capacity.

Single Versus Three-Phase

I’m told that 80% of National Electricity Market customers have a single-phase supply, while two-phase & split-phase aren’t common, three-phase is becoming the new normal as houses grow and EVs proliferate. If you’re not sure what you have, read this explainer.

The vast bulk of appliances are single-phase, but on a three-phase property, they’re spread over three adjacent supplies, and often the house will be wired in three segments because the poles-and-wires network wants loads balanced within 5 kW if possible.

It’s not uncommon for a three-phase house to have a modest 5 kW single-phase battery backup system. They’ll supply 21 Amps, and there’s nothing wrong with that. However, as prices have fallen and appetites rise, more people are opting for full three-phase backup because it can save the cost of rewiring a third of the switchboard.

Things get awkward, though, when a numpty who’s used to selling 8 and 10 kW single-phase systems doesn’t automatically triple their recommendation to customers with a three-phase house.

10 kW Three-Phase Inverters Are Junk

The headline number isn’t the only factor to consider, as we’ve recently outlined, but the thing to remember is:

A 10 kW single-phase inverter is pretty gutsy.

• 10,000W ÷ 230VAC = 43 Amps
• That’s around 68% of your normal 63 Amp grid supply.
• It’s reasonable to expect it to back up the whole house.

However, 10 kW spread across three phases is only 3.3 kW each.

• 3,300W ÷ 230VAC = 14 Amps
• It might run the fridge on one phase, and the lights on another.
• Compared to the mains, you only have a very modest 22% available to any one appliance.

I Recommend At Minimum A 15 kW Three-Phase Inverter

A 15 kW three-phase system will at least provide capacity comparable to the modest 5 kW single-phase machines like the Tesla Powerwall 2. I still wouldn’t try a whole-home backup; a large single-phase air conditioner will overwhelm it. Provided you can do without a few big things, 15 kW makes partial backup quite feasible.

You Need A Power Surge

Electricians are trained to know that an electric motor will impose a huge demand on the supply at the moment it’s switched on. This surge in power, referred to as inrush current, means your 400-watt rainwater pump can draw up to 3,200 watts as it spins up.

The ability to supply this extra energy for a second or two is really important, even if modern air conditioners and washing machines often use “inverter” technology to make them softer on startup current.

Whatever system you choose, make sure it has enough battery. Some inverters will work with a minimum stack of, say, 3 batteries, but they won’t deliver their full rated output unless you have 5 or 6 batteries to really give them some punch.

Does What It Says On The Tin

The best way to judge a hybrid or battery inverter is to see exactly how specific the numbers are on the specification sheet. You’re looking not just for what they’ll surge to, but how long they’ll do it.

For example, the best units are very specific, with large overload figures guaranteed in 30-second, 30-minute, and 1-hour brackets.

Just a sample of the range, Selectronic will peak at either 90% or 140%. They publish numbers you can hold them to, and I’ve measured it first-hand.

Tesla Powerwall 3 offers a stout 10 kW continuous and uniquely ‘murican spec of “185 locked rotor amps”, meaning it should surge to an instantaneous 42 kW! (185 A x 230 VAC) The 160 A fault current figure is a bit more credible at around 36 kW. Neither of these hefty claims is backed by a time, though, so you’re relying somewhat on the manufacturer’s credibility.

Highlighted in red is the important number for most people, which is around 11 kW of single-phase backup from a three-phase inverter

Other reputable brands have nominal and then 10 second surge ratings. GoodWe ESA single-phase has 10 kVA nominal output and some brackets claiming 20 kVA, or 100% extra for ten seconds. However, it’s worth noting that the 30 kVA three-phase units only offer 50% more surge capacity.

For more 30 kW comparisons, the new Fronius Verto+ hybrid is rated at a full 29,990 W with 50,000 W for ten seconds. While some curse the Austrians for being slow to market, we know it will be a solid, well-tested solution. 67% extra from a three-phase inverter is nothing to sneeze at.

You basically need to be an electrician to decipher some of these specifications. Sungrow have a series hybrid that will pass 63 A through when it’s on the grid. It will generate 37.9 A, but you have to calculate 36,500 W ÷ 3 ph ÷ 230 VAC to conclude about 52 A for surge current.

Sungrow 10 kW single-phase offers numbers along the lines of  9999 VA rated output with 13680 VA surge rating, that’s 35% extra for 10 seconds. Their three-phase offering is rated at 25 kVA but has a 45% surge rating.

However, if your prospective inverter has a nominal kilowatt rating and only 20% extra for surges, I wouldn’t buy one. If it mentions nothing more for surge current, just a flat delivery figure, that’s a red flag in my book.

I was tasked with installing this AlphaESS unit 5 years ago. It was rated at 5 kW (yellow), but the backup supply was less than half that figure (red). It would barely start the fridge, but if the lights were on too, it just tripped out. Explaining this to disappointed customers swore me off cheap, lightweight junk.

A Series Of Excuses

We recently had to sort out a dispute between a SolarQuotes-approved installer and an unsatisfied landlord. He wasn’t an electrician, but he understood the issues well enough, and as soon as he mentioned what they’d sold him, I knew what the problem was.

It didn’t matter if the stove was gas or not, the notion that you could put a 10 kW three-phase supply on a food service business, with fridges, range hood, microwave, sandwich press, and a coffee machine; that’s patently absurd.

Part of the email conversation from the installer looked like this passage below, so I’ve added some helpful interpretation in italics.

“Originally, he did not specify the exact loads, and there is limited evidence that he clearly outlined that a backup of a fully operational commercial kitchen was expected.”

• They won the job as part of a resilience grant through Ausnet.
• The idea is that the doors stay open, and the coffee keeps flowing if there’s another bushfire in the Dandenong Ranges.
• They can’t say the place will be resilient when the system installed isn’t reliable.

“Most of the verbal communication has been lost as it was quoted 2 years ago with employees who have left.”

• Sounds like the dog ate my homework and my company documentation.
• They did a site visit and found a commercial chassis switchboard staring them in the face.

The issue is that backing up a commercial kitchen is not a small job.

• Yes. Correct. So they need to design the system properly.
• Or they need to tell the customer there is a limited capacity.
• AND they need to segregate the switchboard to make that limited capacity reliable.

Even if we did it the first time, it’s a significant upgrade, like buying a 4-cylinder car and then saying, “It’s not as fast as I wanted, give me a free upgrade to the V8”

• The customer showed them his tri-axle caravan, yet they sold him a Corolla with a towbar.

The silver lining in this situation is that the parallel-connected, stackable hybrid inverter will be easy enough to swap, though the 20-metre supply cable may need to be enlarged.

They’ll also have a great deal of data now that the system has been up and running for some time, including real-world measurements of peak demand at the site as it currently operates.

• The cable they installed might be good for more than 10 kW.
• Maybe the max demand means they can put a 20 kW inverter in, and that’ll cover the load without replacing the cable.
• However, the demand might be big enough that even a 30 kW system is going to struggle.

In either case, the switchboard might need to be segregated for essential and non-essential loads. Which, in this case, means a fairly involved process of installing another enclosure and removing circuits from the main switchboard.

My bottom line with this installer is to ask, “Do you ever sell 3 kW backup systems?”

Of course they don’t, yet they did site visits, and they looked at a commercial switchboard for the hospitality business… and they designed a system based on a 10 kW three-phase machine. 3.3 kW of backup is effectively useless, and that’s a failure they must own.

They Turn Up On Social Media

Finn recently detailed a real-life problem where customers didn’t realise what they were getting into. I’ve dealt with a rash of similar complaints via the SolarQuotes support channels. One in particular boiled down to a salesman who missed a 7 kW floor heating load, so whether the grid was available or not, the system would overload and shut down intermittently.

Lightweight Hybrid Inverters Are Not Good For Off-Grid

I recently saw an electrician requesting help with an off-grid system that was failing under the load of 4 domestic deep freezers. On paper, it was passable, but in reality, the Sigenergy hybrid inverter proved unfit for purpose and had to be replaced with a proper Victron off-grid system. Thankfully, this bloke was big enough to admit every day is a school day, and sometimes you learn the hard way.

For installers, it’s particularly difficult because they should damned well know better. When things go pear-shaped, it’s not only expensive, but it also causes a lot of reputational damage. There’s often a bit of he-said-she-said; that’s where you find verbal contracts are fickle.

At the end of the day, if an installer has promised unconditional backup, then they’re on the hook under Australian Consumer Law. Either they deliver a solution that works, or they offer a refund that includes funding from the Cheaper Home Battery Program.

I can’t stress it enough; if you’re looking to get blackout protection for your place, make sure everybody is on the same page. Grab your sales representative and write down what you expect the system will, or will not do.

Don’t let a headline kW number decide your backup power. If you’re on three-phase and shopping for a battery, get up to three quotes from SolarQuotes’ vetted installers and ask each one, in writing, exactly how many kW you’ll get per phase, not just on paper. Every quote is backed by the SolarQuotes Good Installer Guarantee.