Thursday, 3:12 PM
The phone rang as I was about to shut down my laptop. It was an installer I'd worked with before, and he was standing on a roof in Mossvale, a suburb in south-west Sydney. He wasn't calling to chat.
'I quoted a standard grid-tied system. The customer now wants backup power for the fridge and a small medical device. The inverter I have on the truck doesn't support batteries.'
That phone call turned into a solar pv with battery storage Mossvale project that had to be signed off by Monday morning. Normal lead time for a hybrid inverter from most suppliers: two to three weeks. He had about 36 hours.
I've handled enough rush orders to know when to listen and when to start checking stock. This was a stock-checking moment.
Why the On Grid Solar Inverter 5kw Couldn't Do the Job
The original plan was straightforward: 6.6kW of panels, an SRNE on grid solar inverter 5kw, and a standard grid connection. That setup is fine for exporting surplus power and lowering the electricity bill. It fails when you add batteries, because a pure grid-tie inverter has no battery terminals and no way to manage charging.
Could we bolt on an AC-coupled battery system instead? Yes, but that means a second inverter, extra wiring, more fussing with communication settings, and a bigger price tag for the customer. For this site, the cleaner move was to replace the grid-tie inverter with a hybrid model.
The unit that made sense was the SRNE 10kw hybrid inverter. Why jump from 5kW to 10kW? Because hybrid inverters aren't always sized the same as pure on-grid units. During a blackout, the inverter has to handle both the house loads and the battery charging. The 10kW gave the installer room to run a fridge, a few lights, and a medical device without riding close to the limit. Bottom line: more margin, fewer callbacks.
People sometimes ask if this is the same as going off-grid. Not at all. If the house had no grid supply, I'd have recommended an SRNE off grid inverter in a separate setup. But this customer was staying on the grid and wanted backup. The hybrid inverter handles both modes in one box. It was the right tool.
How to test LiFePO4 battery capacity before it goes on the wall
Once the inverter was sorted, the installer asked a question I hear a lot: 'How do I prove to the customer that these batteries really are 100Ah?' That's a fair question, because a label alone doesn't tell you much. Some cells sit in warehouses for months, and BMS settings can change how much usable energy you get.
Here is the test procedure I gave him. It isn't fancy. It doesn't need a $500 battery analyzer. You need a charger, a known resistive load, a multimeter, and a timer.
- Charge the LiFePO4 battery fully to the manufacturer's recommended absorption voltage (typically around 3.65V per cell, but verify with the datasheet for your specific pack). Let it rest for one or two hours.
- Record the rested open-circuit voltage. A healthy 12V nominal LiFePO4 pack should sit around 13.2V to 13.4V. This is a sanity check, not a capacity number.
- Connect a load that draws about 0.2C. For a 100Ah battery, that means roughly 20A. Place the multimeter in series to measure the actual current.
- Discharge until the battery's BMS hits low-voltage cut-off (usually about 10V for a 12V pack, but check your spec). Try not to discharge any individual cell below 2.5V.
- Multiply the average current in amps by the discharge time in hours. That gives you amp-hours. Compare that to the rated capacity.
Why does this matter? Because '100Ah' can mean different things at different discharge rates, temperatures, and BMS settings. The test above isn't lab-grade—it doesn't correct for temperature or internal resistance—but it gives you an honest ballpark. And for a customer who's about to spend money on storage, seeing 96-101Ah on a screen is worth more than any datasheet.
That's where the twist came in. The first battery tested at 82Ah. Not 100. The second one hit 97Ah. The third one did about 94Ah. We found a string that was balanced enough to perform as a set, then slowly recharged the weak unit and cycled it twice. By Sunday afternoon, the matched pair was holding 98Ah consistently.
If we had skipped the test, the installer would have mounted a battery that delivered close to 80% of its rated capacity. The customer would have noticed the first time the lights flickered. That's the kind of problem that turns into a warranty dispute.
What I Learned From This Rush Order
First lesson: a hybrid inverter should be the starting point for most solar pv with battery storage projects, not an afterthought. It saves time, money, and a Friday-night panic. The SRNE 10kw hybrid inverter turned out to be the right call because it gave us flexibility without adding a separate battery inverter.
Second lesson: test LiFePO4 batteries before they go on the wall. The procedure above isn't a replacement for a full battery analyser, but it's enough to catch a bad pack before it costs you a site visit.
Third lesson, and this one took me a while to admit: the conventional wisdom that 'new LiFePO4 batteries are ready to install' is only true if the cells arrived balanced and the BMS settings are correct. Sometimes they're not. I've been doing this long enough to know that the extra hour of testing is never wasted.
One honest uncertainty: I'm still not sure why more installers don't specify hybrid inverters from the start. My guess is old habits, because grid-tie only was simpler back before batteries became normal. But once you price the whole system, hybrid usually makes sense. Simple.
For anyone doing this in Australia, remember: grid-connected inverters must meet AS/NZS 4777.2 and be listed on the Clean Energy Council approved inverter list. That was accurate as of March 2024. Verify the current list before you order.
This particular job got finished at 11pm on Sunday. The installer got his sign-off, the customer got their backup power, and I got a reminder that good systems are not picked from a spec sheet alone. They come from checking the real thing.