Backup down under


Solar Enthusiast
Greetings from down under. I live in a home near the small town of Bellingen, in northern NSW, Australia. Glorious country:


That's our home at left centre, and obscured by the tree and slightly off the left of the image is a secondary dwelling where my Mum lives. 12 acres, behind our home (to the left) is bushland. Aside from that, the beach I take our dog to is 20-min away:


We've been here living the dream for a little over five years now.

While semi-rural (~1km out of town) we have 3-phase grid connected power. What you can't see in the pic above is another building which was originally a builder's workshop, hence the 3-phase supply. That building was in ruins when we got here but I have resurrected it and created a mancave escape with 115m² of indoor mancave goodness. :)





Has since been decked out with bed, furniture, storage, workbenches and is a self contained living space / office / workshop / training facility.

I had installed an 11kW grid tied solar PV array, with a Fronius Symo inverter on the primary dwelling:


It's great, has made a massive difference to our power bills and our investment will be recovered inside of 5 years. Our home is all electric except for the hot water in the second dwelling and mancave which each have Rinnai continuous flow gas HW units with 45kg propane tanks. Heating/cooling is all via reverse cycle aircon. Apart from working on the energy supply side with the solar PV, we've also made a few changes on the energy demand side and have managed to reduce our average electrical energy consumption over the past 5 years by 19kWh/day.

That's the good stuff. The downside is rural electrical supply is less reliable than what I was used to when we lived in inner Sydney. With the Fronius meter data I collect, I have been able to keep a complete record of our grid outages and each year we average a dozen longer outages totalling about 36 hours. Plus a lot of shorter brown-out type outages. It's seasonal, with Summer storms being responsible for half the outages.

The longest was a 2-3 days outage after a mini cyclone took out power lines across the district, and it was when we had family staying for holidays with fridges full of all our Christmas food and goodies. Not happy Jan.

It was clear based on our experience over several years that grid outages were going to be an ongoing fact of life here so I did what most probably do and started with a small cheapie generator and lots of extension cords. That was OK and got us through a few outages but it was tedious to set up and put away again, only to have another outage soon after. And once we decided to build the extra dwelling for my elderly Mum to live in, well we were going to need a better solution for some energy security.

So about 18 months ago I decided it was time to have backup power supply wired into the main circuit board. Keeping it simple I started with a 15A power inlet (we are 230V AC here, so that’s enough to handle 3.5kW) which can supply all the home’s circuits via a cutover switch.


While the home is supplied via 3-phase, when on back up supply the home is effectively on a single phase supply. We have no 3-phase appliances or loads, and if we did they would not be connected to the backup side of the cutover switch in any case. I kept the oven and induction stove on the grid-only side - and the oven clock tells us when the grid power comes back on, it also makes a bleeping sound on power up so we know when we can cut back over to grid power.

I bought a pre-loved Yamaha EF3000iSE inverter generator with keyed electric start in near new condition for half the new price. It’ll supply up to 2.8kW continuous without problem. It’s quiet and easy to start/use and this has been our mainstay for power backup and we’ve been very glad to have it.

However, despite how easy it is to use, the wife is still not keen all that keen on using the generator when I’m not home, plus if we were both not home my Mum would have little hope of turning it on.

Hence I wanted to move up another step with our backup power system to make it even easier to use and also provide automation for my Mum for the occasions when we are both away.

So I’ve started an off-grid inverter/battery/solar project. Since this post has been long enough already, I’ll end it here and next post will be about that part of the project.


Solar Enthusiast
Scope of the off-grid backup system:
  • The primary function of the system is to be a very easy to use backup power supply.
    Flip one switch and that’s it. Wife friendly and idiot proof for me.
  • It needs to provide about 6 hours of backup, which will cover 95% of outages we typically get. Generator still available for anything beyond that.
  • Costs need to be kept to a minimum. Else we may as well go for a commercially installed battery system (e.g. Powerwall or similar).
  • Prefer to repurpose pre-loved equipment where practical/sensible.
  • Explore what other options such a system enables (e.g. taking some loads off-grid).
  • It also needs to be fun and educational as a project but not so daunting for a beginner.
Backup energy needs audit
This was pretty easy as I have 2.5 years of 5-min interval data on consumption, so I know in great detail what sort of supply we need for “essential” backup.

The three buildings combined draw an average of ~600W, sometimes a little higher, sometimes a little lower. That’s the general power consumption without use of electric cooking appliances or RCAC for heating/cooling. It does exclude our off-peak hot water tank, which is on it's own dedicated circuit and won’t be covered by this. The HW tank has enough capacity and heat retention for at least 2 days supply, and we have an alternative HW supply in the mancave if needed (it has a bathroom and kitchenette).

So energy supply capacity needed: 600W x 6 hours = 3.6kWh, call it 4kWh for a bit of margin.

Power capacity - while 600W is the average load, there is still a tea-loving wife and mother wanting to boil their electric kettles. We use electric kettles here since they work way better at 230V than at 115V and typically have about a ~2.0-2.2kW draw for a fast boiling time (I’ve measured it with a power monitor). So for a couple of minutes at times I’m going to want to cover loads of about 3kW, maybe a little more for brief periods if a few things happen to cycle on at the same time (e.g. fridges). 4kW would be ample.

Here in Australia we don’t have the depth of market supply for products (new or second hand) there is in the USA. I looked at importing Chinese LiPo batteries but they are just way too much $ for what I want.

As it happens, a guy about 45 minutes drive from me regularly trades in used SLA batteries from data and telecom backup facilities. These companies turn them over fairly regularly and so I took a punt on 4 x 190Ah 12V SLA units by Enersys. That’s a nominal 9.12kWh of storage. Being SLA batteries I’m not expecting to draw beyond 50% DOD, but that’s still 4.5kWh when I was looking for ~4kWh. Heavy buggers at 60kg each!

This was another punt. I decided on one of those generic clone hybrid all in one units. These are like the MPP units you have in the US but are unbranded clones. No doubt much maligned by many. Price and features were right and if it turned out to not be much good, well I wasn’t in the toilet for too much. Specs:
  • 48V / 84A DC battery input
  • 230V AC input for charging or pass though line mode
  • 50A Solar DC input (Voc of only 105V)
  • 4kW / 5kVA AC output (230V / 22A)
It’s my first attempt at such a project and these all in one units just seemed much better option than attempting to build a totally modular system, plus the price made it completely viable compared with getting all the individual components. I totally get the benefits of building modular, but as a beginner this was the better option for me.

Bits n pieces:
This was more of the learning journey but eventually I sorted out the stuff I needed, plus I was donated or loaned a few things to use including, 100A fuse, an energy monitor and a battery balancer.

As it turns out, right under the main circuit board I have a large housing which encloses the pool filter and pump at one end (seen in the image in the previous post). This is all under a 2m wide eave on the SE end of the house so very well protected from the elements and sees very little sun (our sun is in the northern sky). The other end of the housing has the spa pump, filter and spa heat pump servicing an in-ground spa but the heat pump was dead and we never used the spa, so I made the executive decision to decommission it and get rid of all the associated equipment to free up the housing for something more useful. It’s perfectly positioned right under the circuit board and is now where I have the battery bank and inverter.

Here’s the inverter and batteries in situ:


The large orange plug is connected to a 20A AC supply outlet with its own dedicated circuit / RCBO on the main circuit board. This provides the grid AC input for the inverter (and this will be a grid side only circuit). I did it this way so I could also supply the inverter from our generator if needed. Unplug from the wall outlet and plug in the generator (I have a spare 20A socket to make up the cable from the generator).

Inverter and battery monitor:


And with the housing lid on it's all discretely hidden away:


This can supply power to the home via the same power inlet previously set up for the generator. I plan to upgrade the power inlet from a 15A (3.45kW) to a 20A inlet (4.6kW) so it can manage the occasional peak loads the inverter is capable of supplying.

Before installing I had been testing it for a month or so and moved it all from the mancave workbench and into its new home a couple of weeks back. All now in place, tested and working. Batteries are in fine condition and I've done several load tests. I was able to pull 6.7kWh out of them until the first battery hit 10.5V under a 0.2C load, and they all returned to 11.9V after a brief rest. Under actual use I won’t need to take them that far. 50% DOD is equivalent to 6-7 hours of backup. Perfect!

I also tested the UPS function under load and it works great.

Naturally we had a grid outage while I was in the middle of the installation! Figures.

Next steps
Some main circuit board adjustments to be done by a sparky, mostly moving a few designated high power circuits off the backup side of the supply and upgrading the power inlet to a 20A unit as mentioned above (I have the inlet ready). It will be in a permanent state of readiness and this will make the system super simple to use - flip one switch and the backup is active.

It will also mean the off-grid system has the ability (if desired) to provide UPS level backup, which is useful if we are away as power backup can be automatic for Mum in the granny flat. I’ve tested the UPS function under load with the grid power pass through function and it works a treat.

Solar PV:
I’ve been donated 10x 250W Trina panels (yet to pick up) to make up a 2.5kW array. With this particular inverter’s specs I’ll need to put these in a 2S5P arrangement which is an interesting wiring challenge but I think I have that worked out. The array will go on my small flat corrugated metal garage roof. It’s a longish cable run of ~30m to the inverter hence the cabling challenge. I got some used Clenergy rails and mounting feet from my solar installation company for A$100, and via fleabay bought a clamping kit as well as with MC4 connectors, fuses, DC isolator etc.

I’m yet to build the array, this will be part II of the project and should be sometime in the next month or so. So more to update you all with.

Aside from having the solar PV keep the batteries charged instead of using the grid power for that, I’m planning to take the pool pump/filter/chlorinator off-grid. The efficient multispeed pool pump draws about 2-2.5kWh/day depending on the season. I’ve tested the system running from the inverter/battery alone and it works perfectly. Once the solar PV array is feeding the inverter, it will easily handle the pool's energy needs while still having plenty of reserve for covering outages. The battery will pretty much just be providing ballast for the solar.

This will free up about 1MWh/year of my grid tied solar PV energy for other purposes, mostly to be exported to the grid for a little extra export income but it may one day end up being diverted to our hot water system if the off-peak tariff for some reason gets too expensive.

I’ve been repurposing many pre-loved items for this project. The batteries, the solar PV, the rails, the housing and platform and other bits n pieces. The inverter is new, but these new off-grid all-in-one hybrid units are pretty cheap nowadays while being very feature rich.

All up the budget (inverter, batteries, solar PV, racking, DC & AC cabling, fuses, switches, inlets and outlets, plugs and connectors, conduit and all sundry items) was A$2,400, (~US$1,900). There is going to be some external subsidy bringing it down to A$1,600 (~US$1,250). It is lot less than I paid for the Yamaha inverter generator we've been using for our backup, and that was half the new price. For a 2.5kW array with 6kWh+ useable storage capacity with this feature set I reckon that's not bad going.

As for longevity, who knows?

Now I'm just waiting for an outage! My stats tell me at this time of year it's much less likely.


New Member
Great project. I grew up in Urunga/Valla, went to high school in Bello.

To feed back into your house, do you just have a lead from the AC output of the inverter and plugged it into the inlet you have installed? Then flick the changeover switch when you get a blackout?

I'd be interested to know what parts of the system you have built yourself versus what you have had to get an electrician to do. Since you mention an external subsidy, I'm assuming it all has to be pretty legitimate and compliant. I'm looking at doing something similar, and while I'm comfortable doing anything not inside my switchboard, I doubt a sparky would touch anything an amateur has put together. How have you dealt with this?


Solar Enthusiast
To feed back into your house, do you just have a lead from the AC output of the inverter and plugged it into the inlet you have installed? Then flick the changeover switch when you get a blackout?


Well, almost.

As it currently stands I do need to turn off a few circuits on the main board before cutting over to backup. I will be getting the sparky adjust those specific circuits so they can never be supplied power when on backup and hence don't need to be touched at all.

Once I have those few circuits adjusted, then it really will be a simple flick of the cutover switch and that's it. Backup power on tap.

These few specific circuits will only be connected to the grid side. If the grid goes down then those specific circuits go offline, and when grid comes back, they come back on irrespective of whether we have backup power supplying the rest of the home's circuits.

This is how I have the oven/stove circuits set up and it works perfectly - I just want that same arrangement extended to some other circuits for idiot proofing. e.g. I don't want our ducted aircon system to attempt to come back on using the backup power supply as it will overload the off-grid supply. And importantly, the dedicated circuit now supplying the off-grid inverter's AC input. I can't accidentally have the off-grid inverter's AC output supplying its own AC input!

I will however need to improve on using just the socket and power inlet arrangement once I want to connect the the pool pump to the off-grid solar PV supply. It might be we have the off-grid inverter's AC output feed a small circuit board and from that I have the power inlet supplied as well as a couple of off-grid GPOs for the pool pump and anything else I might want to run. e.g. the other side of the wall inside the home will become a walk in robe and I'm pretty keen to put in a dehumidifier in that space which could run off-grid as well.

I'll ask the sparky to set that up for me as circuit boards are a bit beyond my comfort zone for the moment.

I don't want to lose the power inlet as it is a fall back option to be able to plug a generator directly to it, should for some reason this system fail.

I'd be interested to know what parts of the system you have built yourself versus what you have had to get an electrician to do.

I had the sparky install the power inlet, change over switch and do the initial circuit board changes. Otherwise the rest is me.

Aside from the other work I mentioned above I'll have them check the switched AC power outlet feeding the inverter's AC input - it's fed by the AC supply cable which was already in situ from a 20A circuit previously dedicated to the old spa heat pump.

Since you mention an external subsidy, I'm assuming it all has to be pretty legitimate and compliant.

The subsidy is a private/personal one, not based any form of govt or other incentive program. It's just the net effect is to bring the cost down by a third.
I grew up in Urunga/Valla, went to high school in Bello.

Then you'd know why I want to get a dehumidifier! How long since you were here last?