Grid Connected Solar Panels with Battery Backup (aka Hybrid Solar)

A solar panel charging a battery

Batteries + Solar?

A few weeks ago I blogged about my mate who really hates his electricity company. In fact he hates them so much he wanted his new house to have an expensive off grid solar system with battery backup, diesel genset, the works.

I blogged how the cost of going off grid with solar would be over $50,000 compared to about $12,000 for a standard, grid connect 5kW system and that it would be economically barmy to go completely off grid if your home already has a connection to the grid which you can use.

At the end of that blog post I noted that there is a third option for those amongst you who like the idea of off-grid solar (or at least being less dependent on the electricity grid) but who don’t want to drop 50 grand and mess around with diesel generators.

The compromise solution may be to get a grid connected system with battery backup (aka Hybrid Solar).

Hybrid Solar:  A cheaper off grid solar alternative

A hybrid solar system (aka “grid-tie with power backup”, “grid failover” or “grid fallback”) is basically a grid connect solar system with batteries and some funky electronics. The electronics continuously monitor the batteries, the solar panels and the grid and decide:

a) when to charge/discharge the batteries.

b) when to connect/disconnect from the grid

This may sound very similar in cost and complexity to an off grid system. But the crucial point to understand is that, because your system is not totally disconnected from the grid, you save an awful lot of money on hardware. The grid is always there if you need an electricity top up. This means that your system needs fewer batteries and no backup power source (i.e. expensive and smelly diesel genset).

4 Reasons You May Choose To Pay More For a Hybrid Solar System

So why are people increasingly considering adding batteries to their grid connect solar system?

Hybrid Solar Reason #1: To keep the lights on (and the beer cold?)

The obvious advantage of batteries is that you can keep the lights on when the grid goes down. A standard solar system will shut itself down when the grid goes down. Why? To protect any lineworkers on the grid.  Imagine if your solar powered home kept pumping 240VAC out into the transmission lines after a power cut.  Any lineworkers who came to work on the wires in your street would get a nasty, if not lethal, shock!

A bank of batteries, combined with a smart box of electronics can safely disconnect your house from the grid in the event of a power cut, and  also create an “island” of electricity within the confines of your home. Your own little mini grid if you like.

(There is also a newish product out there called the PowerRouter which claims it can do this without any batteries. Of course this particular product will only work if the sun is shining, so you would be out of luck if the grid went down at night, but it is a very clever development nonetheless).

Hybrid Solar Reason #2: To overcome solar system “size limits” imposed by your local electricity network.

I’ve had many emails from people (mostly on the end of SWER lines for the techies out there) who have had their grid connection application for their solar system knocked back.  Allegedly their local electricity network can’t handle the power.

These guys are usually told that they can only have a max solar system size of between 1 and 3 kW.

A Selectronic SP Pro Inverter

SP Pro Inverter

One way to get around this is to use one of a new breed of smart inverters in tandem with your battery bank. These smart boxes can decide when to charge your batteries, and when to export to the grid and can be configured to lots of very useful and cool things. A great (Australian Made!) example is the Selectronic SP Pro inverter.

Imagine you are limited to having a 1kW system due to local regulations. With s Selectronic SP Pro, you could still stick 3kW of panels on your roof. Here’s the trick: you can configure the SPPro to have a maximum export rate of 1kW. Any power from your solar panels over 1kW can be used to charge the batteries. If the batteries are full then you simply dump the electricity through resistors (but this dumping would be minimised in a well designed system).

The result: Everyone is happy. You get your big solar system, and your electricity company gets to stay in the 20th century with its arcane regulations.

Hybrid Solar Reason #3: To get around stingy (or non existent) Feed in Tariffs.

If you live in NSW, as well as being ruled by a premier that appears to think renewable energy is the devil’s spawn, you also get a miserable 7c per kWh for any solar energy you export to the grid.  Compare this to the 25c per kWh saved if you use the energy in your home.

For a typical household who is not home during the day, that can be half or more of your solar energy that is getting the miserly 7c per kWh:

 

A graph of solar energy vs residential demand

The orange solar energy is worth a lot less than the yellow

 

If you’ve got a smart inverter and a bank of batteries you can tell the inverter to charge up your batteries instead of exporting to the grid between 11am and 4pm. Then come 6pm when the sun has gone down, but everyone is home, you can fulfil a lot of the evening’s electricity demand with all that stored solar energy. Your electricity demand profile then looks like the graph below – no precious solar energy is wasted – woohoo!

 

Graph shows Battery stored solar energy being used later in the day

The batteries allow us to use the solar energy when it is most valuable.

 

Hybrid Solar Reason #4:  To Game Your Electricity Tariff. 

Time of use electricity tariffs which charge different amounts per kWh depending on the time of day, can vary from about 12c per kWh at night to around 50c per kWh in the afternoon. If you have a bank of batteries and some smart electronics (you don’t even need any solar panels!) then you can buy electricity when it is cheap and sell it when it is expensive. Hmmmm, that’s got you thinking hasn’t it!

Coming Up Next…

In my next post I’ll go through the design and hardware options you have if you are considering going “grid connect with battery backup”  and I’ll also walk you through some typical costings compared to a “standard” grid connect solar system.

Read Part 2 on Hybrid Solar Design here….

About Finn Peacock

I'm a Chartered Electrical Engineer, Solar and Energy Efficiency nut, dad, and founder of SolarQuotes.com.au. My last "real job" was working for the CSIRO in their renewable energy division.

Comments

  1. Margaret says:

    Hey, this is great! I look forward to further details, including your recommendations for smart inverters and the best batteries, and cost indications.

    I can imagine future potential to use something like this, in conjunction with variable electricity rates based on fluctuations in the output of large-scale wind and solar. Domestic batteries could fulfil the role of storing renewable energy at times of high output rather than using expensive solar thermal storage at the source.

  2. Finn you are right on the mark with the concept of a reasonable battery bank charged during the day while the solar system is supplying the house as well and then use the stored energy during the peak period in the evening and again in the morning. I would suspect that it must be possible to sense the battery banks state of charge and if required the battery bank would be charged at night with off peak power.
    We use about 12-15kWh/day and I would be interested in a batttery capable of say 45-50kWh stored capacity to avoid deep cycling issues.
    How close to commercially available is a system such as I have described? We are in WA and the generous buy back for exported power is 0.07c/unit

  3. Jim James says:

    Great blog. I look forward to the details for off grid solar.
    Thanks

  4. Justin Bunt says:

    Finn, I know of a system as explained above. It is in a demo phase at the moment, has been installed for 6 months. It is getting charged for 2 hours (3am-5am) on the off peak time of use rate and has a small PV system to charge the batteries during the day. We are looking at lithium iron phosphate(lifepo4) batteries which apparently don’t have the depth of discharge problems of lead acid. Which means you can have a smaller battery bank. I would like to know your thoughts on lifepo4 batteries.
    Regards Justin

  5. I am in Victoria and have PV and am on the PFIT (66c/kWH). I am interested in exporting 100% of my Solar power to the GRID and using batteries to supply power to the house during the day (say about 3kWH worth). At present I consume about 1100 kWH per year of the power that is produced by the PV and export about 900 kWH. If I could use off peak power (about 11c/kWH) to store generate that 1100 kWH and thus export an extra 1100 kWH at 66c/kWH) I could get a theoretical $605 saving per year. Of course I wouldn’t expect 100% efficiency of the batteries but a system that could do this that cost less than $5000 could provide a reasonable ROI. This is a different application than the classic Hybrid Solar battery system. It is important than while the system is drawing from the batteries it is still conected to the Grid to enable export. Do you know of any systems that could do this (eg could the SP Pro ?) and if so how much would they cost to buy and install ?

  6. Sonia Anderson, Great Southern Solar says:

    Thanks for your thoughtful reviews Finn. We have quite a number of customers who have chosen battery back-up power systems, and who are very happy to not suffer any more blackouts. One of our customers said that the blackouts in his area used to remind him of living in Africa! Now he really enjoys not knowing about blackouts. I must say, we’re the same in this office – we have battery back-up and only know the grid is down if other people tell us about it!

  7. Lindsay Hart says:

    Hi Finn, thanks for a very good post. It is unfortunate that the boom in Solar recently has also seen an increase in misinformed customers, so your accurate post is well received. Firstly i must state that i work for the company that designs and manufactures the SP PRO, yes right here in Australia. You are correct with the features of the unit plus it can be programmed to only charge the batteries with off peak electricity, or alternatively only bring the grid in when the batteries need it. As electricity companies all around Australia apply their own little caps on export power we have simply made the export amount a variable. I must correct you on one point however as it is important not to mislead the readers. Your comment about not needing solar panels and being able to buy Off Peak and sell at Peak would suggest the ability to export stored energy. Can you imagine if the electricity companies thought we could buy for .10ckWh and then they would pay us .60ckWh to sell it back to them, they would close these systems down very quickly, whilst this is very easy for us to do, we do not allow it for the domestic market, what we export is excess solar, so once the batteries are happy and the house loads are happy then we export any additional available solar. keep up the good work.

    • Hi Lindsay,

      Thanks for the comment!

      So you are saying that it is perfectly feasible technically to charge the batteries with grid electricity at off peak rates and then export the kWh at peak rates, but you disable this function in the software so you don’t upset the electricity retailers?

      Cheers,

      Finn

      • Lindsay Hart says:

        Hello Finn, we had to go to great lengths to convince the power companies to allow us to be approved for grid connection, not necessarily about upsetting them. We look forward to the day that the power companies will allow export of PV or any other renewables at any time of the day, this will certainly suppliment the ageing electricity grid in Australia. Of course for every bit of power you dont use from the grid, you are effectively avoiding that high tarrif. We have the technology right now, lets go.
        Regards, Lindsay

    • Reality in Qld ‘mate’:
      Instead of WIN WIN Lose (your customers)
      Here is WIN WIN WIN

      Origin Energy sells off peak to ME (not YOU) @ 15.595c
      Pay me 44c FIT net if I charge MY batteries with off peak and export on the SP Pro that I paid for to pay your salary.
      Costs Origin: 28.405
      Origin sells it at peak rate 34.923c

      Origin MAKES gross profit of 22.9%

      I suggest that you provide a firmware fix for your existing and potential clients or they will buy Supercombis…

    • Oh yeah, Lindsay, by the way as an “INformed customer” the power sold to the other “customers” at off-peak rate would otherwise (at least for the coal-fired generators) be wasted as they run their plants at two speeds: flat out and stop.

      Customers selling back at peak times provide extra capacity and can be sold at a profit, as my primary school-level calculations in previous post demonstrated.

      People like us who are trying to make a return on investment are probably not earning half-a million $ or more income. Most are middle-class battlers. Most probably borrow against their mortgage to afford it. The “misinformed customers” spend many thousands of dollars for a 5 kWp system, and probably twice that again to purchase technologies like SP Pro (or ,now, more likely, your competition’s products) and the batteries and associated technologies.

      This represents generating infrastructure that power companies do not have to spend as capital investment raised through banks or capital raisings in the equities markets. The customers – a small minority – are the ones providing the generating infrastructure.

      I suggest that you look at the current FIT prices at:
      http://www.energymatters.com.au/government-rebates/feedintariff.php#fit-table

      Victoria is going to $0.25/kWh now.

      Customers need to be able to be able to have a reasonable break-even period and return on investment.

      Your argument is akin to the dopey argument one sometimes gets:
      “Well if we let you do it everyone will want to do it”

      Many lower income customers can’t afford such systems, and those with plenty of money are likely not so desperate to invest in a scheme that will produce marginal profits. They will invest it elsewhere. So that leaves the middle class poor and upper middle class (to feel good about the environment at dinner parties).

      So get your company on the side of those most likely to be your customers, rather than trying to be boy scouts and make decisions for the power companies. If you give me some patronising cop-out – don’t bother posting it.

      p.s. Maybe with your type of “in the box” and tunnel vision thinking you might enjoy a career in politics. Or maybe a board position with the power companies.

    • Lindsay was good enough to call me and fill me in with extra information.

      Selectronics is constrained by the power utilities in order to get their product approved. The hardware and software functions that are disabled are necessary otherwise they couldn’t have had their product approved for installation.

      In all fairness to Lindsay and Selectronics – its the power utilities that are trying to fu&# the bloke trying to make a modest return on his large capital outlay.

      So Selectronics are the good guys as far as I am concerned and I will be looking at purchasing and recommending their products.

  8. Michela Friolo says:

    Hi Finn

    Thanks for your very informative post. We were some of the lucky few in NSW that managed to get the 66cents feed-in tariff. We’ve had the system for 2 years now, and since then, the  rate they charge us for electricity that we use from the grid has increased more than 4 cents a kw. Once the contract is over (though I wouldn’t put it past them to pull another stunt like the recent one where they attempted to retrospectively remove the agreed 66cent buy back) I imagine they will try and pay us a pittance for what we put into the grid during the day and charge us a fortune on what we will need during the night and on days when we are not generating enough.

    Originally I wanted to retro fit a back up system to supply power when we have outages, but I have since thought to save my $’s for when the time comes to stop feeding into the grid and use everything that we generate. My plan was to convert our grid system to a stand alone. But from what you say there might be a less expensive alternative. Could we use the hybrid or backup as a stand alone? Our average daily usage is about 11kw, this will probably be less once we connect the hot water jacket to our wood stove to boost our solar hot water in the winter.

    Like most people, I hate the electricity companies, their rhetoric on supporting solar generation is laughable, and our ministers are as useless as a wheel on a walking stick when it comes to supporting solar energy. I would like to start doing my homework now so when the time comes we’re ready to “cut the cord from the grid” so to speak.

  9. Gordon Thomson says:

    Hi I recently had a 6.5kw solar grid connect system installed. With a Fronius IG PLUS inverter.
    I would like to have a hybrid system now, using the battery power at night etc.
    What do I need to do this? and who would do it? Can I do it myself? I live in NSW rural snowy mountains.
    Regards Gordon

  10. This might be a silly question and thinking very low level ‘grass-roots’ here… my power company charges $100 per quarter just for the connection alone. I assume a fully blown off-grid system would last at least 10 years nowadays before needing to replace batteries which means if I did go completely off-grid I’d be saving at least $4000 over those 10 years purely by not having to pay the ‘connection’ fee. Right? Giving the life expectancy of the panels another 10 years on top of that and that’s another $4000 saving so by not staying connected to the grid I could save at least $8000 over the realistic life of my panel/battery setup.

    That’s pretty much the price I was quoted for a very high quality, very powerful and very silent and very much OTT diesel generator so going completely off-grid would still be far more attractive to me anyway :)

  11. Steve, what type of OTT diesel generator would you buy. There is a german one that is an inverter that is the size of a small suitcase and develops about 2 K.W. As the load drops the rpm drops but the genie still provides perfect sine wave power. It’s mostly used on yachts. Less RPM means less fuel.
    I ran my own 20 KVA 4 cylinder diesel genset in PNG for 8,500 hours because of blackouts and used very little power from the grid and saved big money in the process with a load of 12 amps on two phases and 14 amps on the third.

  12. Finn, love your work. I’m a member of a renewable energy cooperative and would like to talk about technology options like this in a little more detail if that is at all possible (by email or similar)?

  13. Yes Minister says:

    Whats the current school of thought re viability of lithium batteries as compared with lead acid or the supposedly forthcoming sodium ones ?? I understand that lithium ones can be discharged more than lead acid ones without reducing their life, but is that sufficient to offset the higher purchase price. According to a number of ‘experts’ who have cogitated over my electricity usage profile, I don’t need more than 600kwh storage so batteries aren’t going to be the major cost I was expecting. Just to nark the ‘PV system owners are evil’ types, one of my objectives is to export the power I’m presently using & collect the extra 52c FiT :) :) :) Cost-effectiveness is not the only agenda however, sinking the boot into the electricity retailer & the politicians is a major factor as well. A bunch of extra panels & off-grid inverter involve only trivial cost that will be recouped quickly, allowing the existing grid-connect system to export every possible electron. Since I’m not interfering in any way with the terms of the existing contract, I figure there is nothing the grubs can do about my money-grab.

  14. Yes Minister says:

    The mention of tariff gaming (Hybrid Solar Reason #4:) supposedly only applies with time of use arrangements as off-peak appliances are meant to be hard-wired & that would theoretically exclude battery chargers. That said, I’m aware of many folk who do this despite the fact their retailer would undoubtedly throw a wobbly if there was any way of pinning the user down. Whilst I’m currently running as much load as possible on off-peak, its all 100% kosher / hard-wired. In the near future however I’ll be getting another off-grid system purely to supply my own needs. Anyone getting a decent FiT would be better off with a complete additional off-grid system to feed the batteries, that way the whole output of the grid-connect system would attract the nice return. This obviously wouldn’t apply to recent installations with stuff-all FiT

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  1. [...] my previous blog post on hybrid solar systems (aka grid connect with battery backup) I promised to follow up with a post that went into more [...]

  2. [...] my previous 2 posts I looked at the concept and design of Hybrid Solar [...]

  3. [...] jumped out to me is his article on installing grid-connected Solar Panels with a battery backup.  Basically you get your solar panels and your grid connected system, PLUS a bit of clever [...]

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