Are micro inverters & battery backup compatible?

Can you combine micro inverters with batteries? Yes you can!

Can you use a micro inverter off grid? Or even for grid connect with batteries?

With the growth in the use of micro inverters, I’m starting to get more and more emails asking: can micro inverters be used in off grid (or hybrid) solar power systems?

The short answer is yes they can! In fact a number of micro inverter battery backup systems are already operating here and abroad.

The longer answer gets a bit technical – but I’ll try to keep it as simple as I can!

Let’s get back to absolute basics for a minute and focus on off grid systems: The fundamental difference between Off Grid and Grid Connected solar power systems is that Off Grid systems need to store the energy in batteries. The energy produced by your solar panels (or wind turbine, hydro or generator) needs to captured and stored so it’s available when you need it.

Charging batteries with DC

Charging batteries from an energy source needs some sort of control to prevent them being overcharged. Historically, this has predominantly been managed with a regulator, which accepts DC power from your energy source, monitors how the battery is reacting and adjusts it accordingly:

The simplest battery storage system. A panel a regulator and a battery Everything is DC.

If you only ever use DC power, this can work very well. In fact this is the system I use when I go camping to keep my Coopers Pale Ale cold in my DC fridge. Trust me – it works!

Charging batteries with 240V AC

If you have a 240V AC energy source (e.g. a petrol generator or 240V wind turbine) then a battery charger is typically used, which performs the same function, just in a slightly different way:

To charge batteries with AC you use a Charger.

In both cases when the batteries are full the regulator or charger will switch the energy input off.

Once it is stored in your batteries you can use the energy as Direct Current (DC) if you have DC appliances, but in most cases an inverter converts the DC power to 240V AC, just like mains power.

So if you have an off grid system that can be charged from either solar or a backup gen-set, then putting everything together, you have a system that looks like this:

Putting everything together for a proper off grid system.

So, we have our energy stored, a regulator and charger and an inverter to power the loads.

Years ago, inverter manufacturers realised that combining these into a single unit made sense and invented the inverter/charger (also known as an off grid inverter). These were typically designed to accept a variety of inputs and outputs AC input from generators or mains power, DC input from batteries and so on. Typically, solar panels and other devices continued to be connected via a regulator to the batteries.

Modern off grid systems use a single inverter/charger (AKA off grid inverter).

Although it took a while, around ten years ago some manufacturers realised that they could already convert the solar (or other source) input to AC with Grid Tied inverters, so to simplify things a bit more AC Coupling was born, and it looks like this:

An AC coupled system using a grid tie inverter and an inverter/charger (also known as a battery based inverter).

By coupling everything except the battery on an AC Bus, management, monitoring and efficiency was optimised. Although it cost a little more in electronics, transmission losses were minimised, cable sizes were reduced and flexibility improved. One of the major benefits is that if the AC generation source matches the load size, it effectively powers it directly, bypassing the batteries. Another is that if more power is needed, the AC generation source and the inverter charger (drawing power off the batteries) can parallel together, increasing short term capacity. AC coupling can be done at a household level or in much larger micro grids and they can operate with or without a mains connection.

For completeness here is what an AC coupled system looks like with a grid connection (can you tell how much I love block diagrams yet?) :

A grid tied with battery back up AC coupled system.

So, now, we have our energy stored, but energy sources and outputs are all 240V on the input and load sides.

The sophistication of the control and regulation systems required to make this all work smoothly cannot be underestimated; there can be a hell of a lot going at any point in time especially if you have multiple energy sources which can vary their output, and loads which obviously vary a lot too as we go about our daily life.

Luckily, the world has some very clever people and a number of inverter manufacturers highlight their ability to handle AC coupling from solar panels including Selectronics, SMA, Schneider, Outback Power, Magnum and Midnite Solar to name a few.

Although I don’t want to underestimate the sophistication required to make a great inverter/charger there are two major tricks in getting AC Coupling to work successfully.

From a safety perspective, Grid Connect inverters are always searching for a 240V AC reference source and are designed to shut down if it isn’t present simulating a line worker switching off the mains for maintenance, for example. So, in a non-mains connected AC coupled system, the Grid Connect inverters need to be tricked into believing its present by creating a reference point. The inverter charger can do this by creating 240V AC from the battery or if other sources are present (such as a generator) they can too. Either way, as you can imagine, this involves some pretty sophisticated controls to be done safely and reliably.

The second issue is how to regulate the AC energy source to prevent the batteries being overcharged and, to prevent the voltage and frequency from being distorted. If the grid tied inverter and the inverter charger can communicate with each other then the inverter charger can ask the grid tied inverter to gently throttle the power flow up and down as needed.  If these 2 boxes don’t talk to each other then the inverter charger needs to either use a more brute force way to get the grid tie inverter to reduce the flow of power from the solar panels. Different inverter chargers and control systems use different techniques to do this; SMA uses a process of shifting the frequency. Most of the others “throttle” the power in a more traditional way but both systems reduce the energy input into the battery.

Ultimately, there also comes a point where there may be no load, the batteries are full and lots of energy being generated. Some systems can cope with this by throttling, others require control relays to divert energy and in some cases this needs to be “dumped” into a load such as a hot water system or water pump.

A crucial point here is that you can’t just cobble a system like this together. The systems all need to communicate very rapidly, highly reliably and the obviously need to be matched in terms of their ability to handle the power flows. It’s not simple but it absolutely does work and has been gaining favour as a solution for many years.

So, logically micro inverters that present solar as an AC source can indeed be coupled into these types of systems. In the last 2 block diagrams above you simply swap out the solar panel and grid tie inverter for all your AC solar panels. And a quick call to a couple of suppliers revealed that there are already a number of such systems operating in Australia. By coincidence, I also spoke to a local chap the other day who regularly conducts training in off- grid systems and lives off grid in Victoria. While I was on the phone, he literally plugged a micro inverter equipped solar panel into his off grid system and it simply connected and started generating; doing exactly what it was supposed to.

As is the case with non-micro inverter based systems, everything needs to be matched and configured properly but as one example, Enphase have a specific white paper on the issue.

Interestingly, Enphase highlight that one advantage of using micro inverters instead of string inverters is it allows each panel to be individually “throttled” (assuming a compatible inverter-charger), maximising flexibility and efficiency. Recently however, Enphase announced a new generation of smarter micro inverters, which will effectively allow them to make many of the traditional components of an AC coupled system redundant by embedding more of the control and power conversion functions of the inverter/charger into the micro inverter itself.

So if you buy a microinverter based system you won’t be left high and dry if you want to add batteries in the future, you’ll simply need an AC coupled system.

In fact the way technology is progressing it would not surprise me if batteries will soon come with “micro inverter/chargers”. So you may be able to have AC solar panels plugged directly into AC batteries. You read it here first folks! (And I must confess that I actually read it here first last week).