Adding a battery via a Hybrid Inverter

Previously we've only needed an inverter for solar panels, now we need one for our batteries too. So are there any advantages in purchasing one inverter, a hybrid, for both our solar panels and our battery storage?

The Hybrid Solution

Just like solar panels, batteries use DC power, so they too need an inverter to change it to AC for your home. Traditionally this has been done with a second inverter, a battery inverter, in what is called an AC coupled solution. Rather than this two inverter approach, the hybrid inverter combines both the solar and battery inverters into one. This provides many advantages and is a lot simpler, which we'll see in the connection diagram.

Connection Diagram

The very simple hybrid setup. One inverter that connects to both the batteries and the solar panels.

How it works

 Fronius Symo Hybrid

Fronius Symo Hybrid

Hybrid Inverter

The hybrid inverter converts the DC power from the panels and the battery into AC power for the home. It also has the ability to pull AC from the home and store it in the battery. You would only do this if you were looking to buy cheap off-peak power from the grid or if you had another energy source in the house, like an older solar power system. Better hybrid inverters offer superior features, like being able to charge the battery and push power into the home simultaneously (if the panels are producing enough), backup power and excellent monitoring capabilities.

 The LG Chem RESU High Voltage

The LG Chem RESU High Voltage

Batteries

Batteries are coming out rapidly. Leading the pack is the tried and tested lithium ions. Nickel Manganese Cobalt (NMC), a popular lithium ion chemistry, is used in the entire LG Chem range. NMC takes up little space, is light and can be discharged quickly, which is why they're the choice of many electric vehicle manufacturers. Lithium Iron Phosphate (LiFePO4) is another chemical variation of lithium ion. It's preferred by Fronius in their Solar Battery due to it being longer lasting and less susceptible to heat. It is also being touted as the safest lithium battery chemistry on the market by BYD. Both Redflow and Aquion, manufacturers of zinc-bromide flow and salt water batteries respectively, have struggled to compete, despite having high quality products. Battery compatibility with hybrid inverters is also a major issue, and something we'll discuss later.

Fronius 3P Smart Meter

Metering Device

Just like AC coupled solutions, you still need that metering device to tell the hybrid inverter when power is coming from or being exported to the grid. Without this, it doesn't know when to charge or discharge the battery.

Pros

Most Efficient Storage of Solar

If you want to store excess solar power in an AC coupled setup, the solar inverter converts the solar power into AC, only for the battery inverter to convert it back to DC for storage. With a hybrid inverter, it knows when there's excess power and stores it in the battery before it's converted to AC. This saves the two conversions required to store solar in an AC coupled setup. Even with a good inverter you lose around 3% in conversion, so it's a notable efficiency gain.

better use of solar panel oversizing

We always recommend panel oversizing with good inverters, for reasons discussed in our Six Minute Solar video. With a hybrid inverter and a battery, you can actually get more out of this than a standard solar inverter. Here's an example. Let's say your 6.5 kW of solar power is producing 5.5 kW of power. A normal solar inverter would have to clip the 0.5 kW of power above 5 kW, but a hybrid inverter can actually put that in the battery. It could even output 0.5 kW AC and push 5 kW into the battery, provided the battery supported that amount of power. It will essentially produce the AC required for the home, limited by the hybrid inverter's max AC output, and then put the rest into the battery.

Exceeding the 133% CEC Oversizing Limitation

Not only can you get more out of oversizing your panels with a hybrid inverter, you can exceed the 133% limit panel capacity limit imposed by the CEC and claim STCs. Note that you must be installing the battery with the original install to exceed this limit. Without the battery, you would be clipping like a normal solar inverter. Also make sure you're still within the inverter's warranted DC input capacity, which is generally somewhere between 130 and 200% of its AC output capacity.

Backup Power is Easier

When you're incorporating backup on an AC coupled system, you need to establish communication between the battery inverter and solar inverter. This is because the solar inverter may need to reduce its output if it's producing more than is required, the grid is down and the battery is full. Setting up this communication can be difficult, but with a hybrid inverter it's not required. The hybrid inverter knows how much power is required in the home and produces only the required power, from either the solar or battery, in backup mode.

Please note that this does not mean that all hybrid's are capable of backup power, just that there is potentially less involved. You'll still need extra switches and some hybrid inverters won't support it.

Better chance of network Operator approval and Feed In Tariffs

Network operators treat battery and solar inverters the same. So if you have a 5 kW solar inverter and a 5 kW battery inverter, you're going to have a much harder time getting the combined 10 kW approved than one 5 kW hybrid inverter. Some networks won't let you have more than 5 kW per phase (Western Power in WA), some don't mind, and others are happy to allow it but with restrictions. For example, Energex in Queensland permit 10 kW provided a 5 kW grid export limit is set. Also, most energy retailers feed in tariff eligibility is determined on inverter size, so smaller can be beneficial here. Yes, a 5 kW inverter capacity means you can't draw more than 5 kW out of either the battery or solar at one time, but most residents shouldn't need to.

Cons

Battery Compatibility

Hybrid inverters are not compatible with all batteries. You could argue that AC coupling has the same issue, but you're not going to purchase a battery inverter now for a battery down the track. In the case of a hybrid, you might. Because of this compatibility issue, buying a hybrid inverter and saying "I can just add any battery I want later" is not correct, as changing technology might haunt you. For this reason, we don't recommend a hybrid inverter solution unless you're purchasing batteries within a year or two.

Replace your current inverter

If you already have a solar system and you want to retrofit, you've got no choice but to replace your current inverter with a hybrid. Hybrid inverters are more expensive than both solar and battery inverters, so if you're a retrofitter you can expect to pay more. You can add your hybrid inverter system to your existing system, but then it's more like an AC coupled solution.

They're new and hard to find

Our biggest gripe with hybrid inverters is, where are they? Particularly the high voltage ones. The Fronius Symo Hybrid is a good hybrid, but being three phase really limits its usage. Only having one maximum power point tracker for the panels also doesn't make it suitable for upgrading older systems with a battery and adding more panels, something that many people will be very keen to do shortly. SolarEdge make an excellent solution for single phase customers with their StorEdge inverter, and they've been very proactive in making their inverter compatible with many batteries.

All the above solutions are relatively new as well, which some see as a risk compared to the more tried and tested AC coupled solutions. Also, both your solar and batteries are dependant on the one device. For these reasons, we only recommend experienced and premium inverter manufacturers for any hybrid solution.

Summary

While battery inverters are a common and proven way of connecting a battery to your home, hybrid inverters offer many additional benefits over their AC coupled counterparts. In our opinion, they are the superior option. They can also be cheaper than a solar inverter and battery inverter combined, which is a consideration if you don't already have solar or your solar system needs replacing. Retrofitting requires replacing your existing solar inverter though, which is more expensive. Don't be fooled by buying a hybrid inverter now and getting a surprise when you can't add the battery you want in a few years time. Also, while they are an exciting new product that we're looking forward to seeing more of - we need to see some new hybrid inverters, particularly from Fronius and SMA.