Batteries, inverters and surviving the zombie apocalypse

Solar Inverters | The Good Solar Guide Contents | Solar Panels

If you want to add a battery in the future when batteries are cheaper, the good news is that this does not affect your inverter choice now. Any solar inverter can have batteries added later using a technique called AC coupling, which you can learn about here:

If you want to get a battery right now, along with your solar panels, then you will be offered two options:

  1. A single inverter that handles your solar and your batteries. This is called a ‘hybrid inverter’.
  2. A solar inverter for your panels and a separate battery inverter for your batteries.

I’m a fan of the second option, because it allows flexibility. If you want to upgrade your solar panel array, you only have to upgrade the solar inverter – and if you want to upgrade your battery, you only have to upgrade the battery inverter. When your solar inverter fails (it will, eventually) you only have to replace one, and you’ll have more choice because it won’t have to be compatible with your battery.

I’ve also found that the monitoring software that comes with many hybrid inverters is often pretty limited. I prefer the freedom to choose monitoring from a dedicated monitoring software company. We’ll look at monitoring in more detail later in this chapter.

No matter what battery inverter set-up you’re offered, you need to make sure it will give you the backup you expect. I touched on this when describing hybrid solar systems in Step 1. Now I’ll dive into more detail, because it’s important you get the backup you need when the grid goes down and the zombies come out.

Battery inverters and backup

If you’re planning on buying batteries with your solar power system because you want to be protected from blackouts, here’s what you need to know to get a system that will meet your expectations.

Not all hybrid or battery inverters offer backup. To many people, this comes as a big surprise. Imagine you have a big, expensive battery on the wall and the grid goes down. Now imagine that the battery has been designed so that it cannot back up your home. You’re in the same situation as the rest of the street – powerless. That would be embarrassing, especially if you’ve shown off your big, shiny battery to your neighbours.

Believe it or not, many battery systems sold today either have backup as an optional extra or don’t offer backup at all. If you buy a battery without backup, you’re buying it only for economic reasons – to store solar in the day and use it at night. The moral of the story is: you must tell any quoting solar companies that you want backup.

Most batteries will not back up your whole house – only certain circuits. Most reasonably-priced battery systems simply can’t deliver enough power or energy to keep your home running as normal during a blackout. There are two reasons for this:

  1. Not enough power: A typical battery system will have a 5 kW power limit. Your oven might be 3 kW and your ducted air conditioning might be 3 kW. Switch both of those on during a blackout and you’ve overloaded the battery inverter and tripped your power supply, and now you’re fumbling around in the dark. If you haven’t drained the battery of all its energy you should be able to switch the offending appliances off and restart the system, but worrying about tripping your battery inverter during a blackout isn’t ideal.
  2. Not enough energy: This is worse than tripping the power supply. Imagine you have a battery that can deliver 5 kW and store 10 kWh of energy. If (and that’s a big ‘if’) it’s fully charged when the blackout happens and your home is pulling 2.5 kW, your battery will be drained of energy in just 4 hours. But if you’ve only got LED lights, modem, fans, fridge, TV, radio and phone chargers on the backed-up circuits, these will probably pull less than 400 W. That gives you 25 hours of backup supply.

For these reasons, you’ll have to decide which appliances will run off the battery and which you can live without. A good installer will decide with you which circuits you need – these are called ‘critical loads’, and only these will be powered by the battery when you’re in off-grid mode. Usually, these include your lights, fridge, modems, office equipment and a TV. Let’s be honest, how are you going to get through a blackout without The Voice?

Beware of any solar company that claims their 5 kW battery will provide ‘full backup’ for your home. Unless you have a very efficient house with a low peak power draw, this is not going to happen.

Even if your battery or hybrid inverter offers backup, it might not be able to use solar to charge the batteries during a blackout. The nice thing about having a grid connection is that it acts as a buffer. If your batteries are full and your solar panels are generating more energy than your home can use, it’s not a problem. The grid will soak up any excess solar and even pay you for it. Handy.

If the grid goes down, you lose that buffer. If your batteries are full and your solar panels are generating more than you can use, there’s nowhere for the excess solar to go. If you can’t somehow tell the solar panels to throttle back their power output, you’ll get a big bang and your battery inverter may well start to send out flames.

For this reason, many battery inverters will cut power to the solar inverter during a blackout. This stops the solar panels doing any damage, but it means your batteries can’t be charged. Once they’re empty, there’s no more power until the grid is up and running again.

Some battery inverters have been cleverly designed to communicate with the solar panels, so they can throttle them back when needed. If you want to be able to charge your batteries from the sun without a working grid connection, you need to get one of these.

Now, may I ask why you want backup?

  1. Because I like the idea of powering through blackouts, and I’m confident that any blackout will last no longer than a day and a night. They don’t happen often where I live, but the idea of blackout protection feels good.
  2. Because I have blackouts all the time and I’m sick of them!
  3. Because, although blackouts are infrequent, they seem to be getting worse. I’m worried about much longer outages. I don’t trust the government to get supply back the same day – and I want to be able to keep the lights on through outages that last for days, charging my batteries by day and draining them overnight. If the zombie apocalypse comes, I want to be prepared.

If you answered 1, you probably don’t need to charge your batteries without the grid.

If you answered 2 or 3, you need to ask for a system that will charge the batteries without the grid. Make sure any quotes specify this as a feature in writing.

Solar Inverters | The Good Solar Guide Contents | Solar Panels

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