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How does solar battery storage work?

A simple grid connected solar system does not need batteries to function. 

grid connect solar - no batteries

If you think about it - it actually uses the grid as an infinitely large battery. If the solar panels on your roof are creating more power than is needed by your home, then the excess goes into the grid. Later, if your panels can't produce enough electricity, electricity is drawn from the grid to make up the difference.

There are three reasons you may want to add batteries to a grid connected solar system:

1) You want to store excess solar energy generated by day - and use it when the sun isn't shining.

2) You want to have backup power available if the grid goes down.

3) You want to indulge in some sneaky electricity arbitrage (if you are on a time-of-use tariff). e.g. charge the battery with cheap off-peak electricity and use it in peak times. Sounds great in theory doesn't it? Unfortunately I've crunched the numbers and it just makes no economic sense to do this, so I won't waste your time getting into it.

Many old-school installers (and most of the lumbering old utilities who haven't caught on yet) believe that Option 1 (batteries for nighttime use) and Option 2 (batteries for backup) are inseparable, and that all solar battery storage systems must offer both.

However, since Tesla's Powerwall came on the scene smart installers have realised that it can make sense to offer a solar battery storage system that does not offer any backup function.

Why? Because it makes the system a lot cheaper. If you are wondering whether you need backup power, this article may help you decide - but really, if you live in any major town or city, when was the last time you had a blackout at all, let alone one that lasted more than a few hours? 

The rest of this article will give a brief overview of how each type of solar PV system works, for those who are curious (or just want to know why options 1 and 2 are vastly different in complexity and price).

Let's get started!

Solar battery storage system without backup (AKA hybrid solar without backup)

hybrid solar without backupThis is the simplest and cheapest type of solar plus storage system available. Electrically, it treats the battery pack like another solar panel array. So, the battery pack plugs straight into a spare input on your solar inverter.

At the time of writing only the Tesla Powerwall battery pack is designed in a way that makes this possible. The main reason is that it has an output of 375V DC, which is in the range that a standard grid connect solar inverter can handle. All the other battery packs operate at a much lower DC voltage (usually 48V) which requires a special, separate 'battery inverter'.

Here's how Tesla envisage their battery integrating into a solar system. As you can see - it is pretty simple! (Actually, it is probably a little too simple - this blog post explains how I think it will really work.)

How Tesla claim their battery setup will wor

All that is required for this battery integration is:

a) A battery pack with high voltage DC output and integrated charger (like the Powerwall)

b) An inverter that can talk to the battery's management system (BMS) and that has a spare input

c) A way to know how much elelctricity the  home is currently importing or exporting - so the battery knows when it is a good time to charge (i.e. when there is spare solar) and when it is a good time to discharge (i.e. when there is not enough solar to power the nome).

d) Some wire

As you can see - this should make for a quick, cheap and easy installation (apart from the crane required to house a 100kg box on your wall, and the Tesla comms interface which is supposedly a total pig to commission - or so I've heard).

A solar battery storage system with backup (AKA hybrid solar with backup)

This type of system is much more expensive. I'll go through how they work to give you an appreciation of why that is so.

There 2 main ways to skin this cat: AC coupling and DC coupling.

AC coupled systems use 240V AC to charge the batteries. This is the most common way to retrofit batteries to an existing solar system, because all inverters will provide 240V (including microinverters).

An AC coupled system will generally have 2 inverters, the existing one, and an inverter charger (also called a battery inverter). The inverter charger controls the charging and discharging of the battery. It also monitors the grid connection, and if grid power fails it switches almost instantly to battery (+solar) power so your lights won't go out.

As well as an extra inverter, you may need to rewire your home. "Really?" I hear you cry. Yes, really.

The typical Aussie home can pull a lot of power. If your A/C is going, your oven is on, and someone puts the kettle on for nice cuppa, the peak draw could easily be 7-8kW. That will instantly overload all but the biggest, most expensive battery packs. Even if it does not overload, you will probably quickly drain the battery.

For this reason, battery backup systems usually don't try to backup the entire house. The most important appliances like the fridge and the lights are generally wired into a seperate "Backed up AC loads", and the stuff you can live without stays on the "Main AC loads" circuit.

This keeps the battery pack cost down and lets your house run for a lot longer without the grid.

Here's the final of of my trademark block diagrams that show how all this works.

hybrid with backup

As you can see, it is more involved than the design without backup. Here are the extra costs:

1) A special battery inverter that safely switches to solar and batteries if the grid goes down (at least $3k)

2) Modification of home wiring to have a backed up 'Essential AC Loads" and a non backed up "Essential AC Loads" circuit 

3) Enough batteries to cope with both the peak power of your chosen loads, and to run as long as you specify without the grid. The average Aussie home uses about 20kWh per day of electricity, so they'd probably need 7kWh of storage at a minimum to last them through a blackout of more than a few hours. 7kWh of lithium-ion storage will cost about $7,000-$8,000.

So, that's a very high level overview of how the 2 types of grid connected solar battery storage works.

As I mentioned above - think long and hard about the last time you had a blackout that lasted more than a few hours. For some homes that have important medical equipment or require refrigeration running at all times, then obviously backup protection is extremely important. But for the 99.9% of Australians who don't have a critical need for 24 hour power - backup protection is a luxury that few should bother paying for.

One last thing - let's talk about the elephant in the room. Namely - if you are going to get batteries why all this talk about being grid connected? Why not simply go off grid altogether?

 

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