A Solar Powered Life, Part VII: To Invert or Not to Invert

When I selected the various components for my home solar power system, I always had to keep in the back of my mind the words of my wife. “It must be able to run the hair dryer and the coffee machine”. Wanting to ensure that my wife was happy and given these two appliances are usually designed to be plugged into a 240v AC mains electricity socket, I had to make a decision on how to get the 24v DC solar power that was stored in the batteries into the house and useable for these appliances.

Fortunately some very clever people have developed a device called an inverter. An inverter takes the extra low voltage power (in my case 24v DC) stored in a battery (or other extra low voltage power source) and converts it to the usual mains voltage. (In Australia that’s 240v AC.) Because the output of the inverter is the normal mains voltage, you can plug in readily available appliances and run them using the energy stored by the solar power system in the batteries. Cool, the coffee machine and hair dryer were now taken care of and there were smiles all around.

Sometimes, things are so commonplace that you don’t question them. Most houses in Australia are wired up for 240v AC mains electricity (other countries run different mains voltages). However, one question that is rarely asked is, do you need to wire up that house for 240v AC mains wiring? The answer is no, and you cannot be forced to do so. Although the crucial issue is that most readily available appliances are powered through the use of mains electricity.

So why would you not install 240v mains wiring in a house?

I didn’t really want to have to pay for an electrician to either install the solar power system or the wiring in the house. Especially, since I had the knowledge to wire up the solar power system and the 240v mains wiring myself. However, in Australia, this knowledge alone does not provide the legal right to undertake this work. Because I chose to have an inverter and run 240v mains wiring around the house, I was legally forced to employ an electrician. In Australia, any mains wiring must be installed by a licensed electrician, who will issue you with a certificate of electrical safety upon completion of the work. My compromise was that I installed the extra low voltage solar power components and the electrician installed the mains wiring. Had I chosen to not have an inverter and utilised only 24v I would have been able to undertake the entire installation myself.

Is it legal to wire up your own extra low voltage solar power system in a house?

In Australia, as long as the solar power system does not exceed 50v AC or 120v DC it is considered extra low voltage and as long as you comply with the extensive requirements in the Australian Standard (AS4509 Standalone power systems) and you have reasonable competency, then you’re OK. Extra low voltage wiring is still potentially dangerous from a fire and/or electrocution point of view, but not to the same extent that 240v mains wiring is.

Why would you DIY the installation of your standalone power system?

The other reason I installed and wired up the solar power system, was that although the government was offering rebates for the installation of solar power systems in remote areas, (without taking into account my labour) it was actually cheaper to source the components and install them myself. The installation took around 5 days of my time and the components cost about 60% of the quote that I received from a reputable supplier.

What is the motivation for having any mains wiring?

Why would you install any mains wiring at all? It all gets down to appliance availability. I had located an extra low voltage hair dryer, lighting would have been easy, even a laptop or LCD television can be sourced, but it was the coffee machine that was the real stumbling block. Thinking about my wife and coffee (and my own future happiness), I had to factor in an inverter and in my situation this meant installing mains 240v AC wiring.

However, if it was an emergency set up, or a cabin, shed or holiday home, I wouldn’t bother with mains wiring (which I’ll discuss in the next article in the series).

Types of Inverters

There are different types of inverters which perform different functions and they cannot be swapped.

a) Grid Tie Inverters

This is what you use when you connect up solar panels to the electricity grid without using batteries. They’re pretty clever because they convert the output from the solar panels directly into that of mains electricity. In addition to this, they synchronise that output with the electricity grid and sense whether the grid is still active. Once the electricity grid has failed for any reason, the inverter will immediately stop functioning. The reason it stops functioning is to protect linesman who may be working somewhere in the electricity distribution system and will most likely be electrocuted by the unexpected output from the inverter.

With a grid tie inverter, once the solar panels no longer produce any energy, or the householder uses more energy than the solar panels are producing, excess power is drawn from the electricity grid.

b) Standalone power system Inverters

All other inverters are designed to connect up to batteries. These convert the energy stored in batteries to mains electricity. These types of inverters vary significantly in quality, output and efficiency. As a general rule though, the more you pay for an inverter, the better quality, more efficient and the higher output that inverter will have.

Quality

Inverters vary widely in the quality of the mains electricity they produce.

The lowest quality (and therefore cheapest) inverters are square wave inverters. People living in rural areas describe this output as dirty power. Without getting into technical details, this output is good for power tools etc, but can damage other complex and sensitive electrical devices. Also be aware that these can also be sold under the description of modified sine wave inverters.

The highest quality (and therefore more expensive) inverters are the pure sine wave inverters. These mimic the delivery of mains electricity. The quality of the electricity delivered by a good pure sine wave inverter can actually be a whole lot better than the electricity grid itself, which is subject to all sorts of fluctuations and is rarely that stable.

Output

Inverters specify output with 2 measures: Continuous and Peak.

When I think about a small fan heater, I remember that they usually use around 2.4kWh of energy. It’s a pretty scary amount of energy really, especially when you look back at part three of A Solar Powered Life and you remember how little energy my system generates in a day. However, if you wanted to power this fan heater from a standalone solar power system, you’d have to have an inverter that supplies (continuously) an output of at least 2.4kWh (ie. 2,400 watts).

Continuous is the amount of energy that the inverter can supply continuously all day, every day.

Peak output can be many times this continuous number (ie. It could be as high as 9,000 watts), but the inverter can only supply this amount of energy for a very short period (maybe 5 minutes) before over-heating and shutting down.

To put the fan heater into perspective, my system can supply 3,000 watts continuously. This means that if I ran a fan heater, I would only have another 600 watts to provide for all my other household needs.

Efficiency

What is meant by efficiency? If you look at inverters on ebay, they’ll all provide an efficiency percentage figure. This figure indicates the percentage of stored battery energy which is converted into mains electrical output. With a cheapie inverter, this can be as low as 80%, which is not a problem if you’re off camping and hook up an inverter to your car battery for a short period of time to run say, a coffee machine. However, if you want to power a house, 80% is a complete disaster because you’ll lose 20% of your stored energy every time you switch on an appliance. A good inverter will be around 94% efficient.

Before anyone starts writing in to complain about the inadequacies of solar power systems, this happens with the electricity grid on a massive scale. With the electricity grid, not only are there massive losses of energy in the lengthy cables between a city and the generators, but they also transmit the power at really high voltages (in the millions) and then have to convert them to mains power, thus they too have the same efficiency problems as an inverter.

Does your inverter choice affect what batteries you choose?

Some inverters may be small and can be connected up to your car battery (usually a 12v battery), whilst others are quite large and can provide mains electricity to a house, shed etc. Inverters are generally manufactured to connect up to 12v, 24v, or 48 volt battery banks. Therefore, it is the inverter that you choose to use that forces you to make a decision about what voltage and capacity battery bank you will have. If you choose an inverter with a large output capacity you will require a battery with a large output capacity.

Do you have to run an inverter all of the time?

You can also choose to have an inverter on all the time, off, or on standby, and each of these modes will use different amounts of power.

Off – is obviously the most efficient as it uses no power at all.

Standby – is where the inverter is on a little bit just waiting to supply power when it senses a requirement to supply larger amounts of mains quality power. Living with standby is different to being connected to the electricity mains, because when you switch an appliance on, there is a small hesitation before the appliance does what it is meant to do. This hesitation can take a while to get used to. Some complex appliances don’t work very well with standby mode as it doesn’t supply enough power initially for the complex electronics in them to work. The benefit however, is that on standby mode, inverters usually don’t use much power at all.

A good example of this problem is a refrigerator, which has electronics which continually monitor the inside temperature and work out when to switch the compressor motor on or off to cool the contents. These electronics don’t get enough power from an inverter on standby mode to operate at all.

When you look around a household, there are other items which are required to be on all of the time, otherwise they will not function. In my household these appliances include: refrigerator; mains wired smoke alarms; water pump; weather station; and solar hot water pump controller. The common thing with these appliances is that they usually draw little energy in the first place (all up including the inverter itself is about 24Wh). It should also be remembered that any appliance in any situation that is switched on, but on standby, performing no useful function is using energy and this is wasteful.

On – Having an inverter switched on all of the time (without actually doing much), uses up quite a bit of energy which adds up quickly. My inverter, which is quite efficient, uses only 14.4Wh. You may say that that isn’t much power, but over the course of a day (24 hours), it works out to 345.6Wh (or 0.345kWh). When you’re only using 3.0kWh per day then you can see that the system itself uses 11.5% of your total daily requirements.

An inefficient inverter can use as much as 96Wh doing virtually nothing, which works out over a day to 2.3kWh. I couldn’t provide enough stored energy for that particular inverter with my system.

The number that you should look for will say something like “Inverter On – no load – 0.6A at 24v”. Remember that to convert these numbers into watts, you need to multiply the A (amps) by v (Volts). So 0.6A x 24v = 14.4Wh.

Hopefully the relationship with your inverter will be long term, so like any good relationship, it’s worth checking out the small print before purchasing.

Anyway, all this talk of coffee has made me thirsty….

In the next article I’ll describe issues relating to wire, fuses and alternative methods for wiring up a home, shed etc. using extra low voltage.

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I’d appreciate it if commenters kept their comments relevant to this particular article or the series in general. If commenters have ideological issues relating to solar power, I’m happy to discuss them and will do so, however this article is not the forum. Please post these types of comments in the article A Solar Powered Life – Part 6 – The ideological debate. Chances are that your concerns have already been addressed there.