How to size up a solar system

I tell people that roughly sizing up a solar system is easy; solar power can save you around $100/kW (4 panels) per quarterly bill depending on your usage, and a solar system will output around 4 times its size as a daily average.

For example a 3kW system will output around 12kW/h a day and may save you up to around $300 every quarterly power bill.

For many households this rule of thumb is good enough. As solar power prices continue to fall, getting the maths exactly right isn’t as important as it use to be. Many of our customers get a rough sense of what they need and then add on a few more panels to future proof their investment.

From speaking with dozens of Sydneysiders every day, I understand there is a lot of misinformation out there, and this isn’t helped by salesman trying to up-sell customers to large solar systems that they don’t need. So let’s have a look at sizing up a solar system in detail.

Yearly Solar Output

According to the Clean Energy Council Guidelines, a 5kW system in Sydney should output 19.5kW/h a day as an average, or 7117.5kW/h a year. The problem with taking a daily average is that a solar system will output more power in summer and less in winter. This becomes an issue for households that have seasonal usage patterns due to heating, cooling, pool pumps etc. that result in one or two bills a year being significantly more expensive than the others.

Air conditioning and pool pumps often work really well with a solar power system, as an increase in power usage over summer is compensated by increased output from the solar system. Contrastingly, heating in winter will often be turned on over night, making it difficult to run off solar. This can be an issue if you just go off the average and expect a 5kW system to give you 20kW/h a day in June when your winter power consumption is predominantly at night.

Here is an example of what a 5kW solar system in Sydney will output across the year. I have factored in system efficiency to be 90%, and of course the numbers are to be used for illustrative purposes only – your roof and circumstances will almost certainly be different.

When do you use your power?

Perhaps this is an obvious statement, but solar power only works during the day. Without battery storage (which is now available), solar power needs to be used in the home as it is generated, otherwise it automatically feeds out to the grid. You are normally paid around 5-8 cents per kW/h by your energy retailer for the power you sell back to the grid – a fraction of what you pay for your power (but at least it’s something). This means the benefit of solar is to directly reduce your power bill by using it in the home as it is generated.

To find out how much power you use during the day, simply read your meter in the morning and then again in the evening. Subtract one number from the other and that will tell you exactly how many kW/h you have used during the day. Ignore the off peak hot water meter if you have one, and if you have 3 meters for a 3 phase supply, make sure you add the three meters so that you include all of your usage. Of course, you would need to take these readings over many days to get a realistic average, rather than at just one point in time.

The other option if you have time of use billing is to add the usage during the peak billing period with the shoulder billing period. This will give you your usage from 7am until 10pm, which you can then multiply by 0.7 (or so) to give you your day time usage.

Every house has a base load. This will typically include a fridge, freezer, some lights and perhaps some appliances that are left on standby. Like many households in Sydney, no one is home during the day at my place, and we still manage to use 8kW/h during the day time. Divide this number by 4, and a 2kW system would take care of my base load, saving me around $200 a quarterly bill.

On top of the base load you then have the ability to use appliances during the day. Have a look at the average daily usage figure that is on your power bill – how much of this can you move to the day time to run off solar power? Without solar power, you may be running appliances over night during the off-peak billing period, or you may be paying little attention to when you use your power as you pay a flat rate for your power. How much power can you use during the day without too much hassle?

The biggest users of power in a typical home include:

  • Pool pumps
  • Spa
  • Air conditioning
  • Heating (can you turn this on with a timer so you come home to a warm house?)
  • Dryer
  • Dish washer
  • Washing machine
  • Fish Tanks

I generally say don’t worry too much about lights (as long as they are LED, of course), laptops etc. They use very little power compared to the items on the list above.

Also, keep in mind that your power consumption will often change on the weekend, during the 4 weeks of holidays you have each year and the odd day off work here and there.

One final point – keep an eye on the future. A surprisingly large number of people I speak to are getting solar now while they are employed, in preparation for retirement or in expectation of a baby, when the mum (or dad!) will be home all day using power.

So how does this compare to the output of a solar power system?

Well, it depends which way your roof is orientated, and now we are getting into the specifics of maximising the benefit of a solar power installation. I often advise people to go for a split array. Yes, having the panels facing north is best. However this creates a rough bell-curve of output where you get a peak generation period during the middle of the day. Are you going to be able to use most of the solar power most of the time if you have an output curve like the image below?

This is a 3kW system in summer, turning on as the sun comes up and turning off again at around 8pm. As an aside, one of the most important points from this image is the maximum output. A 3kW system almost never gets close to 3kWs at any one time due to the angle and the orientation of the panels. This could be improved with sun tracking, however the cost of installing and maintaining an array that follows the sun does not pay for the increase you will get in output. It is much cheaper to just get a few extra panels.

My advice is to look at google maps (or get out a compass) and see what roof sections you have to work with. Anything north of west or east is usable. Panels facing east will output more power in the morning and panels facing west will output power in the afternoon. Compared to north, panels facing east or west will not output quite as much over the course of a year, however losses are not that great, being more significant in winter when the sun is lower in the sky to the north. The Solaray Team have access to detailed aerial photos, and we can accurately measure your roof and provide advise based on that. For more information give us a call today on 1300 221 586. We can help you size up a system in less than 10 minutes.

In most cases, I think it is a much better design to have two strings of panels giving you two smaller peaks of output. This makes it easier to use most of the solar power most of the time, and this is how you maximise the benefit of your system and hence reduce the return on your investment.

Do you have time of use billing?

Perhaps one of the most important points to consider is how much you pay for your power and when. If you have time of use billing, you are much better off generating solar power in the afternoon during the peak billing period. If your panels face west of north, you will push the output of your solar system into the afternoon, and during summer output will continue up until 8pm. If you have time of use billing, I normally say to take an average of 30 cents per kW/h when estimating the potential benefit to your power bill.

Do you have shade?

According to Renewable Energy World, shading of as little as 9% of a solar system connected to a central inverter, can lead to a system-wide decline in power output with as much as 54%.

The below image is a great example of how micro inverters can significantly increase output. In this image, the system will be working at near maximum output, despite the patchy shade. With a string inverter, the system will be outputting almost no power because of the panels in the bottom right. I have heard a bit of talk about bypass diodes lately. It is important to state that bypass diodes are designed to help stop hot spots on panels from bird droppings and the like. They do not work in the same way as micro inverters and will not help maximise output in shaded conditions like micro inverters do.

If you have patchy shade, please call the Solaray Team for more information. We have shade analysis tools that can help us provide professional advice on whether you need micro inverters or not, and to what extent shade will impact the predicted output of your system. We also have detailed product advice on the Enphase Micro Inverter System.


I hope this information helps you cut through the sales talk. If it all gets too much, just keep it simple; solar can save you around $100/kW (4 panels) per quarterly power bill and will output around 4 times its size, doing more in summer and less in winter. If you want personalised advice, we are here to help. If you have a few power bills ready, and you have read this article I guarantee we will find the right solution for you in a matter of minutes.

We work a little differently here at Solaray; we help you find the right solution – without the sales pressure. We want you to make an informed decision and we want to you to buy the system that fits your requirements.

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