How much solar do you actually need

Start with your average daily usage in kilowatt hours, which is printed on your electricity bill, and size the array at roughly half a kilowatt of panels for every kilowatt hour you use in a day. A home using 20 kWh a day lands around a 10 kW array. That sounds like a lot of panels because it is: the array has to cover the evening as well as the middle of the day, and it only makes power for part of it. The calculator on this page does the same sum properly, hour by hour, and shows you where it lands.

At 475W a panel, which is what most of our quotes are built around, you get a bit over two panels per kilowatt. A 6.65 kW system is 14 panels, 10.45 kW is 22 panels, and 13.3 kW is 28 panels. Allow about 2.1 square metres of roof each once you count the spacing between rows, so 22 panels wants roughly 46 square metres of unshaded roof facing a useful direction.

Because the extra power arrives at midday, which is exactly when most homes are not using much. Past a point the additional generation has nowhere to go but back out to the grid. On the same 22 kWh a day house, going from 6.65 kW to 20 kW of panels, three times the array, only lifts the share of power coming off the roof from about 45 percent to about 52 percent. Everything else gets exported. That is the plateau, and it is the honest reason a battery gets talked about, not a sales tactic.

Roughly what your house uses between sunset and sunrise, which for most homes is somewhere between half and two thirds of the daily total. A home on 20 kWh a day usually wants something in the 13 to 16 kWh range. Undersize it and you are still buying power at 9pm. Oversize it and you are carrying capacity your solar never has enough spare to fill, which the calculator will show you: drag the battery slider up and watch the "about X percent of it gets filled" note stop moving.

The panels work the same, but you use far less of what they make. On our modelling an empty house through the day gets about 41 percent of its power off the roof from solar alone, against about 57 percent for a house that is busy at midday. Add a battery and the two very nearly converge, both landing around 96 percent, because the battery moves the midday surplus into the evening for you. If the house is empty all day, the battery is doing most of the work, and that is worth knowing before you spend anything.

You will see figures like seven or eight quoted for South East Queensland. That is raw sunlight landing on a surface, not power arriving at your meter, and the difference catches a lot of people out. Once you take off inverter losses, heat, cable losses, dust and panel tolerance, a well installed north facing array here delivers around 4.2 kilowatt hours per kilowatt of panels per day as an annual average. Summer runs closer to 5.1 and winter closer to 3.2. We size against the honest number, which is why our systems tend to look slightly larger than the quote next to them.

No, and plenty of good systems do not. North gets the most out of the same panels, but east and west arrays spread their output across the morning and the afternoon instead of stacking it at midday, which can suit a house better than the headline figure suggests. On the same 10 kW array we model about 15,330 kWh a year facing north against about 13,337 kWh split east and west. Losing that much output is often a fair trade for generating when you are actually home.

Usually, but it is the question that most often changes a design. You need unshaded, uninterrupted roof, and things like whirlybirds, vents, skylights, valleys and the setbacks required from roof edges all eat into it. A hip roof cut into small triangles will fit far fewer panels than its total area suggests. The roof space figures on this page are a planning guide. What actually fits gets settled by someone standing on your roof with a tape measure.

Because any dollar figure would be a guess dressed up as a fact. What you save depends on your retailer, your tariff, whether you are on time of use or a flat rate, and what your feed in tariff is that month, and none of those are things we control or can verify from a slider. What we can model honestly is energy: how many kilowatt hours your roof makes, how many your home uses, and when. Once you know that, you can put your own tariff against it, or we will do it with you against your actual bill when we quote.

It is on your electricity bill, usually on the first or second page, listed as average daily usage in kWh. If you have a smart meter your retailer's app will show it day by day. If you cannot find it, a typical Sunshine Coast home with two people sits around 12 to 16 kWh a day, a family of four around 20 to 25, and a big house with ducted air conditioning, a pool and an electric car can be 35 or more.

No. A battery can only give back what your solar had spare to put in it, and it can only be worth having if you actually use that power after dark. Push the battery slider up with a small array and watch what happens: the independence number stops moving, because there is nothing left to charge it with. The sweet spot is usually a battery that empties most nights and fills most days, and you can see that happening in the hour by hour chart.

It is a good sizing sanity check and nothing more. It uses a Sunshine Coast yield of about 4.2 kWh per kW of panels per day as an annual average, a standard household usage shape, typical battery efficiency, and a spread of sunny to wet days rather than one flattering average day. It does not know about the tree over your western roof, the shed that shades you at 3pm in June, how many phases you have, or whether your switchboard has room. Those are the things that move a real number, and they are what a licensed electrician checks on site before we put a quote in writing.

Usually yes, and it is normally a good idea. Panels rarely make their full rated output, so a slightly oversized array fills the inverter for more of the day and lifts your morning and afternoon output, which is when a lot of homes use power. There are rules about how far you can take it and they change depending on whether you are adding a battery, plus your network has its own limits on what you can export. We work that out for your address rather than to a rule of thumb.