Section 6: Planning your solar electricity system

A number of older gas and electric appliances have modern electric equivalents these days. Even without a solar PV system installed, households can save on bills and greenhouse gas emissions by upgrading old appliances with new efficient models. From hot water heat pumps, reverse cycle heating and cooling systems, LED lighting, induction cooktops, washing machines right through to personal heating, there are modern electric appliances that usually cost less to run than older appliances, including gas appliances, especially when your solar PV system powers them some of the time. If you have a plan to replace all of your gas appliances with electric appliances, you will also eliminate the monthly connection charge.

Visit Renew’s Energy Efficiency and Appliances page for more information.

When replacing appliances, comparing the energy consumption of different models is often simple, as many appliances such as fridges, dishwashers and washing machines are required to carry an energy ratings label. The label has two energy usage indicators: the star rating (the more stars, the more efficient the appliance) and the annual energy use in a typical situation (the lower the number, the less energy it uses). Find out more about the Energy Ratings label at the Energy Rating website.

Another step to preparing your home for a solar PV system is to make sure your home is well-sealed to reduce the energy use of heating and cooling systems. If you added up all the areas where air leaks occur you could have the equivalent of a one square metre hole in your wall! In winter, leaks allow your hard-won heat out and the winter cold in; in summer, they allow in the hot outside air and if you are running your air conditioner, your cool air will literally escape out the door.

There’s plenty that can be done to retrofit and fix a draughty home to make it more energy efficient to live in, from sealing doors and windows to installing insulation.

Keep in mind that some types of gas heaters (‘open-flued’) require some ventilation to operate safely. The vents installed in the walls near the ceilings of older homes are generally sufficient. But you should seek expert advice if you are retaining older open-flued gas heaters when draught-sealing a home. Similarly, if you have gas cooking you will need a rangehood with an extraction fan if you are draught-sealing.

Here are a few tips to get you started, or consult an expert about making more substantial retrofits such as insulation or double glazing.

This can be done by applying weather-stripping around the door or attaching a seal to the bottom of the door.

Fitting pelmets to your windows greatly reduces the air circulating down between the window and the curtain.

Downlights can be a source of ‘leakage’. Every place you have a downlight you have a break in your insulation. Remove downlights from your ceiling and bring the fixture within the room ‘envelope’, or install downlight covers.

Modelling by non-profit organisation Renew shows that a larger system can have a shorter payback time, which is the number of years until bill savings recoup the installation cost.

The shorter payback periods were found in particular on larger systems of 4 to 6 kW and were due to higher feed-in tariffs when combined with a lowering in the price of a solar system installation.

Things can change with changes in government policies, FiTs and STC prices (the federal rebate for solar’s contribution to the RET), so evaluate quotes at the time of purchase to estimate your payback period and determine the best system size for your situation. You can also check Solar Choice’s Residential Solar System Prices guide, updated every few months.

If bigger is better, how big should you go? Roof space is an obvious factor. Most people have budget constraints and have to prioritise their spending. Don’t ignore other investments that may pay off even quicker, such as insulation, gap sealing, window shading, LED lights and efficient appliances.

Electricity distributors may limit the size of solar systems connected into their grid. If you’ve got a normal residential single-phase connection, solar systems up to 5 kW in size are usually no problem, however, going larger often requires extra paperwork or may not be allowed. Confirm the process for larger systems with your distributor. The limit is based on the inverter capacity, not the total capacity of the solar array – this is one reason it has become more common to oversize the solar array by up to 33 per cent above the inverter capacity (a 6.6 kW solar array on a 5 kW inverter).

Your distributor may also limit the amount of excess energy that can be fed back into the grid for the feed-in-tariff. This could affect the payback period and the total value you get from the system, so again, verify this with your distributor before you proceed. Refer to “Grid-connected solar explained” on page 14 for more information.

Many people might not be able to afford a larger system. The following advice applies to systems sized to match efficient household electricity use, which will still result in bills savings and help the environment.

First decide how much energy you want the system to produce. Do you want to offset your entire electrical energy use, or just a given portion? You can determine your energy use from recent energy bills or smart meter data, although you should also consider whether your energy use will reduce after any energy efficiency measures, or rise after purchasing additional electric appliances or an electric car.

Think about how much electricity you use during the day when the sun shines and the solar system will be generating. With a feed-in tariff lower than the rate you currently pay for electricity, the solar generated electricity will be of greatest value when it is consumed on-site, as opposed to being exported into the grid.

Think about how this daytime electricity generation can be used to run appliances, even if you’re not at home, such as a heat pump hot water system or even setting a timer on your washing machine.

Ideally a solar site is a north-facing roof or ground space that is not shaded during the day. Panels may also be mounted on other roof areas, and can face north-east or northwest or even completely east or west. However, the more the panels face away from north, the less solar energy they capture and the less electricity they can produce. For panels on an east-or west-facing roof with a 20° pitch, average daily generation will decrease by up to 15 per cent.

Maximising total system generation is not always the key consideration. Some households may instead want to generate more when they are more likely to be using it, to maximise self-consumption of their system. In this case, arrays facing north-east and north-west may be preferable – generating less overall, and less in the middle of the day, but more in the morning and late afternoon/early evening. Other households may wish to maximise feed-in-tariff (FiT) revenue by facing some panels north, but others west or north-west to take advantage of time-varying FiTs. Talk to your installer about the different options.

Where the array consists of panels facing in more than one direction, the array should be electrically split so that each section of the array only consists of panels all facing the same way. In these cases it is usually best to use an inverter with two independent solar array inputs (most modern inverters have this) so that each array section operates independently, without being affected by the other. Using microinverters (explained in Finding the right inverter), which make each panel independent from the others, is another solution. DC optimisers (devices that attach to individual panels to improve their output) could be another option in certain situations – ask your installer.

It is important that the panels are not shaded much during the day throughout most of the year, especially in summer. Because of the way panels are connected together in ‘strings’ (one after the other in series, like fairy lights), if one panel has a shadow across it, its output is reduced and the output of all the other panels in the same string will be reduced by the same amount. If partial shading is unavoidable, the system installer should allow for this, by designing the system so that the shaded panels have the least effect on the rest of the system. This can be done in a similar way to managing arrays facing different directions; by putting all the shaded panels on one string and unshaded panels on another, by using an inverter with two independent solar array inputs; by installing optimisers on shaded panels; or by installing microinverters on each panel.

Microinverters are becoming more common because they maximise power output from solar panels, especially in shade situations. Find out more in Finding the right inverter.

Panels are often set at the roof angle for aesthetic reasons and to simplify mounting. Slight variations in panel angle don’t make that much difference to the annual energy production, so unless you have a radically steep or flat roof angle, mounting panels at the roof angle is usually the best solution. It is important to have a tilt of at least 10° on the panels so that they can be washed clean by rain.

However, the panel angle should be considered if you want to maximise the energy output from your system at certain times of the year, especially if you want to maximise the total annual energy output. To produce as much electricity as possible, angle the panels to maximise summer solar input. To minimise the seasonal variation in generation, angle the panels more steeply to maximise generation during winter, when the sun is at its lowest and weakest. This approach is more common for off-grid systems, but if this is your goal you can work this out with your installer. Speak to your installer to discuss the optimal angle for your situation. Remember, the cost of angling panels more precisely may be more than the value of the additional energy generated.

Shading can impact the performance of your panels. It is important to work with your installer to design a system where shading has minimal impact.