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Section 6: Planning your solar electricity system

There’s a lot to consider when planning a solar electricity system for your home and one factor will influence another.

Household energy use, for example, will determine for instance household energy use will determine what size system you need.

Get the right advice

Remember to seek out independent advice to plan your system. Non-profit organisation Renew offers a range of advice services to anyone looking to invest in energy saving systems for the home.

The Victorian Government also has a range of programs and initiatives to help households plan for their energy needs. Visit theVictorian Energy Saver website for more details.

Work out how much energy you use

Your household electricity use is recorded on electricity bills measured in kilowatt-hours (kWh). Look for a figure that gives average daily usage, and review as many bills as you can to see how this figure varies throughout the year.

Your Smart Meter records your electricity usage every half hour, and this data can give you a much more detailed view of your electricity usage, including how it varies at different times of day and on different days of the week. You can download a spreadsheet of your Smart Meter data from your electricity distributor – however unless you know how to use spreadsheet software (such as Microsoft Excel) to understand what the data can reveal, it may not be useful to you. Fortunately, all the Victorian electricity distributors have online portals that you can log in to and see detailed information about your electricity usage displayed in an easy to understand form.

The distributor is the company that owns the poles and wires in your street and transports the electricity to your home. This is the business you call to report faults and emergencies and is different to your electricity retailers who sends you your bills.

To find out which distribution network you are in, you can look on the back of your electricity bill or go to the Electricity Distributors section of Victoria’s Energy website.

The four Victorian distribution network web portals are:

Energy advice businesses may also use your smart meter data to give you advice about your usage, or to help you plan a solar PV system.

It’s worth also reviewing your gas bills. If your gas use is high then, over time, inefficient gas appliances can be replaced with modern electric ones to make the best use of your solar electricity system, but this means your electricity consumption will be higher.

When planning a solar electricity system for the home it’s useful to know how much energy is used by certain appliances as well. You can use this information to consider replacing older ‘energy guzzling’ appliances, right through to what time of day you might use these appliances, to make the most of the solar electricity generated.

You can use an energy meter such as a PowerMate Lite or Reduction Revolution Plug-in Power Meter to measure the electricity used by certain appliances. Likely problem appliances include old fridges and freezers, older washing machines (especially top loaders if you do warm washes), portable electric heaters such as bar radiators, fan heaters, oil-filled column heaters and panel heaters, older air conditioners, electric hot water systems and pool pumps, which all have more energy efficient alternatives or energy efficient versions on the market now. It’s important to use a good quality energy meter or your readings could be inaccurate, particularly when measuring certain types of loads such as the power supplies in modern electronics.

Examples of household energy culprits and energy use

Halogen downlights are one of the biggest power wasters in the home because they convert nearly all of the energy they use into heat, not light. Replacing 50 watt halogen light bulbs with 10 watt LED bulbs can equate to 80 per cent less energy use for lighting.

This example applies to many appliances in the home and spending a few hundred or even a couple of thousand dollars on more efficient appliances could save you a great deal more than this in energy costs.

As an example, you might monitor your fridge for a week using an energy meter and find that it’s using 4 kWh per day. This is a lot for a domestic fridge, so it might need to be repaired (it might just have damaged door seals or need regassing), cleaned (it might be full of dust at the back) or replaced.

For loads that you can’t monitor directly with an energy meter, such as ceiling lighting and fans, energy use can be worked out with a simple calculation. For example, a 50 watt light used for four hours per day will use 200 watt-hours per day or around 1.4 kWh per week.

Improving energy and thermal efficiency at home

Efficient electric appliances

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.

Make your home thermally efficient to save on running costs

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.

Case study: Five in the house and a power bill of $30 a month…it could be you

Annelies Brandjes is getting power bills of $30 a month in her new home ... Her father, Jan, who has been working on energy efficient buildings for more than 36 years in Victoria and northern Canada’s freezing Yukon region knew adding solar panels was just part of the way to cut energy costs and make the house more comfortable.

Read more

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.

Sealing doors

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

Seal fans and chimneys when not in use

Fitting a DraftStoppa  to your exhaust fans helps seal the home from the ceiling space. These work by using the force of the fan to lift covers up, with gravity keeping things sealed otherwise. You can also buy replacement exhaust fans that are designed to block the opening when not in use. Also, remember to block off chimneys when not in use with a chimney draught-stopper product such as a Chimney Sheep or ecoMaster Chimney Draught Stopper.

Seal off skylights

This can be as simple as fitting a sheet of clear plastic at the bottom of the skylight shaft, or you can employ professional solutions.

Consider pelmets

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

Downlights

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.

Planning an all-electric home

Some households might plan to switch off their gas supply entirely and go all-electric at home. This saves on gas supply charges and means that more appliances in the home can use solar-generated electricity. An all-electric home benefits if the solar panel (PV) system is sized appropriately to cover most electricity use, although any shortfall can be made up with electricity from the grid. All-electric homeowners pay careful attention to efficiency, including the selection of efficient electric appliances such as reverse-cycle heating and cooling and heat pump hot water systems. 

A battery system can further reduce electricity bills by storing excess solar to run electric appliances at night, but before taking this step you should figure out whether the cost of buying and installing the battery, and the foregone feed-in-tariffs, will be lower than the money you will save on bills. If going all electric is something that interests you, visit Renew’s Efficient Electric Homes to find out more.

Working out the right-sized system for your home

This is where you take all the household energy use information gathered, as well as any plans you have to go all-electric to work out what size system suits your needs.

The bigger is better approach

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. Renew has a free online tool that can help with this, or you can get targeted advice via a full solar energy consultation. You can also check Solar Choice’s Residential Solar System Prices guide, updated every few months.

Solar Savvy Tip: Electricity distributors may limit the size of solar systems connected into their grid. Confirm the process for larger systems with your distributor.

Constraints on big solar systems

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.

Smaller systems designed to match energy use

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.

Siting considerations

The company you engage for your solar purchase should do an on-site analysis to ensure your site is suitable for solar panels. The following advice will help you get the most out of your solar installation.

Which direction?

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.

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.

Minimal shading

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.

Getting the pitch angle right

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.

Reviewed 03 March 2021