Sizing your solar panel system


Figuring out the right size of solar PV system to suit your home's energy needs

Catching the sun


  • To size your solar panel system you need to work out how much electricity you use and when you use it
  • A typical home uses 20kWh per day, which equates to a 5kW system
  • The number of panels is irrelevant, it's about the system's overall capacity 
  • Feed-in tariffs for new solar systems are very low, and won't justify the cost of a larger system

If you're thinking of going solar, then you need to know what size of solar system you'll need to run your home (as much as reasonably possible) on solar power.

The size or capacity of a solar photovoltaic (PV) system is the maximum electricity output the system can deliver. But let's clear something up: this isn't about the number of solar panels, it's about the overall capacity of the system. Your system might have 20 x 250W panels, or 25 x 200W panels; in either case it's a 5000W (5kW) system and that's the number that really matters.

This article explains why you need to understand your electricity usage before you can calculate the size of your solar PV system, why roof orientation matters, and reveals the pros and cons of battery storage.

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Understanding your electricity usage

You can't correctly size your solar PV system unless you know how much electricity your home uses. The easiest way to figure this out is to look at past electricity bills, which should tell you how much power you've used in the previous month or quarter. From this you can figure out the average daily usage. This is even easier if you have a smart meter installed; you should be able to see your daily usage either on the bill or by checking your account online.

Your power consumption is measured and billed in kilowatt-hours (kWh).

A typical Australian home uses 15–20kWh per day. But households can vary considerably in their usage; a single-person home will typically use about 8–9kWh per day on average, while a household of five people with a pool could use 33kWh per day.

Time of day and seasonal usage

It's important to consider when you use electricity. Is your home generally empty during weekdays, with everyone at work or school, so that your main power consumption comes in the evening? If so, your solar panels might not be used most effectively, as it's better to use the generated power during the day (or use it to charge a storage battery) than export it to the grid. 

Also consider whether some days are more power-hungry that others; the weekend for instance, when everyone is at home. And do you use more power in summer (running air conditioners), or in winter (running heaters)?

Put all this together and you should have a good understanding of how much power you usually use each day, how much you use on peak days, and the times of day you use most power.

Calculating the size of your solar PV system

Now you know how much power you typically use and the times of day you use it. What capacity will your solar PV system need to cover your power usage?

First, we're assuming you'll have a grid-connected system. This is by far the most common type and it simply means you have solar panels generating electricity during the day, and a grid connection to supply electricity when the solar panels aren't generating enough (at night, for example). See grid-connected vs off-grid for more.

How much electricity you can expect per kW of solar panels?

Solar PV systems are rated in watts (W) or kilowatts (kW). You'll see systems described as 4kW, 5kW, 10kW and so on. (See terminology for the difference between a kilowatt – how the solar PV system is rated – and a kilowatt-hour, the unit by which your consumption is measured and billed.)

1kW of solar panels = 4kWh of electricity produced per day (roughly)

For each kW of solar panels, you can expect about four kWh per day of electricity generation. So a 5kW solar system will generate about 20kWh on a good day (which means plenty of sunshine but not too hot).

It's just a general rule; the actual amount of electricity generated per kW of solar panels depends on your location, the time of year and the amount of sunlight you're getting, the orientation of the panels, how old they are, and so on. In southern regions such as Hobart it could be as low as 3.5kWh per day, while the same 1kW of panels in Darwin could generate 5kWh.

How big should your solar PV system be?

You should aim to cover as much of your household consumption as reasonably possible for a typical day. If your power consumption is (say) 30kWh on some days, but on most days it's 20kWh, it might not be worth adding extra panels just to cover those few 30kWh days. A 5kW solar PV system might be the most cost-effective option, and you'll just have to accept paying for more power than usual from the grid on those occasional high-consumption days.

But solar panels are relatively cheap now, so it's worth talking this through with your installer to see if the sums make sense for a larger system. The larger system might be useful if you want to add a battery, take full advantage of your inverter's capacity, or simply to generate more power throughout the day.

You might think it's better to oversize your system because any excess will be exported to the grid, and you'll be paid for it via the feed-in tariff. But feed-in tariffs for new solar PV systems are generally very low – typically from seven to 12 cents per kWh – which is unlikely to justify the cost of a larger system.

Power usage shifting

Since you're looking at saving on power costs by installing solar, it makes sense to maximise your use of that solar power. So as much possible, your electricity consumption should happen during the day when the panels are generating. Likewise, minimise your power consumption at night. Night-time power is going to come from the grid – which is relatively expensive. Alternatively, night-time usage will come from your storage battery if you have one, and you won't want to drain that any faster than you need to.

So consider running your dishwasher and washing machine during the daytime, using a timer or "delay start" function if they have one. Likewise, try to use air conditioners and heaters during daylight, and again consider using timer functions; this can reduce the amount you need to run them during the evening.

Online calculators

Online "solar calculators" can help you work out the size of solar system you need. And while we don't endorse any in particular, they're worth a look. However, some solar calculators focus on aspects other than system sizing, such as payback times, cost of finance and so on; all potentially useful but it might not be the information you're looking for.

Grid-connected vs off-grid

Grid-connected

By far the most common type in Australia, these systems have solar panels and an inverter, and are connected to the main electricity grid. The solar panels supply power during the day, and the home generally uses the solar power first before resorting to electricity from the grid. The grid connection is used to supply power at night (assuming there's no storage battery connected) and at other times when the solar panels can't generate enough power, such as on low-sunlight days.

Off-grid

This type of system is completely standalone from the main grid. All the home's power comes from solar panels, and possibly some other types of power generation as well, such as wind. These systems almost always use storage batteries to capture unused power from the solar array, for use at night and on low-sunlight days. They often also have a diesel-powered generator for back-up in extended periods of low sunlight and when there's a sudden high demand for power (such as when a pump starts up).

Off-grid systems are usually more complex and expensive than grid-connected systems. They need more solar capacity than a typical grid-connected system, and may also need inverters capable of higher loads to cope with peak demands. Homes that run off-grid need to be particularly energy-efficient and the load demand needs to be well-managed throughout the day.

Off-grid systems generally only make sense for remote properties where a grid connection isn't available or would be prohibitively expensive to install. They should be designed and installed by a supplier with particular expertise in this type of system.

Installation considerations

Roof space

Most freestanding houses will have enough roof area to support however many panels the home needs. Factors that might reduce your available roof area include heavily shaded sections and roofs with unusual pitch. Solar panels are mounted on brackets to ensure correct angling and air circulation, so installers can usually find a way to make most roof spaces work well.

Orientation

It's usually best to have the panels facing north, to maximise the amount of sunlight that falls on them. But that's not always possible and it's not essential. North-east or north-west are often just as good. Your installer should be able to work out the best orientation for your panels given your location, roof space and household needs.

Sometimes a mix of east- and west-facing panels can work best; this may give a slightly lower amount of power generation in the middle of the day, but will produce more in the morning and late afternoon compared to a north-facing array. If you tend to use more power at those times, this orientation might make more sense.

And don't despair if your only available roof space faces south – south-facing panels can still produce about 80% of their rated power. 

Plus, if you already have north-facing panels, you can always expand your solar PV, or add a separate system, on the southerly aspect. Solar panels are cheap enough that this can make economic sense, but you may want to put on a few more panels in the south-facing array to make up for the reduced production.

Inverters

The inverter is a key part of the solar PV system; it's the box on the wall that takes the electricity generated by the solar panels in direct current (DC) and converts it to alternating current (AC) for your household circuits to use in powering your fridge, TV, lights and so on.

The inverter size must match the solar PV array's size; basically, if you have 5kW of panels on the roof, you'll need a 5kW inverter as well. But note that the panels rarely if ever deliver their maximum rated output, due to variable sunlight conditions, loss of efficiency as the panels age, reduced efficiency in extreme heat and so on. So you can actually get away with a slightly lower rating on the inverter compared to the solar PV array (this is sometimes referred to as oversizing the array or overclocking the inverter). But in general, it's best to match the inverter to the solar PV array's rated maximum output.

An alternative to a single inverter unit is to have micro-inverters, where each panel has its own small inverter attached. These are usually more expensive, and have some technical advantages (and disadvantages). Inverters are complicated and we'll cover them in detail in a future buying guide.

What about a battery?

A storage battery will capture the unused solar power generated during the day, for use at night and on low-sunlight days. Installations that include batteries are increasingly popular. Here's a case study of the first Australian home to install a Tesla PowerWall battery.

But for most homes, we think a battery doesn't make economic sense yet. Batteries are still relatively expensive and the payback time will often be longer than the warranty period of the battery. The good news is that battery prices are falling rapidly and in two or three years it will probably be the right decision to include a storage battery with any solar PV system.

The combination of solar and battery is unlikely to meet all your power needs throughout the year; on most days, especially in periods of low sunlight, you'll still need to draw some power from the grid. Even off-grid systems still usually rely on a diesel generator from time to time.

And remember: for most grid-connected systems, having a battery doesn't necessarily protect you in the event of a blackout. You may still lose all power to your home, despite having solar panels producing power and a charged battery ready and waiting. This is because grid-connected systems have what's known as "anti-islanding protection". During a blackout, the grid and any engineers working on the lines must be protected from "islands" of electricity generation (such as your solar panels) pumping power unexpectedly into the lines. For most solar PV systems, the simplest way to provide anti-islanding protection is to shut down entirely. So, when it senses a grid blackout, your solar PV system shuts down and you have no household power at all.

More sophisticated inverters can provide anti-islanding protection during a blackout, but still keep the solar panels and battery operating so that the house has some power. But expect to pay a fair bit more for such a system, as the hardware is more expensive and you may need more solar and battery capacity than you think to run the house for a few hours during a blackout. You might choose to allow only critical household circuits to operate in that situation, such as the fridge and lighting. That could mean extra wiring work is needed. Be sure to discuss this up front with your installer so they can design and quote the right type of system for you.

Talk to installers

It's not always easy to calculate exactly how much capacity you'll need for your solar panels, or how much you'll actually be able to fit on your roof. So while this article will help you do your homework, in the end you should still talk to at least a couple of solar installers to get a detailed quote.

A good installer will work with you to figure out your home's power usage and the right sort of solar system to suit both your power needs and the roof space you have available.

Terminology

Watt (W) and kilowatt (kW): a unit used to quantify the rate of energy transfer. One kilowatt = 1000 watts. With solar panels, the rating in watts specifies the maximum power the panel can deliver at any point in time. For more details, see Wikipedia

Watt-hours (Wh) and kilowatt-hours (kWh): a measure of energy production or consumption over time. The kilowatt-hour (kWh) is the unit you'll see on your electricity bill, because you're billed for your electricity usage over time. A solar panel producing 300W for one hour would deliver 300Wh (or 0.3kWh) of energy.

Our solar panel buying guide explains how solar panels and PV systems work and offers advice about warranties, choosing an installer and more. 


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