What are ‘peak sun hours’?
A peak sun hour doesn't include just any hour when the sun is out in the sky. Instead, it refers to an hour in which your solar panels produce a certain amount of energy.
Each peak sun hour is defined as one hour when the intensity of sunlight (solar irradiance) reaches an average of 1,000 watts of energy per square meter (roughly 10.5 feet).
Summer months and locations farther south will generally see more peak sun hours than wintry times and areas farther north. That's because regions closer to the equator are also closer to the sun.
On the other hand, in latitudes farther north, when the sun is closer to the horizon, the sunlight is filtered through more layers of the atmosphere. In those places, the sunlight isn’t as strong by the time it reaches your solar panels, which results in lower peak sun hours.
Peak sun hours also vary depending on where your solar panels are placed. In other words, the peak sun hours' value for one part of the house might be different than its value for another part of the house. These factors can affect the number of peak sun hours:
Direction: The direction your solar panels are facing affects the intensity of the sunlight, as well as the total amount of sunlight received.
Shading: Even if all the solar panels face the same direction, they might experience sun hours at different times based on when a tree (or other obstruction) casts shade on them.
How do we calculate total peak sun hours?
One peak sun hour = 1000 W/m2 of sunlight. However, when calculating the total amount of peak sun hours received at any location, you don’t just consider hours with 1000 W/m2 of solar radiation. Instead, you need to add the total amount of solar irradiance received by the location. You then express that in terms of the equivalent number of hours with 1000 W/m2.
It may sound complicated, but the concept is actually relatively simple to apply. For example, if a given location receives a total of 4,500 Wh/m2 of solar radiation over the course of a day, then that location gets 4.5 peak sun hours.
Regular sunlight hours refer to any time the sun is shining during the day - the hours between sunrise and sunset. However, it doesn’t tell us anything about the strength/intensity of the sun's radiation during those hours, and thus, is unhelpful when it comes to designing a solar panel system.
That is why the concept of 'peak sun hours' has been developed. It precisely measures the amount of irradiance that will hit the solar panels, thereby allowing us to calculate the expected electricity generation.
We know that during peak sun hours, a solar panel should theoretically produce 1,000 W /m2. But, solar panels do not work at 100% efficiency. All solar panels these days come with a Standard Test Conditions (STC) rating, which is the amount of energy those panels can actually produce during peak sun hours.
What is a solar photovoltaic (PV) system?
A solar photovoltaic (PV) system, often referred to as solar panels or solar power, generates renewable electricity by converting energy from the sun. The solar panels generally sit on a house or shed roof facing north so that they get good access to the sun, though sometimes panels are installed to face in other directions, if there is limited roof-space facing north or limited northerly solar access. Some west-facing PV panels can also be useful, as they generate more electricity on a summer afternoon, when you might be using an air conditioner.
A solar PV system is different from a solar hot water system, which uses the sun's energy to heat water rather than generate electricity.
Types of solar photovoltaic (PV) systems
In Australia the solar photovoltaic panels are usually connected to the electricity grid and generate DC (direct current) electricity. A device called an inverter is used to convert this DC electricity into the 240-volt AC (alternating current) electricity which is required to run the electrical appliances in your home.
The electricity generated by the PV system is delivered directly to your home for consumption and any excess electricity is exported to the electricity grid. Your house will draw electricity from the grid when it is night or when your PV solar system cannot generate enough electricity to meet your consumption.
It is also possible to install solar PV systems with battery systems but these are currently much less common than grid-connected systems. Solar PV systems with batteries allow for storage of PV-generated electricity to use at night or at times of low sunshine, and in some cases allow electrical appliances to operate during power outages. If the battery installed is large enough it is possible for a house to completely disconnect from the electricity grid, although these systems usually also rely on a diesel generator back-up and are costly to install.
Benefits of solar PV systems
Installing a solar PV system will allow you to generate renewable energy and reduce your electricity bills. Some of the electricity you generate will be used in your home to power electrical appliances and some will be exported back to the electricity grid.
In Victoria a typical house consumes an average of around 12 kilowatt hours of electricity per day. Over one year, a 1.5 to 3 kilowatt solar PV system can generate around 45–90% of this, though the amount generated by the system varies throughout the year as the amount of daily sunshine changes.
The amount of the PV generated electricity used in your home will depend on the size of your PV system, how much electricity you use, and how much of this you consume during the day. The higher your daytime consumption, the higher amount of PV generated electricity you will use. Typically around 30 to 50% of the electricity generated by a PV system is consumed in the home and 70 to 50% is exported to the grid.
This reduces your annual electricity bill:
by reducing the amount of mains electricity you draw from the grid (and how much you are therefore billed for this by your retailer)
because you are paid for the electricity you export to the grid (see feed-in tariff below).
How much will you save?
The annual output of a solar PV system depends on a range of factors including its size (or rated output in kilowatts), orientation, and your location – there is more sunshine in northern Victoria, so the annual output of the PV system is larger. The typical annual output for common system sizes in Melbourne and Mildura are shown below: