How to Check Solar Output?

Solar power is measured in capacity using the term watts (W) or kilowatts (kW). Its power output (in real time) is measured in several ways the most useful been W and kW.

These two factors are technical considerations and do not relate to actual metered output which is measured in kW hrs (in most cases). Kilowatt hours are a measurement of power over time giving a real indication of what has gone out. Remembering that the kW hr measured on the inverter will not necessarily end the same as the kW hr number on the bill due to home power usage.

Why is this important? Because as a rather large investment it is good to be able to prove normal operation by glancing at a screen or bill and ensure that everything is still returning as normal.

The best way to determine actual solar output is two fold
1 Look for the daily total – sometimes called E Total and check its reading around dusk. A 5kW system in SEQ will produce 12-28kWhr per day depending on the weather and will return an average of 20kWhr per day.

2 Look at the overall total and divide it by the number of days installed. This check is worth doing once per quarter or year to determine that averages are been met as its quite difficult to gauge this off the day to day readings.

If you have a different size system the average daily production per kW installed is 4kWhr. So a 3kW x 4 = 12kWhr per day average across Australia.

To determine its output on the bill is difficult as it doesn’t show power used from the solar. But if you look at the total kW hr produced by the inverter over the billing period and minus the export amount shown on the bill that will provide an understanding of usage from solar vs usage vs export.

All this export usage is complicated cant I just let it run?
Yes but it will be far from the best benefit. There are two ways too run a import export solar system. If one has a high FIT it is best to schedule as much power usage as possible outside of the solar production primary hours (10-3pm ish). If one has a low ie the new 8c FIT then the best method of operation is to use power during those hours. This means shifting the operation time of things like dishwashers, washing machines, pool pumps, storage hot water systems and so on.

My inverter never peaks but I have the same or more panels?
This is a very common concern but has very little meaning in terms of production or return. But generally speaking it is very rare for a kW to kW matched system to reach maximum output due to losses in the design – many of which cannot be avoided. For example dirt alone can account for 10-20% – temperature will also have a noticeable effect as panels are tested at 25 degrees Celsius. Pitch and orientation is another factor as is internal losses in the inverter and cabling.

Most of these factors there is little anyone can do to change them beyond better designs (cabling) and higher quality components (panels and inverters) however dirt and temperature vary with conditions and have a dramatic role to play in output.

Solar panels run at around 50 degrees on the roof and are very hot to touch in full summer sun. Once they reach 25 degrees they loose 0.4 or so percent of output for every extra degree of temperature. As an example at 50 degrees a 250W panel will loose around 10% of its output therefore making it a 225W panel. Temperature also comes into play with inverter output as most inverters will drop their output during high temperature conditions for protection reasons. In this case a 5kW inverter can become a 3.5kW inverter. For this reason the highest output of a solar system may not be in the middle of summer and elevated locations with good airflow will achieve higher readings. Inverters should not be installed in low airflow environments and panels should be installed with a good air gap between and under rows. All those panels crammed up to look like one big sail? It might look cool but its actually bad design.

Shading is a Dirty Word
We mention these in the same paragraph because dirt will have the same effect as shade and shade is the dirty word in the solar industry. For every few hundred systems out there that are working exceptionally well there are some that have been installed in locations which will result in terrible output and a negative return on investment!

Basically speaking when a solar panel is shaded by whatever be it dirt, a cloud, a TV aerial, a tree or a power line the shaded component will become an electrical load and will draw power from the system thereby dramatically and in many cases logarithmic-ally reducing the output of the solar system. In very simple terms if you shade one you will loose two – if you shade two you will loose four and so on.

For these reasons it is important to place solar panels in locations that are not shaded and not exposed to large amounts of dirt – for example installing solar panels next to a coal mine will not produce high output and panels may need to be cleaned after say construction was completed across the road and so on (higher amount of dust moving through the air).

Cloud Edge Effects
From a dirty word to the holy grail of solar production – the cloud edge effect. This is the point at which panels which may have been cooled through rain and cloud are suddenly exposed to full sun after a middle of the day storm. It is generally the point during which a system will reach its highest output especially if its during the summer months.