04/02/2022
10 steps to testing solar panel output and how to avoid disappointment.
We've often seen posts on various groups where solar panel users question why their panel is only outputting a fraction of its rated power. Whilst there are many reasons this may occur, including poor quality equipment, the test conditions and setup are often a significant reason of a poor result. Here are a few tips how to ensure you get an accurate test.
1. Test under STC (standard test conditions) or close to as possible - Technically this is the solar cell (not environment) at 25 degrees Celsius and in an area of solar irradiance of 1000w per sqm. Realistically these conditions are difficult if not impossible to achieve 99% of the time. Practically speaking point your panels directly at the sun and conduct your test during the middle of a completely clear day. A test we recently ran found the exact same setup in full sun, at the same temperature produced 185w at midday and only 140w at 4pm.
2. Have all your equipment out and ready to go before you put your panels out in the sun. This is to prevent heat soak which will begin as soon as the panels are in the sun. Once the cells are above 25 deg c they will be above standard testing temperature and you can expect a drop in output.
3. Use a quality MPPT solar controller. If you have a PWM controller or a poor quality MPPT controller you can expect a lower test output.
4. IMPORTANT - Ensure that the battery that you connect to is NOT fully charged (ideally less than 50%). If you battery is close to being fully charged your solar controller may taper off the current going to the battery and as such you will get a lower reading. Think of it as a water tank. If it's close to full and you want to pour water in but not spill a drop, you'll pour more slowly to prevent a spill. If it's already full you'll stop pouring water!
5. Connect a load to the battery if possible. If you connect a load to the battery that draws more current than the panels you are testing, you can prevent the issue highlighted in step 4.
6. Ensure your solar controller is set to the correct battery type and is in it's bulk phase of charging.
7. Use a good quality shunt to measure the power output. Whilst many solar controllers will display the power output via a Bluetooth app or a LCD screen on the controller, a high quality shunt will help get an accurate reading.
8. Be patient. Some solar controllers take a short period of time to start charging and will slowly ramp up charging current. This may take from a few seconds to a few minutes.
9. Expect variations. In our testing in the real world, if on a good day you can get 80% of the panel rating into a battery as a charge current that's pretty good. By the time you take into consideration conversion inefficiencies, temperature, solar irradiance, cable runs, connections, item quality etc etc etc it is unlikely that you will get a test result that matches the panel rated output.
10. Before running your test (if you really want to nerd out) check the forecast solar irradiance in your local area. There are a few websites online that have this data available however we use solocast https://toolkit.solcast.com.au/live-forecast . This does require creation of an account (pretty sure it's free). This data will help set your baseline expectations. ie if the forecast solar irradiance is only 600w/m2 then there is no way a panel that is rated at 100w under STC will produce 100W when you test it.
Last word - Having a realistic expectation of your solar system and sizing appropriately is probably the best way to avoid disappointment with your setup.
