When researching solar energy, you may have heard the phrase “voltage rise”. This is quite a simple concept, but with important implications for you and your system if not considered properly.
Before you read this, I recommend you read my Electricity Basics article to understand Ohm’s Law and learn more about voltage.
Voltage rise is the difference between two different voltages – for example, if the grid is 230V, and your inverter is 235V, there is a 5V voltage rise. Alternatively, there’s a 5V voltage drop from the inverter to the grid.
For electricity to flow from the inverter to the grid (to export excess solar energy generated), there must be a small voltage rise from the grid to the inverter, to “push” the energy from your inverter to the grid. According to Australian regulations (specifically AS/NZS4777), your inverter should be a maximum of 2% higher than the nominal grid voltage. In Australia, this is 230V – 2% of 230V is 4.6V – that is, a 4.6V “voltage rise” from the grid and the inverter.
So how do you ensure this is the case? By using Ohm’s Law, V=IR. If you have a single-phase 5kW inverter on your solar system, it will generate a current of ~21.7A (5000W / 230V = 21.7A). This means, to ensure the voltage is no more than 4.6V, the resistance between the grid and your inverter must be a maximum of ~0.21Ω (21.7 x 0.21 ≤ 4.6).
How do you control resistance? In our metaphor in Electricity Basics, we used pipes to contain the water. In electricity, we use wires (cables). Without going into too much detail, there are three ways to adjust the resistance of the cable: alter the thickness/diameter of the cable (much like the water pipe analogy, thicker/wider has lower resistance); use a different material (gold has a lower resistance than aluminium, for example); or change the length of cable (longer has more resistance).
So, if the goal is to have a low resistance, why not just go for a short, thick, gold cable? Firstly, money! A thicker cable costs more money, and of course gold isn’t the cheapest metal. Secondly, it’s also a lot more difficult to work with, as they have different bending radii. And thirdly, it may not be possible to move your inverter right next to your switchboard – you may need to use a long cable. This is why it’s important to use a knowledgeable electrician or engineer who can calculate the correct cable for your system.
Another related concept is grid voltage. Remember, the inverter must be at a higher voltage than the grid to export electricity. I said before, the grid nominal voltage in Australia is 230V, but that is more of a target than a reality; sometimes the grid voltage can rise to 240V, or even 250V! The problem with this is that high voltage can damage electronics. If the inverter detects an excessively high voltage (e.g. 253V), it will down-regulate its output power, or, in extreme cases (e.g. 260V), turn off. This doesn’t just mean you can’t export electricity to the grid, it means your entire solar system effectively shuts down, meaning you can’t even consume your own solar energy. You can mitigate this slightly by having as minimal voltage rise as possible – if the grid is 250V, a 1% voltage rise of 2.5V would not down-regulate the output (the inverter would see 252.5V, <253V), but a 2% voltage rise of 5V would (the inverter would see 255V, >253V).
The other thing we can do is support the power networks, to upgrade their infrastructure and support modern electricity technologies.