DC Over-Sizing PV Solar in Ireland

Why Use DC Over-Sizing?

There are two main components in any solar panel system; the (DC) panels and the (AC) inverter. Conventional wisdom states that the inverter should be rated for the same power output as the panels. However, it’s becoming more and more common to have a higher power rating for the panels than the inverter. This is called “DC over-sizing”. Other names for this technique include “inverter over-clocking” and “using a high DC-AC ratio”. Whatever you want to call it, there are two main reasons to use DC over-sizing:

Reason #1 for DC Over-Sizing: Save Money With a Smaller Inverter

Let’s say you want to choose an inverter for your 5 kW of solar panels. Conventional wisdom might suggest you use a 5 kW inverter, or possibly even larger. However, you could save money by using a 4 kW, or even 3 kW inverter. Sound sketchy? Actually, as long as the inverter is specified to take 5kW or more of input power, then it can be totally fine to do so. What about the lost power? Well, it’s far less than you might think (more on that later).

The amount of money you’ll save by dropping an inverter size or two won’t be dramatic. But consider this: you may be able to buy a high-end 3 kW inverter for the same price as a mid-range 5 kW inverter. And the high-end model is likely to serve you much better in the long run.

There may also be additional savings when it comes to the electrical installation. This is because cable thickness requirements will be more relaxed as the inverter size decreases. Required AC isolator sizes will likewise be smaller.

Reason #2 for DC Over-Sizing: Grid Regulations on Inverter Size

The second reason for DC over-sizing is grid regulations. In particular, ESB Networks generally limit Irish homeowners are generally limited to 6 kW of total inverter power. This can be a single 6 kW inverter, or a pair of 3 kW inverters, for example. Either way, DC over-sizing is the only way for most Irish homeowners to install more than 6 kW of solar panels. Luckily, some inverters, such as the SMA Sunny Boy 3.0, allow for up to 100% DC over-sizing. These inverters thus allow for up to 12 kW of solar on an Irish rooftop.

Is DC Over-Sizing a Good Idea?

DC over-sizing is widely used on small and large scale solar installations. But if you’re still feeling a little unsure about the idea, that’s natural. After all, DC over-sizing is counter-intuitive on the surface. But dig a little deeper, and it really starts to make sense. I’ll explain in a moment. First though, we need to take a very brief and easy look at solar panel and inverter power ratings:

Solar Panel Power Ratings

Solar panels are rated based on their power output under standardized conditions in a lab. This is a great for making accurate and fair comparisons between different brands of panels. On the hand though, the power output of solar panels varies all the time when they’re in real world action. So for example, a 400 Watt solar panel could actually deliver 100 Watts, 200 Watts, 400 Watts, or even 500 Watts at any given moment. This number depends mostly on light intensity and temperature; bright light and low temperatures are best.

Solar Inverter Power Ratings

In contrast to solar panels, inverters are generally rated based on their maximum power output capacity. So for example, if your solar panels are producing 3000 Watts of power and you connect them to a 2000 Watt inverter then the inverter will give you a power output of 2000 Watts. This means that you’ll be “losing” 1000 Watts of power that could have been produced given a larger inverter. This situation is called “clipping” and it’s the main reason why some people are reluctant to use DC over-sizing:

How Big Are Clipping Losses in Ireland?

You’ll need to take a couple of factors into account when estimating clipping losses. Firstly, you’ll need to consider is how much you’re over-sizing by. For example, a system with 200% DC over-sizing will have much greater clipping losses than a system with 5% over-sizing. Another factor is where in the world your solar panels will be located. Generally, the further you are from the equator, the less severe your clipping losses will be. See below a table of estimated clipping losses in Ireland depending on DC:AC ratio.

It’s Often Worth Tolerating Some Clipping Losses

As the table shows, you can use a substantial amount of DC over-sizing without much drop in annual power production. This can help you design solar PV systems for maximum price/performance. Better still though, it means that you can go well beyond the Irish 6 kW limit on single-phase inverter power output. For example, if you install 9 kW of panels with your 6 kW inverter, clipping losses will be only 6%. Or in other words, your system would be as productive as one with 8.5 kW of panels and an 8.5 kW inverter!

Clipping Losses Come At Good Times

Another point to consider with DC over-sizing is that clipping only occurs when your solar panels are producing a particularly high power output. Under these circumstances you’re likely to have surplus power anyway. Better still, DC over-sizing means your inverter will be closer to its efficiency sweet-spot under low-light conditions when you need every ounce of power that the inverter can give you. Overall, the generation profile of a DC over-sized system tends to be very suitable for matching power demand.

DC Over-Sizing and East-West Panel Orientation

You can push DC over-sizing even further if your panels are not facing due South. In this case, it will be less common for the panels to produce spikes of very high power output. For example, if you have an PV solar system where half of the panels face East and the other half face West, then you will never have a situation when all of the panels are receiving maximum sunlight all at once. This means that you can include a certain amount of DC over-sizing without any significant clipping losses.

When Not to Use DC Over-Sizing

While DC over-sizing can be a good idea, there are times when it’s better not used. One example is when you want to maximise solar energy output but you’re limited by roof space. In this case it makes sense to use a large inverter so that you can squeeze every Watt of power from the limited number of panels that you have space for.

AC Over-Sizing to Leave Roof For Future Expansion

This is another time when you might want to avoid DC over-sizing. You may want to avoid DC over-sizing now so you can easily expand your solar PV system in the future. In fact, you might even want to do the opposite by going for AC over-sizing. An inverter with extra AC capacity will let you simply “plug in” more solar PV panels at a later date.

Won’t DC Over-Sizing Destroy My Inverter?

In general, inverters are limited by voltage and current, not by total power. These limits often allow substantial room for DC over-sizing. In fact, some inverter manufacturers specifically allow or even encourage DC over-sizing. For example, some SMA allow up to 100% over-sizing. If in doubt, check the inverter datasheet.

Even if DC over-sizing is safe and allowable, it still makes your inverter work harder. This could have some impact on inverter lifespan. On the other hand, quality is key to inverter lifespan. If your inverter comes from a good brand then it should last many years, DC over-sizing or not. Second-tier inverters are much more likely to give trouble, and DC over-sizing will only make this problem worse.

The table outlines the DC-oversizing specifications of selected solar inverters. Note, you should check with the manufacturer as these values are subject to change.

Conclusion

DC over-sizing can boost the price/performance ratio of solar panel systems in some cases. Moreover, it can help you to go well beyond the 6 kW (AC) limit for many single-phase solar panel systems in Ireland. Just make sure to pay close attention to the inverter datasheet and use a reputable brand of inverter.