Solar Panel with a higher power output than the inverter.

DC Over-Sizing PV Solar in Ireland

Why Use DC Over-Sizing?

There are two main components in any grid-tied solar panel system; the (DC) panels and the (DC-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:

  • Save money with a smaller/less expensive inverter
  • Regulations may limit the size of inverter that you’re allowed to install

Is DC Over-Sizing a Good Idea?

DC over-sizing can help you get the very best price/performance ratio from your PV solar system. In fact, multi-million euro solar farms use substantial amounts of DC over-sizing as standard. So let’s take a look at the factors that will determine if DC over-sizing is right for you. First though, we need to understand what solar panel and inverter power ratings mean:

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.

DC Over-SizingEstimated Clipping Loss
0<.1%
10%<1%
20%<2%
30%3%
40%4%
50%6%
60%8%
70%9%
80%11%
90%13%
100%14%
200%29%
400%47%
Estimated clipping losses in Ireland. Note this example assumes a solar PV array facing due South with a 35 degree tilt. Losses will be smaller for arrays facing other directions. License: CC-BY. Credit: energyd.ie

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.

Graph of solar PV power with Dc over-sizing.  The output graph shows clipping and converted energy.  The graph demonstrates that clipping losses are relatively small and come at times when there is a surplus of solar PV energy available.  The graph is for the simulated output of a solar PV system in Dublin, Ireland, on the 26th of July 2005 (a day with unusually high clipping).
Clipping tends to occur most in the middle of bright days, at times where there’s a surplus of power available anyway. Image License: CC-BY. Credit: energyd.ie

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. Enphase is a good example of this. Likewise, SolarEdge allows over-sizing up to 100% depending on the model. 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.

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.

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