Solar clipping happens when solar electric (photovoltaic) panels provide more power than an inverter can handle. Clipping is lost power production, but there may be some advantages to it. We'll explain, and talk about some pros and cons.
In solar PV systems, solar electric panels generate DC electricity. Most homes use AC electricity. The inverter converts DC electricity to AC electricity, and has a limited AC capacity, typically between 3 and 7.6 kW.
A typical DC to AC ratio (array to inverter) is around 1.1 -1.2 to 1, i.e., the array is slightly larger than the inverter capacity. This means that DC power from the array is maxed out on a bright sunny day, there is energy lost because the inverter is not capable of converting all the DC power into AC power.
In the picture below, solar clipping occurs between 12:00 and 13:50 (noon and 1:50pm). (Notice the “flat top” in the middle of the day.)
How frequently does clipping occur?
In a properly designed system, clipping occurs for a only a small percent of each sunny day, so energy lost due to clipping is minimal.
In our examples, it occurs in 8.8% of sunny days. Clipping depends on your location (latitude) and the size of your solar panel array compared to the AC output of your inverter.
Clipping Examples over 30 days
In the picture below, you can see that our system reaches 7.6kw six times over 30 days in November and December. Here in Wisconsin, November and December historically have low levels of solar insolation. (There are not many sunny days, and the days are short.)
Solar clipping loss occurs on 6 of 30 days in the graph. This means that 20% of the days saw clipping, but only for a short period of time. In spring and summer, clipping will take place for longer periods.
Does solar clipping damage the system?
No, clipping doesn’t damage the solar array or inverter. The electricity gained at the beginning and end of the day generally outweighs the clipping losses.
Clipping doesn't damage your battery storage, if you have battery backup.
How much solar clipping is normal?
Clipping is not “normal”. Some systems may clip and others may not. It's simply the difference between how much solar power you have available and how much you can process.
I know it may sound obvious given what we've shared in this post, but we didn't know about solar clipping when we had our system installed. We counted on the installer to size everything, since he's been doing this for 10 years.
As power readings came in, we were only seeing around 6kW of power production. With a software update, that went up to just over 7kW, but never close to 10, even though the array size is over 10kW. This is how we discovered solar clipping.
How much power do we lose?
The picture below compares solar array power output. In this example we have 10.6kw of solar panels.
The solid red line is a 7.6kw inverter using the 10.6kw of panels. The red dash line is a 10.6kw inverter with 10.6kw of panels. The blue line represents a 10.6kw array on a less sunny day.
The brown line represents the same less sunny day with a 7.6k array. Both the blue and brown lines never reach the 10.6kw or even the 7.6kw inverters.
While we lose energy when the sun is at max, we gain energy at the beginning and end of the day with more panels. In the example above we actually only lost 1.29kwh (and gained 2.84kwh).
We could (theoretically) lose up to 3kw per hour at peak, but in the real world, the loss is closer to 1.5kw per hour. As long as we're below 7.6 kW, we gain roughly 30% more energy per hour.
Because our array is oversized for the inverter, we get 30% more power on more than 80% of the days in summer, spring, winter and fall. We gain more power from this larger system in the morning and afternoon, but then lose energy when the panels are at maximum power production (clipping).
3 Reasons you Might want Solar Clipping
- Solar project located away from the equator in an area with less available sun.
- Morning/Afternoon Gains and Cloudy Day Gains
- Increased power production over the life of the system. Solar panels lose efficiency over time, between .6% and 1% annually. We expect our 10.6kw array will produce approximately:
- 10.2kw at 5 years
- 9.95kw at 10 years
- 9.4kw at 15 years
- 9kw at 20 years
Planning for Efficiency Losses and Less Sun
If you want the system to be fully powered for 20 years, you want to be at least 13% oversized because of lifetime loss and sun variability. At latitudes farther from the equator with less available sun, up to 30% excess panel capacity is advised.
There are numerous resources to calculate how much “sun power” you will have available, such as the map below. Most inverter companies have software that allows you to plug in your location and electric load to automatically suggest system sizes.
The list below gives examples of 30% arrray oversizing (which will result in solar clipping) for various inverter sizes:
- 3kw inverter will need a 4kw solar PV array.
- 5kw inverter will need a 6.5kw solar PV array
- 7.6kw inverter will need a 10.6kw solar PV array
We only show three inverter wattages, because most inverters only come in a few wattage ratings. We used a Pika (Generac) inverter. There are also good inverters from: SolarEdge and Outback. The Telsa ones are nice but crazy expensive.
NOTE: Most inverters larger than 10kw produce 3 phase AC power.
Most homes use single phase power and cannot easily use 3 phase. It's possible to convert 3 phase to single phase electricity, but there is power loss during conversion. A converter also adds significant cost and complexity to the solar electric system.
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We have more solar electric posts in progress as we get to know our system and “work out the kinks”. (After talking with tech support from the inverter company, we think we might be able to boost power utilization by adjusting the battery connection, but the installers are booked through the end of the year.)
Leave a comment below with questions, or to share your experience.