Why is my DC and AC size different on my Solar Proposal
Some clients have noticed that although they bought a system size based on the solar panels, their monitoring always shows a value lower than this. Even on the sunniest days. Don't worry, this does not mean the system is malfunctioning necessarily.
We are going to try to explain this is two ways. First we are going to try to relate it to something else in life then we will get into the more technical explanation.
Like any production process you won’t always get 100% efficiency 100% of the time!
Your solar system is your own personal power plant! Like any manufacturing process in a power plant (or any manufacturing plant) there will be times where you are not using your raw materials as effectively as other times. In this case the raw material is the sun's rays that get manufactured to DC current (by your panels) and ultimately converted into the usable AC current (by your inverters). Sometimes your power plant is working really efficiently and other times it's not working as efficiently (maybe a crew member called in sick or you ran out of a key ingredient ;)) Our modelling accounts for all of this so over the year the amount of energy produced should be close to the expectations we set in the proposal.
The more technical answer to: Why does my monitoring system show lower values that I expected?
The simple answer is that the power being supplied to the inverters is higher than the output rating of the inverter. There is a very sound reason why this is done globally in the industry. But first, let's discuss some basics.
Your solar power system consists of a number of components, but let's focus on the big ones.
Firstly you have the solar (photovoltaic or PV) panels that produce Direct Current (DC) and have a power rating. This power rating is it's PEAK output which is why you may see it expressed as Wp or Watts Peak. This means that the panels essentially output up to this much power in certain conditions. If the rays of the sun are hitting the panels at an angle or are shaded by clouds, then the output of the panels will be less than the peak output.
Next, you have an inverter. The purpose of the inverter is to take the DC power produced by the panels and convert it to Alternating Current (AC). This is the kind of electricity that your 3 prong wall outlets supply in your home. This is also the value that your monitoring shows you. It is important to know that the power the inverter is able to output is limited by how much power it receives from the panels. It cannot produce more electricity than the panels are supplying to it. Therefore a lot of technology goes into inverters and they are designed to be able to handle the fluctuations in the power being supplied by the panels.
If you have noticed, there are times of the day, typically the morning and evening, where the sun has a very acute angle to the panels. Or you may have an overcast day. During those times where the intensity of the sun's rays is low, the panels still have the ability to produce power. It will just be less than its peak output.
This is true more often than you may realize. In fact, when you look at historical weather data and take the average sunny hours per day, it's only about 4.5hrs per day*. This means that for the rest of the time the panels are likely producing very little or not at all. This is the reason why it is an industry best practice to have the panels produce more power than the inverters can output. This is called Oversizing and refers to a ratio/percentage of the DC size to AC size.
If you were to size the inverter to match the panel, then you'd be paying a premium to have the inverters produce maximum output for only those few hours per year. The rest of the time the inverters would be under utilized. By increasing the supply from the panels, you'll be maximizing the output more often and for longer durations. Therefore, when done correctly, the inverters will turn on sooner and start generating power for longer durations throughout the day.
There are limitations of course. You don't want to Oversize the system too much. Typically for Canada, you design the system with an Oversizing of about 110% to 140%.
In any case, when you are considering your system, understand that the modeled production has already accounted for all of this. Therefore, even if your system does not output the peak DC output of the panels, the amount of power (kWh) it produces in a year should be close.