Hooray! You’re interested in solar! You start talking to installers and utilities, but it seems like they speak a whole different language. So, here’s a brief introduction to some of the terms and concepts you may encounter on your solar journey.
How to speak solar
Building Blocks of Solar
Cells, Modules, and Arrays
Solar cells—also called photovoltaic (PV) cells—convert sunlight into electricity. They are the building blocks of solar modules. Solar modules are composed of solar cells that have been wired together and set within a frame. The two most common sizes of modules are 60-cell and 72-cell. Both are usually about 3.25 feet wide, while the height usually ranges from 5.5 feet to 6.5 feet. Sometimes people use the term “panel” to refer to a module. A solar array includes all the modules/panels connected in your PV system.
From DC to AC
Solar cells produce electricity in the form of direct current (DC): electrons flow in one direction all the time. Our electrical system uses alternating current (AC): the flow of electrons reverses direction many times per second. That means that your PV system will include:
- Solar modules to produce DC electricity.
- Inverter(s) to change the DC electricity into AC electricity.
Two main types ofinverters
- A string inverter is a centralized inverter, usually installed near your electrical service entrance (meter or circuit breaker panel). It is usually the less expensive option, and if there is a maintenance issue, it is generally easier to troubleshoot. You can expect to replace your string inverter about halfway through the lifespan of your system (typical warranty period for sting inverter is 8-12 years).
- Microinverters are individual inverters that are installed with each module. They are typically a more expensive option than a string inverter, and they are less easily accessed in case of maintenance issues. Microinverters do have several advantages, however. They offer better performance for arrays that are partially shaded or have modules at different angles (such as a roof with multiple slopes). They make future expansion of the array easier. They allow for module level monitoring: you can see how much electricity each module in your array is generating. Finally, they often offer longer warranties (25 years).
kW and kWh
Kilowatts (kW) measure the RATE of energy consumption or production. (It’s like the speedometer on a car.) Kilowatt-hours (kWh) measure the QUANTITY of energy consumed or produced. (It’s like the odometer on a car.) Here are a couple examples:
- If you use a 1 kW microwave oven for 2 hours, it consumes 2 kWh.
- If a 10 kW solar array operates (under standard test conditions) for 2 hours, it produces 20 kWh.
The size of a PV system is usually measured in kW. In other words, system size is measured by the rate at which it can produce electricity. So you will hear installations described with words like “a 9 kW system” or “a 40 kW system.”
This next part gets a little gnarly. When people talk about the production of the solar modules, they are talking about kW DC (because modules produce DC electricity), while when they talk about the electricity output from the inverter, they are talking about kW AC (because the inverter changes DC electricity into AC electricity). The inverter output (in AC) will be slightly lower than the module production (in DC).
When you are interconnecting with the grid, what matters in system sizing is the kW AC (not kW DC), because the electricity going onto the grid is the AC output of the inverter, not the DC output of the modules.
Let’s talk about interconnection!
In this guide to solar, we are focusing on grid-connected PV systems, which are the simplest, most common, and least expensive option. You use electricity produced by your PV system, and any extra electricity your system produces goes out to the grid. During times when you are using more electricity than your system produces, you purchase that electricity from the grid.
In Minnesota, if your grid-connected PV system is under a certain size, your utility must pay you (with a credit) for the extra electricity you produce and put back on the grid. This is called net metering. The utility must give you a credit (per kWh) for your excess generation. The amount of that credit is regulated and is based on the utility’s average retail rate; it may or may not be identical to the rate that you pay for electricity, but it will usually be in the same ballpark.
The net metering size limit for a PV system depends on the type of electric utility you have. If your utility is an investor-owned utility (Minnesota Power, Xcel Energy, or Otter Tail Power), your annual production limit is 120% of your annual consumption and your system size cannot exceed 1 MW (1000 kW). If your utility is an electric cooperative or municipal utility, your PV system size must be smaller than 40 kW. If your system is larger than this, you will not be eligible for net metering. (For a sense of scale, a 40 kW array is about the size of a tennis court and can meet the needs of about 4 average Minnesota homes.)
The main disadvantage of a simple grid-connected system is that if the grid goes down (you lose power from your utility), your PV system automatically shuts down too. There is a simple safety reason for this: it is essential that your PV system not put electricity onto powerlines when utility workers are repairing them. If you want to have power when the grid goes down, you need a system that is grid-connected with battery back-up. If you don’t have access to the grid at all (you are not connected to an electric utility) but instead produce all of your own electricity, you have a standalone system. Both of these options are more complex (and more expensive to install) than a simple grid-connected system.