Solar Inverter

Solar energy doesn’t provide electricity in a format that your table lamp could be powered by. Inverters change the power produced by your solar panels into something you can actually use.

Think of it as a currency exchange for your power. You might have a fistful of yen, but until you stop and exchange it for USD, you can’t pay for lunch stateside.

Your home is wired to conduct alternating current (AC) power. The electricity produced by solar panels is initially a direct current (DC). Inverters change the raw DC power into AC power so your lamp can use it to light up the room.

Inverters are incredibly important pieces of equipment in a rooftop solar system. There are three options available: string inverters, micro-inverters, and power optimizers.

Solar Inverter Types, Pros and Cons

String Inverters

String inverters have one centralized inverter — or, keeping with the metaphor — one central currency exchange station.

This is a standard inverter, and it works just fine if you don’t have any encroaching shade from nearby trees or a big chimney. It’s also great if you have all of your solar panels facing the same direction.

String inverters are standard in the industry, and they’re the least expensive.

Pros:

  • Lowest cost
  • Standard inverter
  • Performs well with no shade

Cons:

  • Overall production decreases if one panel is damaged or shaded
  • No ability to monitor each panel individually
  • Not optimal if your solar panels are facing different ways
  • Increasing power needs are more difficult and may require second central inverter installation

Micro-inverters

Micro-inverters have a small unit to convert power underneath or built into each individual solar panel. Think of it as having mini currency exchange stations on every nearby street corner.

This gives each panel the ability to function at peak performance, independent from its neighbors. Even if the panel next to it has a tree branch shading it for most of the day, all the other panels can convert at full capacity.

They also enable you to monitor the performance of each individual panel. This is helpful for spotting any issues with a single panel so you can have it repaired before it slows down the whole system’s productivity.

Any drop in efficiency only affects one panel. These can be more expensive than string inverters, but it could pay off by getting more power from your system overall.

Micro-inverters also make it easy to increase power usage if you want to. Say you buy an electric car and you’ll need more power to charge it every night. Adding more solar panels and inverters is easier and less expensive than adding an additional central inverter for a string inverter system.

Read more about string inverters and micro-inverters here.

Pros:

  • Shade from a nearby tree won’t reduce the whole solar panel system power output
  • Individual panel monitoring available
  • Increasing power needs are easier and less expensive than installing a second central inverter
  • Good for rooftops where solar panels may face different directions

Cons:

  • Higher initial cost
  • Not necessary if all panels face the same way and are not shaded

Power Optimizers

These are somewhere in between string inverters and micro-inverters both in how they function and in price.

As with micro-inverters, power optimizers have a component (the “optimizer”) underneath and within each solar panel. But rather than change the DC to AC right there on site, these inverters optimize the current before sending it to one central inverter. This is more efficient than a string inverter, as any sluggish production from one panel doesn’t slow the whole system, but more cost-efficient than a standard micro-inverter setup.

Imagine being able to cut to the front of the line at the currency exchange office. It’s not quite as quick or convenient as having your own exchange office a few steps from your home, but there’s no waiting around once you get to the central office.

Micro-inverters and power optimizers are gaining popularity and prices are dropping as the technology advances.

We have more details on power optimizers in this post.

Pros:

  • More efficient than string inverters
  • Less expensive than micro-inverters
  • Individual panel monitoring available

Cons:

  • Higher initial cost
  • Not necessary if all panels face the same way and are not shaded

Ultimately, best inverter for you depends on your roof shape and size, nearby trees, how much energy you need, and your budget.

What to Look for in a Solar Inverter

To recap, there are three kinds of inverters: string inverters, microinverters, and power optimizers. They all transform the power your solar panels generate from direct current (DC) to alternating current (AC). This makes the energy usable for your home.

Here’s a few things to look for when shopping for inverters…

Solar Inverter Warranties

Most people feel more comfortable purchasing electronic devices with warranties. Solar inverters are no exception. Most inverters have warranties ranging from anywhere between 5 and 10 years, though some can be extended to 25 years.

When you’re looking at a company, make sure you know what’s included in the warranty and what’s not. For example, some power optimizers might not include the central inverter under the warranty.

Also make sure you understand the terms of a warranty. Is the device covered in case of an internal glitch as well as in the case of external damage? Will you be charged for labor or shipping if you have to send parts in? These are all important questions to ask.

Solar Inverter Operating Temperatures

As with most electronic equipment, inverters operate best when they’re running cool. Operating temperature is the safest temperature range an inverter maintains.

Inverters will naturally generate some heat themselves as they do their job. Since they’re typically in an uncontrollable environment outdoors, they’re exposed to a wide range of temperature fluctuations.

Obviously, conditions aren’t always ideal and some times an inverter will have to work harder than others. The higher the operating temperature (the more heat it can handle), the better.

Solar Inverter Efficiency

There are two numbers to look for in solar inverter efficiency: peak efficiency and weighted efficiency.

Peak efficiency will give you the efficiency of your inverter when it’s running optimally. It’s good to know what the best-case scenario is, but it’s also worth noting that it won’t always be hitting that level. Some days it might only reach peak efficiency for an hour or two, or maybe not at all.

Weighted efficiency figures in the variables like DC input levels. This gives a more accurate gauge as sunshine, temperature, and other environmental elements affect inverter efficiency throughout the day.

Solar Inverter Key Terms to Know

  • Clipping/Scalping – This is the term used to describe the energy output that is lost due to undersizing an inverter. Any given inverter has a maximum power rating (at the residential level, measured in W or kW). When solar supplies DC power in excess of that inverter’s maximum power rating (what the inverter can handle), the resulting power is “clipped.” It’s important to consider the solar panel arrays’ maximum power output and select an inverter with the correct size, model, and type in order to avoid excessive clipping. It’s normal for the DC system size to be about 1.2x greater than the inverter system’s max AC power rating. For example, a 12 kW solar PV array paired with a 10 kW inverter is said to have a DC:AC, or “Inverter Load Ratio,” of 1.2. When one takes into account real-world, site-specific conditions that affect power output, it may make sense to size the solar array a bit larger than the inverter’s max power rating, as there may be very few “power-limiting days,” or instances of clipping for that system.
  • Inverter Efficiency – A percentage that tells us how much DC power input to an inverter comes out as usable AC power. No inverter is 100% efficient, although some come close in favorable conditions. In the conversion from DC to AC, power is lost in the form of heat. While inverter efficiency is an important factor to consider in the selection process, there are other factors to consider that also affect a project’s economics, such as warranty, price, expected life, serviceability, and monitoring functions.
  • Maximum Power – This is the highest amount of power allowed to feed into an inverter, which is a function of the inverter’s specifications or the maximum power a solar panel can produce. This will occur at the optimal trade-off between voltage and current along a given panel’s I-V (current and voltage) curve.
  • Maximum Power Point (MPP) – A solar system’s power output will vary with conditions, such as how much sunlight it receives, temperature, and other factors. A fixed-tilt, stationary, roof or ground-mounted solar PV system might only produce its maximum rated power during a limited period of the day. Every specific solar cell has its own unique I-V curve, which relates its maximum power output to variations in current (I) and voltage (V).
  • Maximum Power Point Tracker (MPPT) – A device that periodically tracks characteristics of a given panel, string of panels, or system, and optimizes and varies voltage and amperage accordingly in order to produce maximum power.
  • Micro-Inverter – A device that converts direct current (DC) produced by a single solar panel into alternating current (AC). Micro-inverters are commonly connected to and installed at the site of, or behind, each individual solar panel in an array. Most micro-inverter makes are installed in the field, while some come panel-integrated by the manufacturer. Popular brands of micro-inverters include: Enphase, Chilicon, APS, ABB, SMA, and SunPower.
  • Optimizer – A DC-DC converter, optimizer, or “panel optimizer,” is a module-level power electronic device that increases the solar system’s energy output by constantly measuring the MPPT of each individual panel. The panel optimizers relay performance characteristics via a monitoring system to facilitate operations and any necessary maintenance. In essence, optimizers support flexible system designs and arrangements – with multiple panel orientations, tilts, azimuths, and module types in a given string. Because optimizers are a DC-DC, or DC-coupled, technology, systems using this technology will generally be compatible with DC-coupled energy storage or battery backup solutions, like the Tesla Powerwall. Perhaps the most advantageous benefit of using a DC optimizer is panel-level MPPT, or max-power point tracking. The result is increased energy harvest from a panel system, especially when subject to periodic or sweeping shade.
  • Peak Power Point – Another term for Maximum Power Point (MPP).
  • String Inverter – The device that converts direct current (DC) electricity produced by groups of solar panels (called strings), into usable alternating current (AC) electricity. String inverters are considered a “mature” solar technology that has proved effective, safe, and reliable. Residential, 240V AC string inverters usually carry manufacturer’s warranties of about 10 years. When installed to a manufacturer’s specifications, code, and best practices, a string inverter may require service or ultimately replacement during a photovoltaic system’s lifespan. Check with Pick My Solar to understand what is covered by your solar system’s workmanship warranty, as well as the inverter manufacturer’s warranty. Depending on what one’s goals, budget, and preferences are, string inverters can be a great option for your solar PV system.

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