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Inverters 101

Inverters 101

Solar Inverters.  

If you're exploring the idea of going solar, no doubt you've heard that solar inverters are essential components for most solar electric systems.  Buy why is that?  Why are solar inverters so important? And what does a solar inverter do anyway?  Are all inverters the same? This page’s focus is to layout some of the basics involved with the different types of inverters and try to answer some of these questions along the way.  

What is an inverter? 

As you probably know, there are two forms of electricity commonly used today.  Solar panels produce the form known as direct current (or DC) electricity.  DC is also the form of electricity most batteries store.  On the other hand most homes and businesses use alternating current (AC).  AC faces less voltage drop over long distances, making it a convenient form to move from power plants to end users.  An inverter's main function is to invert DC electricity into AC electricity.  Inverters span a range of sizes, from small ones that allow you to run your laptop off your car's cigarette lighter, to larger and more robust inverters designed to work with home or industrial solar electric systems.  But there really isn't a generic solar inverter.  Why? Because there are different types of solar electric systems, which require different types of solar inverters.

Grid-direct inverter.  

A grid-tied or grid-interactive solar electric system is the simplest solar system and requires a specific grid-direct inverter.  In a typical residential grid-tied system, the solar PV modules send DC current to the grid-direct inverter, which inverts the DC electricity to AC and then sends the AC to home's electric service panel to feed any electricity demand in the house.  If the current house loads are less than the AC output from the inverter (like in the daytime when no one is home), the grid-direct inverter routes the electricity back to the utility (causing the meter to run backwards).A grid-direct inverter must produce a clean, pure "sine-wave" of AC electricity that matches the utility grid's wave form.  What's a sine wave?  An alternating current electrical wave has the shape of a sine wave.  If you see an inverter spec sheet that says  "modified sine wave," then you know that inverter cannot replicate the utility's form of electrical wave.  That means that some devices will not function (or at least not as well as they could) on electricity from a modified sine wave inverter.  Today, essentially all grid-direct inverter replicate's the grid's sine wave shape.Grid-direct inverters must also comply with UL-1741 anti-islanding safety requirements of the NEC (National Electric Code).  "UL" means Underwriters' Laboratories.  The applicable NEC provision requires that when the utility grid goes down, a grid-direct inverter must automatically disconnect the PV array from the electrical system.  This is a safety requirement to help ensure electricity from the PV array doesn't flow back onto the utility lines and injure any utility workers repairing the lines during a power outage.  This also means that while the solar panels may be producing power during a day-time power outage, the inverter stops the supply of electricity to the house.

Stand-alone, off-grid inverter.  

A stand-alone inverter need not concern itself with the grid because it is only used in solar electric systems where there is no grid electricity present.  In this situation, the PV array supplies DC electricity to a DC charge controller that charges a battery bank. When the house demands electricity while the sun is shining and the battery bank is full, the stand-alone inverter tells the charge controller to skip the battery bank and route the DC electricity from the solar panels straight to the inverter which then supplies AC electricity to the house loads.  When the array isn't producing (at night), the  inverter inverts DC to AC electricity that is stored in the battery bank via the charge controller.  Today, most off-grid inverters are really "inverter/chargers" because they also include the functionality to allow charging of the battery bank from an AC source (usually a generator). Why?  Because unfortunately, the sun doesn't shine all day, every day in most places.  After more than a few days without good sun (clouds, rain, snow), the batteries discharge to their limits and can't be fully charged by the solar panels.  Connecting a propane or gas generator to the inverter/charger inputs allows for convenient recharging of the batteries.  Some inverter/chargers even automatically start the generator when the inverter senses the need.In the past it wasn't uncommon for off-grid inverters to produce a lower quality wave like a "modified sine wave" or a "square wave" since there was no need to match the utility's  wave form.  But technology has improved and to ensure AC loads in the house or business are appropriately powered (especially today's sensitive electronics), off-grid inverter/chargers commonly produce clean sine-wave electricity.

Grid-Tied with Battery Back-up.  

A grid-tied with battery back-up solar electric system combines components and functionality of a grid-tied system and a stand-alone battery-based system.  As a "hybrid" system, it is the most complex system with the most components.  The inverter for this system is also more accurately called an inverter/charger because it can connect with an AC source to charge the battery bank when the sun can't do the job.  But in the case of a hybrid system, the utility grid is present so the primary AC source used to charge the batteries is the grid.  Some models allow a secondary AC source (a generator) to be connected as well.  Like a grid-tied inverter, this hybrid inverter also sends excess solar-produced electricity back to the grid (as long as the batteries are fully charged).  And when the grid goes down, the hybrid inverter disconnects from the utility grid (like a grid-tied inverter does) and pulls DC electricity from the battery bank, inverts it to AC and sends to a special "emergency" or "back-up" load center or panel.  To avoid excessively large and expensive battery banks, a home owner must be selective and only choose important loads to be backed up.

String vs Module level inverters. 

For grid-tied solar electric systems, there is another choice to be made.  Do you want a single "central" inverter to which all strings of solar panels connect?  Of do you want to attach an individual "micro inverter" behind each solar panel that inverts DC to AC at the panel?  Or do you want a hybrid style central inverter connected to DC optimizers attached to each solar panel?  How to choose?

String inverters.  

String inverters are the original workhorse style of the solar grid-tied inverter. These types of inverters are tried and true.  And today, they are generally cheaper than module-level micro inverters or DC optimizers.  They do lack the ability to optimize and "maximize" the power produced from each solar panel.  And they also can't reduce the impact one shaded or impaired panel may have on the array's production.  For example,  shade on one panel may reduce the production of the entire string of panels in that array. A string inverter can maximize power production of an individual string but not at the module level.


These devices are little mini inverters that attach behind the module and invert from DC to AC at the module level.  This means that that AC wiring connects the solar array to the house,  which is cheaper and less subject to voltage drop than DC wiring.  But more importantly, having a mini inverter connected to each module means that each module can be adjusted to produce the maximum amount of power possible (called maximum power point tracking or MPPT).  So if one panel is shaded, it does not reduce the power produced by the string In general, microinverters should produce a bit more power than string inverters, which is a good thing!  Finally, for some of us, our inner geek appreciates the ability to monitor individual panel production on our home laptop or phone. But beyond satisfying our curiosity, monitoring also makes it possible to quickly fix or replace any components minimize downtime.

DC Optimizers. 

These types of inverters are a bit of a hybrid. Like a microinverter, a DC optimizer attaches to the rear of each solar panel.  And they enable maximum power point tracking (MPPT) for each panel as well.  But the module-level optimizer doesn't invert to AC at the panel.  Instead each optimizer connects back to a central inverter where AC electricity is actually produced. These devices can be a bit cheaper than a true microinverter while providing  similar benefits.

Solar Inverter Pages.  

Hopefully this introduction has helped you become a bit more familiar with the terminology and the  choices available for solar inverters.  Check out here or drill down in the  menu above.  Give us a call so we can help get you the right inverter for your project
Module Versatility – 72 Cell Solar Panels

Module Versatility – 72 Cell Solar Panels

Why choose a 72 cell solar panel?
SPS Project Spotlight – Small Hunting Cabin Solar System

SPS Project Spotlight – Small Hunting Cabin Solar System

SPS Project Spotlight - Small Hunting Cabin Off-Grid Solar System

Yesterday a customer came into our office looking for a nice power solution for his hunting cabin. Initially, he had some questions about a pre-configured portable solar kit that he had noticed online. These plug and play solar kits can be a great choice for particular applications, but after speaking with him for a few minutes, the custom design wheels started to turn in our heads!

After  reviewing the customer's expected loads (what he planned to power), showing the customer some solar system components and examples of a few of our off-grid solar charging systems in our warehouse, we were able to get a great idea of exactly what he needed. Keeping low cost and high reliability in mind, our first system design ended up being a 540 Watt pole-mounted system with a 1000W 24VDC -  120VAC inverter. After providing him a quick quote and basic custom wiring diagram, we look forward to the next step with this customer and his installer to see if he would like to look into other options, such as using a larger inverter or possibly an inverter/charger for automated generator use.

As with many off-grid systems, there will probably be a few tweaks to our first design, but we are more than happy to work with our customers until they get exactly what they need and leave with a smile on their face and an extremely reliable system that won't leave them in the dark!

Here's the skinny on what we quoted for this customer, and some notes on what the customer can expect from the system.

Small Hunting Cabin Off-Grid Solar System - Parts List:

QTY: 2 - Peimar 270W 60 Cell Poly Solar Panel SG270P
QTY: 1 - Side-Of-Pole Solar Mount 2X 60-Cell Panel (SOP-Y)
QTY: 3 - HelioLug U Lug PV Lug with Hardware UL 2703
QTY: 1 - 50 ft MC-4 PV Cable Extensions
QTY: 1 - MidNite Solar 3 Circuit Combiner
QTY: 1 - MNEPV 20A PV Breaker
QTY: 1 - MidNite Baby Box
QTY: 2 - MNEPV 30A PV Breaker
QTY: 3 - Two Wire Cable Grip 6mm
QTY: 1 - MidNite Kid 30A MPPT Charge Controller Black MNKID
QTY: 1 - Samlex PST-1000-24 1000W 24V Inverter
QTY: 1 - 2-0 Inverter Cable 48in Red
QTY: 1 - 2-0 Inverter Cable 48in Black
QTY: 1 - Samlex DC-FA-200 Battery Post Fuse Block

Off-Grid Solar System Description.

This is a pretty straight forward 540 Watt 24 Volt system. The two solar panels will be mounted on a side of pole mount from our friends at General Specialties, one of the top-notch U.S. made racking companies we've happily worked with for a number of years. The panels are wired in series, so only a single solar panel extension wire will be required and the panels are fused at the array inside a Midnite Combiner box. The 20A PV breaker is intentionally oversized, the solar panels do have a max series fuse rating of 15A, but here in CO we have seen odd breaker issues in the winter time on pole mounted systems due to the high irradiance and an anomaly we call snow bounce. This 20A breaker is installed inside the Midnite Combiner. Not included in this parts list or wiring diagram, but something that we will also recommend is a lightning suppressor, which would also be installed at the combiner. Our favorite lighting arrester or surge suppressor for this is also made by Midnite Solar, the MNSPD-300-DC.

Wires from the pole will land in the Midnite Baby Box, a handy indoor breaker enclosure, which holds the two 30A breakers for the solar input and output from the MPPT Charge controller. An MPPT charge controller was used not only to step down the voltage from the panels wired in series, but will be better for wintertime production, which is generally when these small hunting cabins are utilized the most. This will feed a 24V battery bank, which the customer currently has but may be interested in upgrading, I'll make some notes below on a couple battery banks that would pair up nicely with the kit.

A 1000 Watt 24V - 120VAC Pure Sine solar inverter from Samlex can power their AC loads, and can either be hardwired to their electrical panel or used standalone with its two standard GFCI receptacles.

A system like this should charge around 1500 - 2700 Watt Hours into the battery each day in the winter time, this is assuming proper panel orientation and 3-5 hours of good sunlight.

We recommended a nice starter battery bank for this cabin solar system that includes four Trojan T105 225Ah 6V batteries. These would be wired in series for a 225 Amp Hour 24 Volt bank, with a total storage capacity of 5,400 Watt Hours.

Off-Grid Solar System - Wiring Diagram

Here's the wiring diagram we worked up for them to give them an idea of how all of the parts work together, we like to keep our parts list in basically the order you see them in our diagrams to make things as easy as possible!

We hope that this customer project spotlight will help inspire your own solar project!  Give us a call to explore what kind of custom solution we can provide for you!

Thanks for reading!

Solar Panel Tariff Exclusions Update

Solar Panel Tariff Exclusions Update

In January, the U.S. Trade Representative (USTR) announced 30% tariffs on all imported solar panels. This caused a lot of concern for not only large scale and residential solar projects, but also smaller industrial systems using small solar panels . As of September 19th, the following products were excluded from the import duties. We have great options for small panels from several manufacturers and are happy to say that we have not had any significant price hikes for small industrial panels and systems.

  • 45-W off-grid solar panels
  • 4-W solar panels
  • 60-W panels
  • 120-W flexible and semi-flexible panels used for motor vehicles and boats
  • 90-W frameless solar panels in colors other than black or blue
  • IBC and busbar-less solar cells
  • IBC and busbar-less solar panels
  • Modules using only U.S.-made solar cells

Seems like sort of a strange list! SunPower, a U.S. company that produces modules in the Philippines and Mexico, seems to be the biggest winner with these exclusions. The USTR received more than 50 exemption requests. Sunpower had announced that in April that it would acquire SolarWorld, who has a 500 MW capacity solar panel production plant in Oregon. This acquisition should go through by the end of the year.

Many other companies are going to set up new, or retrofit older PV production facilities in the United States to avoid the tariffs currently in place, as well as smaller companies working with OEM plants. So far, some of the biggest noticeable hits that the tariffs have had seem to be on large scale projects.

Here at SolarPanelStore.com we did see a small hike in panel pricing, but now see the prices declining again back to where they were at the beginning of the year, and have plenty of great panel options starting around $0.60 / W.