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Installing solar panels is a great way to reduce dependence on fossil fuels and also cut down on household fuel bills.
More and more people want to go further, and live completely off-grid, with no connection to the national power system.
While solar panels are clean, long lasting and relatively cheap to install, there are a couple of downsides.
The biggest concept to understand is that the electricity generated by a solar panel is different from that produced by the massive power generators that feed the utility grid and inverters are required.
- What does a solar inverter do?
- Why Do Solar Cells Need An Inverter?
- How Does A Solar Inverter Work?
- What Is A Grid-Tied Solar Inverter?
- What Is An Off-Grid Stand Alone Solar Inverter?
- What Are The Different Types of Solar Inverters?
- Questions relating to inverter operation:
What does a solar inverter do?
A solar power inverter converts direct current (DC) generated by solar panels to alternating current (AC) used in domestic premises. There are two broad types: stand alone for off-grid use and grid-tied, which synchronize their voltage output to national grid power lines.
What Is The Difference Between DC and AC Voltage?
A solar panel is basically an array of semi-conductor cells and like all semi-conductors, they either use or create direct current (DC).
It’s just how they work. DC is the same kind of electricity stored in batteries, and like batteries all DC devices have a positive and negative terminal which must not be mixed up!
Solar Panels and Batteries Are Direct Current Devices
All domestic electricity are supplied with alternating current (AC), which means that the voltage is never the same value at any instant in time.
This is because the voltage is generated at the power station by a rotary device in which magnets sweep across a magnetic field in a circular motion.
The voltage rises from zero to maximum and back down again. Plotted over time it appears in the from of a perfect sine wave (see drawing below).
Sine Wave Associated With Domestic Alternating Current
As you can see, the voltage rises from zero three times in a cycle, reaching both positive and negative maximum values.
This gives us a problem: solar panels produce a type of electricity the appliances in our house can’t use.
Video – What is a solar inverter and how does it work?
Why Do Solar Cells Need An Inverter?
The straight line characteristic of solar panel DC current needs adapting electronically so that it can do useful work. This is the solar inverter‘s job – to create a sine wave voltage from a straight line voltage.
An Inverter Converts DC to AC Power
Basically, the inverter has to change a straight line into a varying voltage that alternates around zero 60 cycles a second (Hz) in the U.S. and parts of Europe, or 50 cycles a second in the UK.
How Does A Solar Inverter Work?
First of all the inverter needs to create a negative value of the voltage coming in from the solar panels.
Circuits driven by electronic components called MOSFETS driving an oscillator to produce a square wave. This square wave is alternating current in it’s strictest sense, but it’s completely unsuitable for domestic use.
A Square Wave Is A Simple Alternating Voltage
This very rough shape will need to be shaped by the nverter using filter circuits using inductors (coils of wire) and capacitors (store packets of electricity) progressively until the sine wave is formed.
The shape or ‘pureness’ of the sine wave produced is an indication of the quality and expense of the inverter.
A ‘choppy’ sine wave will run some devices OK, but can interfere with radio and other electronic devices. It’s best to buy the most expensive you can afford for trouble-free operation.
What Is A Grid-Tied Solar Inverter?
When a solar panel installation is connected to the utility power grid, it is called a grid-tied system.
It’s permanently connected to the electricity supply and is completely invisible to the home-owner, except that the electricity injected into the grid offsets the cost of their usage and saves money.
Grid-Tied Solar Inverter System
In this case, the inverter has a bit more work to do! Imagine that the grid electricity is at maximum positive when the inverter output voltage was at maximum negative.
If they were connected together there would be an almighty bang!
The two voltages are said to be out of phase, or unsynchronized. The grid-tie inverter’s job is to synchronize it’s ac output to the power grid. Once this is done, it connects the two supplies.
Grid-tied Inverter Connects To The Power Grid Safely
What Is An Off-Grid Stand Alone Solar Inverter?
An off-grid inverter is concerned solely with converting DC current from solar panels to AC current used in domestic appliances.
It isn’t concerned with synchronizing the output to the power grid sine wave because it is never connected.
What Else Do I Need For Off-Grid Solar Panel Electricity?
Solar panels can only produce electricity during daylight, and most of that is generated around mid-day. In many areas you can only count on 4 to 5 hours of bright enough sunshine to give you the panel rated output.
This is why a bank of solar batteries is essential for off-grid living, particularly if you run appliances such as a refrigerator at night.
It can more than double the expense, but it’s really necessary. The only other option is to only run appliances in the day-time.
Off-grid Solar Inverter With Batteries
Table – Difference between pure sine wave and modified sine
Pure Sine Wave Inverter
Modified Sine Wave
Perfect sine wave output
Practically square wave
High quality electronics
Minimum electronic components
Low level hum
Can be noisy, can hear definite buzz
Does not interfere with other electronic gear
Can cause interference on TVs, cell phones
Safe for all appliances
Some appliances may overheat
90% to 95% efficient
75% to 85% efficient - wastes power
2 to 3 times more expensive than modified
What Are The Different Types of Solar Inverters?
As the interest in solar power accelerated over the past decade or so, all components of a solar power system have improved in capacity and efficiency.
In the case of solar inverters, the topology of domestic solar installations has evolved to become more efficient.
Central Inverters – How does a central inverter work?
Before home solar power was common, commercial installations all used one big inverter at a central location to convert DC to AC. This has some disadvantages:
- DC losses in long cable runs have more losses
- if one or more panels are shaded by passing clouds, the whole system output suffers
- if solar system output is very low, central inverter efficiency can be very low
Hoem solar systems inherited the convention of using a central inverter, but this practice has largely dissapeared for all but the smallest installations.
How do string inverters work?
A solar power installation may consist of several groups of solar panel arrays. These may be located seperately on different roofs with different orientation and title angles.
Each group of panels are connected together and are called a ‘string’. A string inverter collects the DC power from the string and converts it to AC before transmitting the power to the main control center.
One of the advantages is that if one string of panels is shaded, the other strings are not affected, so the reduction in output power is not so severe.
What does a micro inverter do?
Microinverters take the idea of using string inverters to minimize shading and other losses to another level.
Microinverters are mounted on the back of each individual solar panel and convert DC to AC locally before transmitting the power to the home interior.
The latest microinverters can convert the output from several panels.
String Inverters (DC to AC in one location)
Micro-inverters (DC to AC at each panel)
High DC voltage transmitted across roof
Normal AC voltage easier to transmit
Single point of failure stops system
Micro-inverter failure affects only one panel
Bad/shaded panel affects system performance
Bad/shaded panel has minimal effect overall
Questions relating to inverter operation:
What are the best solar power inverters?
The best solar power inverters produce a pure sine wave of ac power. The quality of the wave-form is always a function of quality and price.
It’s a mistake to cut corners with inverters – the result is a choppy wave-form that may cause appliances to wear out quicker and also interfere with telephones, radios and TVs.
Finely balance motors and pumps may seem to run well, but ‘modified sine wave‘ inverters (the name given to choppy wave-form types) can cause juddering that isn’t obvious to the naked eye or ear, hence it won’t last as long.
What size solar power inverter do I need?
Solar power inverter size is a function of the load to be supplied. Once this is calculated in kilowatts (kW) as a steady load, it’s usually best to purchase an inverter with same capacity that you need.
Appliances with motors on board pull more current when they start a cycle This is called the surge current. A refrigerator for example may draw 1.5 to 2 times as much current when the compressor motor starts.
How does a solar power inverter work?
All power inverters work in the same way, by chopping up incoming DC voltage generated by solar panels, adjusting the values and shaping them into an AC sine wave.
The more basic, ‘modified sine wave’ inverter, doesn’t output a perfect sine wave and are cheaper. They are great for general use, but can cause problems with sensitive electronics or stepper motors, which can judder or overheat.
Pure sine wave inverters are the best quality inverters and are more expensive.
What is the purpose of solar inverter?
The purpose of a solar inverter to convert direct current (DC) into the alternating current (AC) required by home appliances.
Solar panels and batteries generate or store DC, and all electronic ciruits use DC.
Do solar panels need an inverter?
Solar panels need an inverter if it’s intended to run AC household appliances from the output.
For some purposes, such as running DC motors and power DC appliances designed for RV use, inverters are not required.
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