Buying an inverter may seem simple as its X amount of kW but its not that easy, here are some key points to buying a solar inverter.
Power output.
inverters are sold on the basis of the power output, this is measured in KW, but there are two types of inverters LF and HF and the application you use the inverter for will impact its ability to work. LF which is low frequency and big and heavy and are great for inductive loads. That means motors for the most part. HF are good for resistive loads, cookers and lights.
The second part to buying an inverter for its power. The starting point is the peak loads that you have. Start up loads are normally higher than the running power loads. There is consideration to combined load and combined startup. You can look at “power now” on your smart meter to find your peak load, and it is a good idea to go 30% over this peak.
Power input.
There are two power inputs on most modern inverters, PV (Solar) and AC (Grid/Generator)
The input from PV is often limited by current (ISC/A/I) Your solar panels will give you the ISC they produce and the sticker and information of the inverter will provide you with its limit. Exceeding the limit creates heat and some inverters will give the current limit and absolute limit.
Different inverters which have built in charge controllers will have different limits and this can vary between models. Importantly check the voltage limit as they cannot be exceeded ( OCV).
This means that you need to know how many solar panels you will have, and both the ISC and OCV to keep within spec.
You should look for an inverter with a higher input voltage, typically 450v when working with larger project, small projects may find they have a limit of 150v
Grid or generator power input is important as you need to be able to charge your battery. The value is expressed in amps on the DC side. The DC side being the battery voltage.
For example the Sunsynk 8kw inverter has a charge current of 190 amps or 9.1kW
When using AC to charge that’s 39.9 amps.
We have wrote a post about battery charging and there are 3 posts in relation to cycling and charging.
Ensure that your inverter charger can out put the charge you need in the specified time, this will be your cheap rate energy window, and also within the solar window depending on how you set up your solar array.
Cheap inverters.
Cheap inverters are something that can be used if you know what they are doing and how they work, The cheap cheap inverters tend to not be pure sine wave, these square wave or modified square waves can plague most electronics and cause things to not work.
Typically the rating is lower, and the peaks somewhat pie in the sky, the claimed constant is more the peak power and so you should keep far below the rated power.
It is perplexing as Pure Sine Wave inverter controllers are very low cost, so there is not need to have poor inverters and in the same charge so much for a PSW.
Micro inverters.
Micro inverters work differently, only in that they are AC from the panels, The inverter plugs into the panels and is rather small and puts out its AC power there. This means that you run a AC line to the inverters which is smaller that running the PV cables.
Micro inverters are great for shading and tune themselves per panel to get the most from your solar array. The down side to micro inverters is that they do not scale well when it comes to cost.
Micro inverter can also be used for “extra power”, some inverters have Aux ports that can have a micro inverter attached to it, this comes in handy to add more solar in winter or to supplement your array and off setting the power delivery, for example a west array for the summer evenings.
Hybrid inverters.
Grid tied inverters are only good as small inverters, hybrids are necessary if you want to save money on your power bill. Rather than exporting the excess solar, you charge the battery and use that stored power later in the day and night to power your home.
Solar power is not even though the day and the power can and does jump up and down a lot in seconds. Most software averages and charts this result which can show a more fluid power delivery over the day, this is lies.
As you can see in this real life live data, over the 15 minutes, the power goes up and down, therefore a battery will act as a buffer smoothing out the power to meet the needs over a longer period.
Misinformation.
Many sellers of inverters are giving misleading information about hybrid inverters. Unfortunately many people find out too late and the term is rather broad. Some of the “hybrid inverters” are grid attached, they can also be just a UPS. They can also be off grid and some will short your homes power due to the PE bond. So do take care in what you order.
Grid attached inverters.
Grid attached inverters do not send power to the grid but allow power to pass though from the grid to your home. They can work in different modes, such as UPS, which mean that when the grid power is down, your home still has power. They can also work in SUB and SBU modes. This means that your solar power runs the house, if solar is not enough it will take power from the battery, and if both those are not enough, then it will draw from the grid.
The one thing to look for with grid attached inverters, is the pass though. Most homes in the UK will have either 65amp or 100amp feeds, grid attached can be as low as 20 amp, but are typically 35 and 65 amps, Thats 8kW and 14.9kW in ac power.
The grid attached inverter does not need a G98 or G99 application as they are now working with the grid (in parallel) and they are often lower cost than hybrid inverters as they do not have to meet a lot of standards and requirements. They can also have unlimited power!.
Grid Tied inverters.
Grid tied inverters will not have a battery function, they are more suited for “power offset” but you only save what you can use, and generate. I would only suggest these types of inverter for smaller scale applications. The power is during the day, and if you are out, your home will not use more than 300w of power on average, therefore excess of this is giving your money away.
Retro fit inverter.
Retro fit inverters are nothing more than a AC coupled inverter. They are usually fitted to grid tied systems and they have a CT clamp that detects export power and diverts that power to the battery. They then add power from the battery via the inverter. We do sell as goodwe, sunsynk and victron units can be used in the same way.
Parallel inverters
Inverters can work together, and these are parallel inverters, a cable between the two or more makes sure the inverter work in sync. While you can buy 3 phase inverters, they are normally over 12kw, you can use parallel inverters to make 3 phase power, or you can use them to boost single phase. Not all inverters can work in parallel.
You may want parallel inverters as redundancy, load balancing or split circuits. For the most part a key consideration here is that inverters do use power even if they are doing nothing, the power use can range a lot from 30w to 65w which adds up over the day and this could be over 2kwh of power. When using the inverter in single phase, it is a good idea to shut down the extra inverters to save power, such as over night or when they are not needed.
Off grid inverters.
There is a massive amount of off grid inverters available on the market, You will find a wide range to suit most needs. The more load you have the higher the voltage you will want and this voltage is important when it comes to charging too.
a 20 amp charger at 12v is half the power of 24v and therefore you may be limiting the solar input into your loads.
You also need to use “power factoring” when you are using solar as the output current may exceed what your control can offer, meaning that you lose power. click on the link to see more about charge controllers.
Charge controllers and PV inputs.
Not all inverters comes with charge controllers, many AIO come with PV inputs, so take care as you may have to buy one or more charge controllers if your inverter does not support this.
Most inverter PV inputs are 1, 2, 4 and they are effectively combiner boxes in some models and individual MPPTs in others.
When the PV is combined this in effect gives you parallel solar arrays. with separate MPPTs they run individually to get the most from each array. How many arrays you have will dictate the mppts that you will need and having more so that you can add more later, is a good idea.
START UP.
This is often missed by many buyers, you do need to pay attention to the start up voltage ( and current) of the inverters, most separate charge controllers do not have this issue, but inverters do. The start up is NOT the same as the MPPT range, the mppt range is soley for optimsed power. Start up voltage is the voltage needed to make the hardware work, but there is also the current. smaller inverters and micro inverters will have around 65v and 26v respectively for start up. mid sized inverters start at 150v. most will need a current of 0.1a from the array in order to have stable voltage.
We have had many a call of “it doesn’t work” where people have plugged in AC power and the inverter does not work. They do not work on AC but DC, we have also had this in the evenings where the sun does not provide enough current for the start up of the inverters.
Optimal Voltage.
The way in which inverters work can differ, we call this topology. inside the inverter there is a H bridge which makes AC power, however it can have “pre booster” voltage which rises the DC voltage before or/and after the H bridge.
The wave from of AC power, is 120v + and 120v- Therefore in DC terms the current switches back and fourth; This is the job on the H Bridge.
If you split the voltage you can see that half voltage (65v) applied twice, will give you the half wave voltage. Therefore you obtain the start up voltage of 65v.
Full size inverters will need power (bus voltage) of around 370v (dc). This voltage is then split without conversion saving conversion losses.
The higher voltage is converted to current on the output transformer making the inverter more powerful.
Power from the array is either boosted (raised) of Bucked (lowered) which alters both the voltage and the current.
When rising the voltage the current will drop and the same the other way. It is a good idea to plan for optimal voltage as it will save you power, but also keep in mind the voltage drop in the panels from heat.
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