We have sized the PV array and the battery bank, we have selected a controller we now need to specify the off grid solar inverter as it is the component which will make our PV system usable by AC appliances.
To answer the common question about taking your home off-grid, I generally say to most people “if you already have the grid connected to your home then it makes no sense to go completely off-grid because the cost is too high; you will need a very large battery system to store energy for 3-5 days of bad weather along with a auto-start back-up generator for the winter months (in colder area’s)”. There is also the cost of fuel to run a back-up generator, maintenance, etc. so for most people it just doesn’t make economic sense. Why not use the grid as your back-up power source and feed your excess solar energy into the grid for some return?
What is off grid solar inverter?
Off grid solar inverter is used in isolated systems where the inverter draws its DC energy from batteries charged by photovoltaic arrays. Many off grid solar inverters also incorporate integral battery chargers to replenish the battery from an AC source, when available. Normally these do not interface in any way with the utility grid, and as such, are not required to have anti-islanding protection.
8 tips to select your off grid solar inverter
There are 8 tips to consider when selecting the proper off grid solar inverter, let us see as following:
1. The off grid solar inverter output voltage and frequency
This will base on your load requirements and usually its same as your country standard supply voltage/frequency.
- Off grid solar inverter’s output voltage should correspond to the load nominal voltage. 240Volts in Europe and Africa and 120V in USA.
- The inverter should maintain a frequency of 50Hz in Africa and Europe and 60Hz in USA
2. The inverter power ranging
The off grid solar inverter comes in size ratings all the way from 50 watts up to 50,000 watts, although units larger than 11,000 watts are very seldom used in household or other PV systems. The first thing you have to know about your inverter is what will be the maximum surge, and for how long. (More about 230 volts pumps etc later).
- Surge: All off grid solar inverters have a continuous rating and a surge rating. The surge rating is usually specified at so many watts for so many seconds. This means that the inverter will handle an overload of that many watts for a short period of time. This surge capacity will vary considerably between inverters, and different types of inverters, and even within the same brand. It may range from as little as 20% to as much s 300%. Generally, a 3 to 15-second surge rating is enough to cover 99% of all appliances – the motor in a pump may actually surge for only 1/2 second or so.
- General Rules: The off grid solar inverter with the lowest surge ratings are the high-speed electronic switching type (the most common). These are typically from 25% to 50% maximum overload. The highest surge ratings are the transformer based, low-frequency switchers. Surge ratings on these can range up to 300% for short periods. While high-frequency switching allows a much smaller and lighter unit, due to the much smaller transformers used it also reduces the surge or peak capacity.
- Pros and Cons: Although the high-frequency switching type doesn’t have the surge capacity of the transformer based, they do have some definite advantages. They are much lighter, usually quite a bit smaller, and (especially in the lower power ranges) they are much cheaper. However, if you are going to run something like a submersible well pump, you will need either very high surge capacity or you will need to oversize the inverter above its typical usage, so that even at maximum surge the inverter will not exceed its surge rating.
You can read more about Prostar PSW3K-Pro 24v 3000w off grid solar inverter without battery
3. Inverter output wave form
There are 3 major types of inverters – sine wave (sometimes referred to as a “true” or “pure” sine wave), modified sine wave (actually a modified square wave), and square wave.
Pure Sine Wave
A pure sine wave is what you get from your local utility company and (usually) from a generator. This is because it is generated by rotating AC machinery and sine waves are a natural product of rotating AC machinery. The major advantage of a pure sine wave off grid solar inverter is that all of the equipment which is sold on the market is designed for a sine wave. This guarantees that the equipment will work to its full specifications. Some appliances, such as motors and microwave ovens will only produce full output with sine wave power. A few appliances, such as bread makers, light dimmers, and some battery chargers require a sine wave to work at all. Sine wave inverters are always more expensive – from 2 to 3 times as much.
Modified Sine Wave
A modified sine wave off grid solar inverter actually has a waveform more like a square wave, but with an extra step or so. A modified sine wave off grid solar inverter will work fine with most equipment, although the efficiency or power will be reduced with some. Motors, such as refrigerator motor, pumps, fans etc will use more power from the inverter due to lower efficiency. Most motors will use about 20% more power. This is because a fair percentage of a modified sine wave is higher frequencies – that is, not 60 Hz – so the motors cannot use it. Some fluorescent lights will not operate quite as bright, and some may buzz or make annoying humming noises. Appliances with electronic timers and/or digital clocks will often not operate correctly. Many appliances get their timing from the line power – basically, they take the 60 Hz (cycles per second) and divide it down to 1 per second or whatever is needed. Because the modified sine wave is noisier and rougher than a pure sine wave, clocks and timers may run faster or not work at all. They also have some parts of the wave that are not 60 Hz, which can make clocks run fast. Items such as bread makers and light dimmers may not work at all – in many cases appliances that use electronic temperature controls will not control. The most common is on such things as variable speed drills will only have two speeds – on and off.
There are very few, but the cheapest inverters are square wave. A square wave off grid solar inverter will run simple things like tools with universal motors without a problem, but not much else. Square wave inverters are seldom seen anymore.
This is a measure of how much power from the batteries your inverter delivers to your home when it’s operating in perfect conditions. A good peak efficiency rating is around 94% to 96%.
You can read 7 ways to increase your home energy efficiency
5. Self-consumption, or no-load current draw
How much power will your off grid solar inverter consume just sitting there? Obviously you want this to be as low as possible.
Surge capacity. How much short-term overload can the off grid solar inverter handle before it “trips?” Some appliances, like pumps or fridges, need as much as 2x–3x their running power to start up.
6. Battery charger output
Many off-grid solar inverters include a battery charger, which is used to recharge your batteries during the winter months with a backup generator. The battery charger will have a rating, usually measured in amps. Most decent off-grid inverters will have a battery charger in the range of 50-100 amps DC.
7. The inverter built in solar charge controller will be MPPT or PWM.
MPPT is technically better as its able to convert solar panels high voltage to lesser voltage, so it charges batteries with low losses (high efficiency), but it costs higher than PWM solar charge controller. In the other side, if we can size PWM solar charge controller type properly according to solar panels specs, then we can confirm it will work as well as MPPT solar charge controller type.
8. Solar array max output voltage and MPPT trackers.
The PV max DC output voltage must not exceed the inverter max DC input voltage listed in its specs, and MPPT trackers refer to how many inputs the inverter accepts, it will be indicated based on how large is your solar array, and it will ensure better maximum power point tracking performance.
This is the primary reason why people in blackout-prone regions are interested in off-grid solar. When connected to the grid, you are dependent on an external supplier — the utility company — for all your power.
When the grid shuts off, so does your power supply.
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