Off Grid? Inverter?
Many time when we start working with solar power, we often hear the term “off grid”, or “off grid inverter”. What’s an off grid inverter? What’s “off grid”? What’s “inverter”? What is the difference between a solar off grid inverter and a normal “inverter”?
What's an "inverter"?
In electricity worlds, there are two types of electic currrents: Direct Current “DC” and Alternatic Current “AC”.
DC is the electricity that comes from some sources like the solar generators, or the batteries in your laptop or mobile phone: It has a fixed direction, and fixed voltage with reference to zero, for example, the simple battery is 1.5 Volts DC, it means it gives a 1.5v DC when measuring from positive + to negative -, and -1.5v DC when measuring from negative to positive. So it has a direction that is fixed and doesn’t change.
AC is the electricity that comes from many sources, like generators, and it is the main form of electricity that is used widely today in power generation and transmission. It is also the type of electricity that is used in almost all of our home appliances. For example, when we say our plug at home is 200v, it means it is giving us a stable 220 volts AC, and we can plug our devices and use them.
May be you have noticed there are some devices that come with a small “adapter”, like cell phones or laptops. These devices are actually running on DC electricity, but because our homes are standardized with AC plugs, we use those small “adapaters” to change the AC electricity from 220v AC (110vAC in the US) to any DC volts needed by our device (5vDC for phones, or 14 or 19vDC in most of the laptops). Those small devices are called “inverters”, they “invert” the AC electricity to DC electricity of a fixed voltage to be used as required, they “invert” the electricity from one form to another.
Why do we need an inverter for solar systems?
Solar panels produce DC current, with variable voltage. Actually, the voltage increases and decreases as the solar radiation is variable throughout the day. So we need a device that will be able to catch this DC, variable voltage, converts it to stable voltage DC to charge the batteries, and also be able to invert it to a useful AC that we can use in our homes.
Some years ago, solar systems used to have seperate devices for each task: a solar “charger” that will get this variable voltage direct current coming from the solar panels, and “converts” it to fixed voltage DC to charge the batteries, and another seperate device that will get this DC from the batteries and “inverts” it to fixed voltage (110 or 220v) alternating current, so we can use it to power our appliances.
Those two devices are now combined into single device, an “inverter-charger” or simply inverter, which is widely used for off grid solar systems to get the variable VDC from the panels, and converts them to fixed VDC to charge the batteries, and also inverts it to fixed VAC for output
Types of solar charger
Solar chargers are not all the same, there are some variations. The main types are the PWM (pulse width modulation), and the MPPT (Maximum Power Point Tracking). The difference is huge but yet simple, with no technology being “best” to the other. However, MPPT chargers are recently the most dominating types in the market.
PWM chargers
Pulse Width Modulation (PWM) is the conventional type used in most of the applications in other chargers outside of the solar world. It is the type of chargers that is used in conventional car batteries chargers, and most of the industrial chargers, where there is a fixed current coming in, and a fixed voltage needed coming out, the charging current is fixed during the “bulk charging” phase, then once the battery is charged, the charger gives “pulses” each fixed amount of time to make sure the battery stays at full charge.
For example, suppose we have a 12 vdc battery, this battery needs to be charged from a 220vAC source at home, a PWM charger will get the AC at 220v, inverts it to 12vdc and charges the battery. However, what happens in real world, this battery is fully charged at 14vdc, so the charger gives it a “bulk charge” at a higher current and volt, until it reaches ~80% of its capacity, then it lowers the voltage to a “floating voltage” of 14 or 14.2 vdc, and it keeps “pulsing” it at this voltage to ensure it is full.
When those chargers were first used in the solar industry, they were very sufficient: solar panels produced 14 to 16 volts to charge 12v batteries, or 28-31 volts to charge a 24v system, so the charger design was mainly ignoring the extra volts, and charges the batteries with their needed voltage, adjusted previously in the settings.
MPPT chargers
With the advance in solar panels technology, and due to the nature of the electricity coming out of the solar panels, there was a need for a new type of chargers that is especially designed for solar panels.
Solar panels produce DC electricity that changes in Current and Volt all the time, the characteristic curve that describes that is known as I-V curve of a solar panel. This I-V curve has a “maximum power point”, where the maximum volt and current coming out of the panel at a certain time produces the “maximum power”. MPPT chargers are programmed to track this maximum power point, hence the name “Maximum Power Point Tracking”.
The MPPT charges then calculates the current and volt coming out of a solar panel, and converts them to power that can feed the hungry battery and charge it.
Can we use the same amount of panels for a PWM and MPPT chargers?
Simply: NO!!!
As we have discussed, a PWM charger charges the batteries at a fixed voltage, and ignores any higher voltage coming in, also it has a maximum current rating. On the other side, MPPT charges take all the power in, and converts them to power out at the fixed voltage needed by the battery.
Let’s take an example:
Supposedly we have two chargers, PWM & MPPT, each is rated 40A/24v, maximum Voc of the chargers are 100vdc, that means that the two chargers can charge a battery at a maximum of 40 Amperes @ 48 volts (DC of course), and we cannot have any panels in series that can give out more than 100v
What’s the maximum power of solar panels that each can handle? Given that we have a solar panel that is 30V & 10A, 300W?
PWM charger: the charger will “ignore” any voltage higher than 24v, so one panel in series only can be used, however, we can use up to 4 panels in parallel for a total of 40A, so that means we can use a total of 4 panels for a total of 1200w
MPPT charger: the charger maximum power capacity is 40A*24v=960W. So we can use up to 3 panels, because our maximum allowed Voc is 100vdc, so we can use all the three of them in series. We can also use them in parallel, since the current will be 30A, and voltage is 30V, higher than the 24v for the needed charge
In this example, actually there is no much difference between the two chargers, let’s take a larger system and see what happens:
Suppose we have a 48v battery bank, and we need to hook up a 5kW solar array to it, which is better of the two types? Let’s say we have a 540W solar panel, with 49VOC, and 12A
Case 1: PWM charger: 48v, 60A
We can have 2 panels in series, for a total of 98 volts open circuit, and 5 strings in parallel, for a total of 10 panels, 5.4kW
Case 2: MPPT charger: 48v, 60A (2.88kW max power), so we round it up to 3kW maximum power allowed:
We can have three panels in series, and a total of only 2 strings in parallel, or 2 panels in series and 3 strings in parallel, that’s a total of 3.24kW, which is slightly more than the allowable power.
In that example, a PWM charger might look better, however, don’t forget that the charger “ignores” any voltage above its output voltage, so it mainly uses only 48v from the panels, which means it actually uses 48v*50A=2.4kW, while the MPPT charger is actually using all the power it is receiving.
