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The difference between a pwm solar inverter and an MPPT solar inverter
I often see a lot of solar inverter manufacturers selling solar inverters with PWM charging and wonder why they do so. In 99% percent of cases, PWM charges make no sense even though it is cheaper than MPPT. Then why are PWM solar inverters so prevalent?? I can only assume that customers are not fully educated about the advantages of MPPT over PWM chargers despite is higher price and manufacturers play on that to entice customers. With that in mind, I decided to write a small article on MPPT and PWM solar inverters
So how do I decide what is good for me? Lets start with an example. Say I am looking out for a 1KW solar inverter and I have an array of choices from low cost ones based on PWM and high cost ones based on MPPT. What should I choose?
A 1KW solar inverter will likely have 1KW of panels with around 2 batteries of 12 volts each. Let me assume that each panel is 250W of 24V each. In this case, the total voltage adds up to 96V.
Before you read the below, please keep in mind that Power (kw) = Voltage(V) X Current (Amps)
Since power is voltage multiplied by current, 1KW i.e 1000w will have a current of 1000W divided by 96V= 10.42 Amperes. When a MPPT is charging the batteries it requires only 24V (ie. 2 batteries * 12 Volts). In this case, when the MPPT steps down the voltage to 24V i.e to 1/4th of 96V, it also increases the current by 4 times ( remember Power = Voltage * current, so if voltage goes down, current steps up proportionately, and vice versa). This means, the entire current is now available for batteries to be charged, or if more current is generated than required, it can be used rerouted intelligently by your MPPT to run the load too.
In case of a PWM, the 96V is stepped down to 24V by the 10.42 Amperes of current remains at 10.42 Amperes rather than being stepped up to around 42 Amperes. The rest of the energy is dissipated as heat. That means, only 1/4 of the current generated by your solar panels is being used and given that solar panels are close to 75% of your overall solar installation, this is an incredible waste of energy. In addition, high heat dissipation reduces the life expectancy of the components within the solar inverter thereby reducing reliability of the system too.
So when does PWM make sense?
In case the panel voltage is the same as your battery voltage, PWM starts making sense that too only in small capacity inverters. So if your battery bank was 96V in the above example, PWM may have made some sense. But since panel voltages and battery voltages rarely match this scenario is hardly ever achieved. In case of small solar inverters like 100W wherein the panels used are just 100W it is better to save money on a PWM solar inverter and put in the extra amount toward buying solar panels rather than buying a MPPT solar inverter.
Even if you do choose an MPPT solar inverter, it is important to understand what is the efficiency of the MPPT and what underlying algorithm is used in the MPPT. But that is a different story for a different day.