MPPT vs PWM Charge Controllers

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MPPT (Maximum Power Point Tracking) charge controllers are designed to harvest the maximum amount of power from the solar array. It is generally accepted that even the most basic MPPT controllers will provide an additional 15% of charging capability compared to a standard PWM regulator, with the more advanced MPPT regulators to 30%. In addition to this efficiency, there are several other important differences and advantages between PWM & MPPT technologies.
PWM Charging
Traditional solar regulators featuring PWM (Pulse Width Modulation) charging operate by making a connection directly from the solar array to the battery bank. During bulk charging when there is a continuous connection from the array to the battery bank, the array output voltage is ‘pulled down’ or 'clipped' to the nominal battery voltage (where MPPT will use the full voltage curve). The battery voltage adjusts slightly up depending on the amount of current provided by the array and the size and characteristics of the battery.
The Vmp (maximum power voltage) is the voltage where the product of the output current and output voltage (amps * volts) is greatest and output power (watts = amps * volts) is maximized. Module wattage ratings (e.g. 100W, 205W) are based on Pmp (maximum power) at Vmp under standard test conditions (STC). Using a nominal 12V system as an example, the battery voltage will normally be somewhere between 10 – 15 VDC. However, 12V nominal solar modules commonly have a Vmp(STC) of about 17V. When the array (having Vmp of 17V) is connected to the batteries for charging, the batteries pull down the output voltage of the array. Thus, the array is not operating at its most efficient voltage of 17V, but rather at somewhere between 10 and 15V.
Because these traditional controllers rarely operate at the Vmp of the solar array, energy is being wasted that could otherwise be used to better charge the battery bank and maintain power for system loads. The greater the difference between battery voltage and the Vmp of the array, the more energy is wasted by a PWM controller during bulk charging.  Imagine a 36V PV panel connected to a 12V nominal battery bank using a PWM controller. You would clip / waste 20-24 volts of the panel voltage that makes up the usable energy! Where MPPT would use the full voltage curve and boost the current to equal the panel wattage.
MPPT Charging
MPPT controllers are designed to quickly and accurately determine the Vmp (maximum power voltage) of the solar array. MPPT controllers ‘sweep’ the solar input to determine the voltage at which the array is producing
the maximum amount of power. The controller harvests power from the array at this Vmp voltage and converts it down to battery voltage, boosting.
Because power in is equal to the power out of the controller (assuming 100% efficiency, neglecting wiring and conversion losses), it follows that a down conversion of voltage corresponds to a proportional charging current in the process.  increase in current. Power (watts) is equal to the product of voltage and current, therefore, if voltage is reduced current must be increased to keep the input/output power equal. Assuming 100% efficiency: 
Input Power = Output Power : Volts In * Amps In = Lower Volts Out * Higher Amps Out
For example:
A 100W panel (Vmp of 17V) is used to charge a battery at 12V with a MPPT controller. In ideal conditions, 5.88A of solar current flow into the MPPT (100W / 17V = 5.88A). But the output voltage (battery voltage) is 12V, meaning current flow to the battery is 8.33A (100W / 12V = 8.33A). You can see that the greater the voltage difference between the Vmp and the battery, the more “boost” current the battery will receive.
A consequence of getting more “boost” when the voltage difference is greater: the less charged the batteries are (lower battery voltage), the more “boost” current they will receive. This is precisely the time when batteries will benefit from an increased amount of charging current. 
The preceding discussion of PWM vs. MPPT may cause some to wonder why a PWM controller would ever be chosen in favor of an MPPT controller. There are indeed instances where a PWM controller can be a better choice than MPPT and there are factors which will reduce or negate the advantages the MPPT may provide. The most obvious consideration is cost. MPPT controllers tend to cost more than their PWM counterparts. When deciding on a controller, the extra cost of MPPT should be analyzed with respect to the following factors:
1. Low power (specifically low current) charging applications may have equal or better energy harvest with a PWM controller. PWM controllers will operate at a relatively constant harvesting efficiency regardless of the size of the system (all things being equal, efficiency will be the same whether using a 30W array or a 300W array). MPPT regulators commonly have noticeably reduced harvesting efficiencies (relative to their peak efficiency) when used in low power applications. Efficiency curves for MPPT controller are printed in their corresponding manuals and should be reviewed when making a regulator decision.
2. The greatest benefit of an MPPT regulator will be observed in colder climates (Vmp is higher). Conversely, in hotter climates Vmp is reduced. A decrease in Vmp will reduce MPPT harvest relative to PWM. Average ambient temperature at the installation site may be high enough to negate any charging advantages the MPPT has over the PWM. It would not be economical to use MPPT in such a situation. Average temperature at the site should be a factor considered when making a regulator choice.
3. Systems in which array power output is significantly larger than the power draw of the system loads would indicate that the batteries will spend most of their time at full or near full charge. Such a system may not benefit from the increased harvesting capability of an MPPT regulator. When the system batteries are full, excess solar energy goes unused. The harvesting advantage of MPPT may be unnecessary in this situation especially if autonomy is not a factor.