What is a solar charge controller?
A solar charge controller, also known as a solar regulator, is essentially a solar battery charger connected between the solar panels and battery. Its job is to regulate the battery charging process and ensure the battery is charged correctly, or more importantly, not over-charged. DC-coupled solar charge controllers have been around for decades and used in almost all small scale off-grid solar power systems.
Modern solar charge controllers have advanced features to ensure the battery system is charged precisely and efficiently, plus features like DC load output used for lighting. Generally, most smaller 12V-24V charge controllers up to 30A have DC load terminals and are used for caravans, RVs and small buildings. On the other hand, most larger, more advanced 60A+ MPPT solar charge controllers do not have load output terminals and are specifically designed for large off-grid power system with solar arrays and powerful off-grid inverters.
Solar charge controllers are rated according to the maximum input voltage (V) and maximum charge current (A). As explained in more detail below, these two ratings determine how many solar panels can be connected to the charge controller.
Current Amp (A) rating = Maximum charging current.
Voltage (V) rating = Maximum voltage (Voc) of the solar panel/s.
MPPT Vs PWM solar charge controllers
There are two main types of solar charge controllers, PWM and MPPT, with the latter being the primary focus of this article due to the increased charging efficiency, improved performance and other advantages explained below.
PWM solar charge controllers
Simple PWM, or ‘pulse width modulation’ solar charge controllers have a direct connection from the solar array to the battery and use a basic ‘rapid switch’ to modulate or control the battery charging. The switch (transistor) is open until the battery reaches the absorption charge voltage. Then the switch starts to open and close rapidly (hundreds of time per second) to modulate the current and maintain a constant battery voltage. This works ok, but the problem is the solar panel voltage is pulled down to match the battery voltage. This in turn pulls the panel voltage away from its optimum operating voltage (Vmp) and reduces the panel power output and operating efficiency.
PWM solar charge controllers are a great low-cost option for small 12V systems when one or two solar panels are used, such as simple applications like solar lighting, camping and basic things like USB/phone chargers. Note, if more than one panel is used, they should be connected in parallel, not series.
MPPT solar charge controllers
MPPT or ‘maximum power point trackers’ are far more advanced than PWM controllers and enable the solar panel to operate at its maximum power point, or to be more precise, the optimum voltage for maximum power output. Using this clever technology, MPPT solar charge controllers can be up to 30% more efficient, depending on the battery voltage and operating voltage (Vmp) of the solar panel. The reasons for the increased efficiency and how to correctly size an MPPT charge controller is explained in detail below.
As a general guide, MPPT charge controllers should be used on all higher power systems using two or more solar panels, or whenever the panel voltage (Vmp) is 8V or higher than the battery voltage - see full explanation below.
What is an MPPT or maximum power point tracker?
An MPPT is basically an efficient DC to DC converter used to maximise the power output of a solar panel. The first MPPT was invented by a small Australian company called AERL way back in 1985, and this technology is now used in virtually all grid-connect solar inverters and many solar charge controllers.
The functioning principle of an MPPT solar charge controller is rather simple - due to the varying degree of sunlight (irradiance) landing on a solar panel throughout the day, the panel voltage and current continuously changes. In order to generate the most power, the maximum power point tracker sweeps through the panel voltage to find the ‘sweet spot’ or the best combination of voltage and current to produce the maximum power. The MPPT is designed to continually track and adjust the voltage to generate the most power no matter what time of day or weather conditions. Note, generally only high-end MPPT controllers can detect partial shading, or are able to track multiple power points. Using this clever technology, the solar panel efficiency increases and the amount of energy generated can be up to 30% more than a PWM solar charge controller.
PWM Vs MPPT Example
In the example below, a common 60 cell (24V) solar panel with an operating voltage of 32V (Vmp) is connected to a 12V battery bank using both a PWM and a MPPT charge controller. Using the PWM controller, the panel voltage must drop to match the battery voltage and so the power output is reduced dramatically. With an MPPT charge controller, the panel can operate at its maximum power point and in turn can generate much more power.
Best MPPT solar charge controllers
See our detailed review of the best mid-level MPPT solar charge controllers used for small scale off-grid systems up to 40A - click on the summary table below. Also see our review of the most powerful, high-performance MPPT solar charge controllers used for professional large-scale off-grid systems here.
Battery Voltage options
Unlike battery inverters, most solar charge controllers can be used with a range of different battery voltages. For example, most smaller 10A to 40A charge controllers can be used to charge either a 12V or 24V battery, while most larger capacity or higher input voltage charge controllers are designed to be used on 24V or 48V battery systems. A select few, such as the Victron 150V range, can even be used on all batteries voltages from 12V to 48V. There are also several high voltage solar charge controllers, such as those from AERL and IMARK which can be used on 120V battery banks.
The maximum solar array size that can be connected to a solar charge controller is generally limited by the battery voltage. As highlighted in the following diagram, using a 24V battery enables much more solar power to be connected to a 20A solar charge controller compared to a 12V battery.
Based on Ohm’s law and the power equation, higher battery voltages enable more solar panels to be connected. This is due to the simple formula - Power = Voltage x Current (P=V*I). For example 20A x 12.5V = 250W, while 20A x 25V = 500W. So using a 20A controller on a higher 24V volt battery, as opposed to a 12V battery, will allow double the size solar array to be connected.
20A MPPT with a 12V battery = 260W max Solar recommended
20A MPPT with a 24V battery = 520W max Solar recommended
20A MPPT with a 48V battery = 1040W max Solar recommended
Note, adding more solar or oversizing the solar array is allowed by some manufacturers to ensure an MPPT solar charge controller operates at the maximum output charge current, provided the maximum input voltage and current is not exceeded! - see more in the oversizing solar section below.
Solar panel Voltage Vs Battery voltage
For an MPPT charge controller to work correctly, the solar panel operating voltage must be at least 4V to 5V higher than the battery charging (absorption) voltage, not the nominal battery voltage. On average, the real-world panel operating voltage is around 3V lower than the optimum panel voltage (Vmp).
All solar panels have two voltage ratings which are determined under standard test conditions (STC) based on a cell temperature of 25°C. The first is the maximum power voltage (Vmp) which drops under cloudy conditions or when the solar panel temperature increases and the second is the open-circuit voltage (Voc) which also decreases at higher temperatures. In order for the MPPT to function correctly, the panel operating voltage must always be several volts higher than the battery voltage under all conditions.
In the case of 12V batteries, the panel voltage drop is not a big problem as most (12V) solar panels operate in the 18V to 22V range, which is much higher than the typical 12V battery charge voltage of 14.4V. Also, common 60-cell (24V) solar panels are not a problem as they operate in the 30V to 40V range.
In the case of 24V batteries, there is no issue when 2 or more solar panels are connected in series, but there is a problem when only 1 solar panel is connected. Most common (24V) 60-cell solar panels have a Vmp of 32V to 37V - While this is higher than the battery charging voltage of around 28V, the problem is when the panel temperature increases on a hot day, the panel voltage can drop by up to 6V, and end up below the 28V battery charge voltage, thus preventing it from fully charging. Another way to get around this, when using only one panel, is to use a larger, higher voltage 72-cell or 96-cell panel.
When charging 48V batteries, the system will typically need at least 2 panels in series but will perform much better with 3 or more panels in series, depending on the maximum voltage of the charge controller. Since most 48V solar charge controllers have a max voltage (Voc) of 150V, this allows up to 3 panels to be connected in series. The higher voltage 250V charge controllers can have strings of 5 or more panels which is much more efficient on larger solar arrays as it reduces the number of strings in parallel and in turn lowers the current.
Note: Panels in series can produce dangerous levels of voltage and must be installed by a qualified electrical professional and meet all local standards.
sizing a Solar charge controller
The charge controller Amp (A) rating should be 10 to 20% of the battery Amp/hour (Ah) rating. For example, a 100Ah lead-acid battery will need a 10A to 20A solar charge controller, and a single 150W to 200W solar panel to generate the 10A* charge current needed for the battery to reach the battery adsorption voltage. *Note: Always refer to the battery manufacturers specifications.
Before sizing a charge controller and purchasing panels or batteries you should understand the basics of sizing an off-grid solar power system. The general steps are as follows:
Estimate your loads - how much energy you use per day in Ah or Wh
Determine the battery size needed in Ah or Wh
Determine how many solar panel/s you need to charge the battery (W)
Choose the Solar Charge Controller/s to suit the system (A)
- Estimate the load -
The first step is to determine what loads or appliances you will be running and for how long? This is calculated by - the power rating of the appliance (W) multiplied by the average runtime (hr). Alternatively, use the average current draw (A) multiplied by average runtime (hr).
Energy required - Watt hours (Wh) = Power (W) x Time (hrs)
Energy required - Amp hours (Ah) = Amps (A) x Time (hrs)
Once this is calculated for each appliance or device then the total energy requirement per day can be determined as shown in the attached load table.
- Sizing the Battery -
The total Ah or Wh load is used to size the battery. Lead-acid batteries are sized in Ah while lithium batteries are sized in either Wh or Ah. The allowable daily depth of discharge (DOD) is very different for lead-acid and lithium, see more details about lead-acid Vs lithium batteries. On average, Lead-acid batteries should not be discharged below 70% SoC (State of Charge) on a daily basis, while Lithium (LFP) can be discharged down to 20% SoC on a daily basis. Note: Lead-acid (AGM or GEL) batteries can be deeply discharged but this will severely reduce the life of the battery if done regularly.
For example: If you have a 30Ah daily load you will need a minimum 100Ah lead-acid battery or a 40Ah lithium battery. However, taking into account poor weather, you will generally require at least 2 days autonomy - so this equates to a 200Ah lead-acid battery or an 80Ah lithium. Depending on your application, location, and time of year, you may even require 3 or 4 days autonomy.
- Sizing the Solar -
The solar size (W) should be large enough to fully charge the battery on a typical sunny day in your location. This is not simple as there are many variables to consider including panel orientation, time of year & shading issues. This is actually quite complex, but to simplify things we can roughly work out how many watts are required to produce 20% of the battery capacity in Amps. Oversizing the solar array is also allowed by some manufacturers to help overcome some of the losses - see more details below.
Solar sizing Example: Based on the 20% rule, A 12V, 200Ah battery will need up to 40Amps of charge. If we are using a common 250W solar panel, then we can do a basic voltage and current conversion - 250W / 12V battery = 20.8A. So we would need at least 2 x 250W panels to get close to 40Amps charge. Remember there are several loss factors to take into account so slightly oversizing the solar is a common practice.
- Solar Charge controller Sizing (A) -
The MPPT charge controller size should be roughly matched to the solar size. A simple way to work this out is using the power formula:
Power (W) = Voltage x Current or (P = V*I)
If we know the total solar power in watts (W) and the battery voltage (V), to work out the maximum current in Amps we re-arrange this to work out the current (I) - so we use the rearranged formula:
Current (A) = Power (W) / Voltage or (I = P/V)
For example: if we have 2 x 200W solar panels and a 12V battery, then the maximum current = 400W/12V = 33Amps. In this example, we could use either a 30A or 35A MPPT solar charge controller.
Due to the various losses in a solar system, it is common practice to oversize the solar array to enable the system to generate more power during bad weather and under different conditions such as high temperatures where power derating can occur. The main loss factors include - poor weather (low irradiation), dust and dirt, shading, poor orientation, and cell temperature de-rating (refer to the power temperature co-efficient on the solar panel spec sheet for more details). Learn more about solar panel efficiency and cell temperature de-rating here.
The various loss factors can add up as high as 20%. For example, a 300W solar panel will generally produce 240W to 270W in summer due to temperature power de-rating, and in winter or due to the low irradiance, depending on your location. For these reasons, oversizing the solar array beyond the manufacturers ‘recommended or nominal values’ will help to generate more power. Oversizing by 150% or more is even possible on some higher-spec MPPT solar charge controllers. However, not all solar charge controllers are designed to handle the excess power when the solar is operating at full capacity and this can damage some controllers. Therefore, it is important to always check the manufacturer allows oversizing - Morningstar and Victron both allow oversizing but take note to always check the manufacturer’s specifications.
Note, you must NEVER exceed the maximum input voltage (Voc) or maximum input current rating of the solar charge controller!
IMPORTANT - Oversizing solar beyond the manufacturer’s recommendations is allowed on some higher-end MPPT solar charge controllers such as those from Victron and Morningstar. Oversizing can void your warranty and could result in damage or serious injury to persons or property - always ensure the manufacture allows oversizing and never exceed the maximum input voltage or current limits.
More about Solar Sizing
As previously mentioned, all solar charge controllers are limited by the maximum input voltage (V - Volts) and maximum charge current (A – Amps). The maximum voltage determines how many panels can be attached (in series), and the current rating will determine the maximum charge current and in turn what size battery can be charged.
As described in the guide above, the solar array should be able to generate close to the charge current of the controller, which should be sized correctly to match the battery. Another example: a 200Ah 12V battery would require a 20A solar charge controller, and a 250W solar panel to generate close to 20A. (Using the formula P/V = I, then we have 250W / 12V = 20A).
As shown above, a 20A Victron 100/20 MPPT solar charge controller together with a 12V battery can be charged with a 290W ‘nominal’ solar panel. Due to the losses described previously, it could also be used with a larger ‘oversized’ 300W to 330W panel. The same 20A Victron charge controller used with a 48V battery can be installed with a much larger solar array with a nominal size of 1160W.
In comparison to the Victron MPPT charge controller above, the Rover series from Renogy does not allow solar oversizing. The Rover spec sheet states the ‘Max. Solar input power’ as above (not the nominal input power). Oversizing the Rover series will void the warranty. Below is a simple guide to selecting a solar array to match various size batteries using the Rover series MPPT charge controllers.
20A Solar Charge Controller - 50Ah to 150Ah battery
20A/100V MPPT - 12V battery = 250W Solar (1 x 260W panels)*
20A/100V MPPT - 24V battery = 520W Solar (2 x 260W panels)*
40A Solar Charge Controller - 150Ah to 300Ah battery
40A/100V MPPT - 12V battery = 520W Solar (2 x 260W panels)*
40A/100V MPPT - 24V battery = 1040W Solar (4 x 260W panels)*
* Remember only ‘some manufacturers’ allow the solar array to be oversized, as long as you do not exceed the max voltage or current rating of the charge controller - always refer to manufacturers specifications and guidelines.
Voltage issues - High Voc in cold climates
In cold climates, the open-circuit voltage (Voc) of the solar panel can increase significantly, up to 5V or even higher, which may result in the Voc of the solar array going above the maximum voltage limit of the solar charge controller and damaging the unit. See example below.
Example: A Victron 100V/50A solar charge controller will have a maximum solar ‘open circuit voltage’ or Voc of 100V, and a maximum charging current of 50 Amps. If you use 2 x 300W solar panels in series with 46Voc, then you end up with 92V. This seems ok, as it is below the 100V maximum. However, in extremely cold conditions the panel voltage can go much higher than the Voc. This can be calculated using the ‘voltage temperature co-efficient’ of the solar panel (this is typically 0.3% for every degree below STC - 25°C cell temperature), but to simplify things, you can generally add 5V to the panel Voc **. In this case, we would end up with a Voc of 102V. This is now greater than the max 100V input voltage limit and could damage the unit or void your warranty.
Solution: There are two ways to get around this issue. 1. Use an MPPT solar charge controller with a higher input voltage rating such as the Victron 150V/45A. Or a much easier option 2 - Connect the panels in parallel instead of series, so the maximum voltage will now be around 46V + 5V = 51 Voc, no worries if you are using a 12V or 24V battery system. Remember in this case the input current will double so the solar cable should be rated accordingly.
Note: Assuming you are using a 12V battery and 2 x 300W panels, the MPPT charger controller output current will be roughly - 600W / 12V = 50A max. So you should use a 50A MPPT solar charge controller.
** Up to -10°C. Guide only - check the lowest recorded temperature at your site.
solar charge controller Price guide
The older, simple PWM, or pulse width modulation, charge controllers are the cheapest type available and cost as little as $40 for a 10A unit, while the more efficient MPPT charge controllers will cost anywhere from $80 up to $1500, depending on the voltage and current (A) rating. All solar charge controllers are sized according to the charge current which ranges from 10A up to 100A. Cost is directly proportional to the charge current (and maximum voltage - Voc) with the higher the current controllers being the most expensive.
A general guide to the cost of different size solar charge controllers:
PWM 100V Solar controllers up to 20A - $40 to $120
MPPT 100V Solar controllers up to 20A - $80 to $200
MPPT 150V Solar controllers up to 40A - $200 to $400
MPPT 150V Solar controllers up to 60A - $400 to $800
MPPT 250V Solar controllers up to 80A - $800 to $1200
MPPT 300V Solar controllers up to 100A - $900 to $1500
This is to be used as a guide only. Before making any purchases or undertaking any solar/battery related installations or modification, you must refer to all manufacturers specifications and installation manuals. All work must be done by qualified personal only.