Since solar charge controllers are so important for a solar generator system, I put together multiple combinations of solar panels and solar arrays from 50W to 400W to calculate the right charge controllers needed for most people.
In general, a 10A MPPT charge controller can be used with a single 50W (12V) or 100W (12V) solar panel to charge a 12V battery. A 20A, 100V MPPT can be used with 150W (3x 50W) or 200W (2x 100W) of 12V solar panels in either a series or parallel configuration to charge a 12V battery.
Most single 150W and 200W solar panels rated for 12V can also be used with a 20A, 100V MPPT charge controller.
The amperage (A) and voltage (V) ratings on a charge controller tell you their limits; however, in order to find a solar panel setup that works within these limits, a lot of math needs to be done.
In short, this is because solar panels produce more power as the temperature gets colder outside.
What size charge controller do I need?
Below I have examples of charge controllers that pair with specific solar panel wattages from 50-400W. These numbers are based on the lowest estimated temperature outside being -40°F (-40°C).
I set the temperature very low because I wanted to make sure that my charge controller suggestions would work for most people. Your residence may never get to -40°F or even 0°F, but setting the temperature low allows for a buffer of safety so your charge controller can work even on the coldest days during the year.
I used a charge controller calculator from explorist.life to factor in this temperature for sizing the right charge controllers. I recommend using this for yourself for your own solar setup.
The calculations below are for 12V solar panels used with MPPT charge controllers.
- For a 50W solar panel, a 10A charge controller with a 30V minimum solar input is ideal for charging a 12V battery. Most 10A MPPT controllers are rated for much higher than 30V, so using one capable of 75-100V is typical.
- A 100W solar panel typically needs a 10A charge controller with a 30V minimum solar input to work with 12V batteries.
- A 150W (3x 50W) solar panel array can use a 20A charge controller for 12V batteries (10A for 24V batteries). Connecting the panels in series works for both 12V and 24V batteries as long as the MPPT is capable of handling 80V. A parallel solar panel connection should only be used with 12V batteries.
- A 200W (2x 100W) solar panel array should use a 20A MPPT charge controller for 12V batteries and can be connected in series or parallel. A 10A controller can be used with 24V batteries but should only be connected in series when using 12V solar panels.
- A 300W (3x 100W) solar panel array typically needs a 30A charge controller for 12V batteries and a 15A controller for 24V batteries. Connecting the panels in series works with both battery types as long as the MPPT can handle 90V of input. A parallel connection should only be used for 12V batteries.
- A 400W (4x 100W) solar panel array is best used with a 40A charge controller for charging 12V batteries. For 24V batteries, a 20A controller can be used with the panels in series. With both 12/24V batteries, a series connection should have an MPPT with at least a 120V input capability.
Below I have specific charge controllers that can be used for all of these solar panel setups, starting with ones for 12V batteries.
Sizing your charge controller for a 12V battery
My calculations below are based on the averages of three 50W solar panels and three 100W solar panels.
| Solar Panel Wattage | Solar Panel Array Max Voltage | Max Amps (Controller to Battery) | Minimum Charge Controller Sizing | Suggested MPPT Charge Controller |
|---|---|---|---|---|
| 50W | 26.18V | 5.93A | 10A, 30V | Victron 75|10 |
| 100W | 27.07V | 9.92A | 10A, 30V | Victron 75|10 |
| 150W (3x 50W) | Series: 78.53V Parallel: 26.18V | 17.78A | Series: 20A, 80V Parallel: 20A, 30V | Series & parallel: Renogy Rover 20A |
| 200W (2x 100W) | Series: 54.14V Parallel: 27.07V | 19.85A | Series: 20A, 60V Parallel: 20A, 30V | Series & parallel: Renogy Rover 20A |
| 300W (3x 100W) | Series: 81.20V Parallel: 27.07V | 29.77A | Series: 30A, 90V Parallel: 30A, 30V | Series & parallel: Victron 100|30 |
| 400W (4x 100W) | Series: 108.27V Parallel: 27.07V | 39.70A | Series: 40A, 120V Parallel: 40A, 30V | Series: Victron 150|45 Parallel: Renogy Rover 40A |
All data based on use with a 12V battery. Only 12V solar panels were used in this table. Data gathered from charge controller calculator from explorist.life with est. lowest temp. of -40 deg. F (-40 deg. C).
As you can see, both series and parallel connections work for the 150-400W examples above. But for a series connection with four 100W solar panels, the voltage is higher than 100V. Due to this, I recommend a larger 150V charge controller with 45A.
This MPPT compared to the others in the table is much more expensive. That being said, I recommend using a parallel connection if you decide to use 400W of solar.
Charge controller sizing with a 24V battery
Using 12V solar panels with 24V batteries requires you to use a series connection. This increases the voltage with every additional solar panel in your array while the amperage stays constant.
The opposite is true in parallel solar panel connections, where the amperage increases with every additional solar panel while the voltage stays the same.
A single 12V solar panel, as well as multiple 12V panels in parallel, do not work optimally for charging 24V batteries. This is because 24V batteries charge at a rate of 28.8-29.4V, not 24V. The maximum average voltage of the three 100W solar panels in my testing is 27.07V at -40°F, which is not sufficient to charge a 24V battery.
Below you’ll find my charge controller recommendations when using 12V solar panels connected in series to charge 24V batteries.
| Solar Panel Wattage | Solar Panel Array Max Voltage | Max Amps (Controller to Battery) | Minimum Charge Controller Sizing | Suggested MPPT Charge Controller |
|---|---|---|---|---|
| 150W (3x 50W) | 78.53V | 8.89A | 10A, 80V | EPEVER 10A 100V |
| 200W (2x 100W) | 54.14V | 9.92A | 10A, 60V | Victron 75|10 |
| 300W (3x 100W) | 81.20V | 14.89A | 15A, 90V | Victron 100|15 |
| 400W (4x 100W) | 108.27V | 19.85A | 20A, 120V | Victron 150|35 |
All data based on use with a 24V battery with solar panels connected in series. Only 12V solar panels were used in this table. Data gathered from charge controller calculator from explorist.life with est. lowest temp. of -40 deg. F (-40 deg. C).
As you can see, the minimum amperage rating of the charge controllers is exactly half that of the 12V batteries. This is because the MPPT controller simply allows twice the charging voltage into a 24V battery (28.8V vs. 14.4V) without needing to convert the volts into additional amps.
How to size your charge controller
A properly-sized charge controller needs to factor in the following from your solar panel:
- Total wattage (W)
- Open circuit voltage (Voc)
- Current at maximum power (Imp)
In addition, several solar panels have something called thermal characteristics. We need two pieces of information from here:
- Temperature coefficient of Voc
- Temperature coefficient of Pmax
The next area to look at is your battery’s voltage, which is typically 12V or 24V. This is important because 24V batteries need a higher voltage and fewer amps from a charge controller compared to a 12V battery.
The final step is to find out the lowest temperature your solar panels will be exposed to. Obviously, this is not precise, but you can estimate it. This is needed because when the temperature gets colder, the voltage from solar panels increases.
Since an MPPT controller uses excess volts and translates them into amps to charge your battery as efficiently as possible, too small of a controller can be ineffective or malfunction in cold weather.
In order for the charge controller to work effectively for years, we need to factor in the increased voltage that the panel produces in cold conditions.
With all of this information, we can estimate the correct charge controller size for not only a single solar panel, but for an entire solar panel array.
Collecting the data and using the charge controller calculator
Below are the solar panels I used to get the results shown in my previous tables. I used three 12V solar panels rated at 50W and did the same for 100W models.
| Solar Panel | Wattage | Source |
|---|---|---|
| Renogy RNG-50D-SS | 50W | Manual |
| SUNER POWER SP-50W-M | 50W | Amazon |
| HQST-50P | 50W | Amazon |
| Renogy RNG-100D-SS | 100W | Manual |
| RICH SOLAR RS-M100 | 100W | Manual |
| HQST HSP100D-L | 100W | Amazon |
I used the data provided for these models and averaged them out. I simply put the information into the charge controller calculator to get the averages at -40°. My results are bolded below.
50W Panel Avg:
- Avg. Open-Circuit Voltage (Voc) = 21.67V
- Avg. Maximum Power Current (Imp) = 3.26A
- Avg. Temperature Coefficient of Voc = -0.32%/°C
- Avg. Temperature Coefficient of Pmax = -0.41%/°C
- Avg. Max volts @ -40° = 26.18V
- Avg. Max amps @ -40° = 5.93A
100W Panel Avg:
- Avg. Open-Circuit Voltage (Voc) = 22.9V
- Avg. Maximum Power Current (Imp) = 5.28A
- Avg. Temperature Coefficient of Voc = -0.28%/°C
- Avg. Temperature Coefficient of Pmax = -0.38%/°C
- Avg. Max volts @ -40° = 27.07V
- Avg. Max amps @ -40° = 9.92A
For more information on the creator of the charge controller calculator as well as a visual breakdown of how the calculations work, check out the video below.
