How this calculator works
A charge controller sits between the panels and the battery. A modern MPPT controller turns the higher panel voltage into the battery voltage, so the current into the battery is the array power divided by the battery voltage. The controller must handle that current with some margin (we use 25%), and its maximum input voltage must stay above the voltage of the panels wired in series, which rises in cold weather. Panels in series add their voltages and keep the current, and strings in parallel add their currents.
Worked example: four 200 W panels make 800 W. On a 24 V battery that is 33.3 A, 41.7 A with the margin, so a 50 A controller. On 12 V it is 66.7 A and a 100 A controller, and on 48 V it is 16.7 A and a 30 A one. Two 24 V Voc panels in series make a 48 V string, which needs a controller rated for at least 60 V in a cold climate. A single panel on 12 V needs 15 A. Six 400 W panels in strings of three on a 48 V bank make 2,400 W and need a 70 A controller rated for 184 V. Five panels do not divide into strings of two (2.5 strings).
Charge controller size by array watts (MPPT, 25% margin)
Amps for 12 V, 24 V and 48 V battery banks.
| Array | 12 V battery | 24 V battery | 48 V battery |
|---|---|---|---|
| 200 W | 30 A | 15 A | 10 A |
| 400 W | 50 A | 30 A | 15 A |
| 600 W | 70 A | 40 A | 20 A |
| 800 W | 100 A | 50 A | 30 A |
| 1,000 W | over 100 A | 60 A | 30 A |
| 1,500 W | over 100 A | 80 A | 40 A |
| 2,000 W | over 100 A | over 100 A | 60 A |
| 3,000 W | over 100 A | over 100 A | 80 A |
Tips before you buy
- Size the controller on the charge current, which is the array watts divided by the battery voltage, with a margin of 25%.
- The input voltage matters as much as the amps: the string voltage in the cold must stay under the controller's maximum PV voltage, or it can be destroyed.
- Wire panels in series to raise the voltage and cut the current, which allows thinner wire over a long run.
- A higher battery voltage means a smaller controller for the same array, so a 24 V or 48 V bank can charge from more panels.
- Put a breaker or fuse between the controller and the battery, and between the panels and the controller where the manual says.
Tools & materials for this project
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Frequently asked questions
What size charge controller do I need for 800 watts of solar?
On a 24 V battery bank, a 50 A MPPT controller (33.3 A, 41.7 A with the margin). On a 12 V bank you need a 100 A controller, and on 48 V 30 A.
How do I calculate charge controller amps?
Divide the array watts by the battery voltage, then add a 25% safety margin. 800 W on a 24 V battery is 33.3 A, so 41.7 A with the margin.
What is the maximum input voltage for the controller?
The sum of the open-circuit voltages of the panels in series, times a cold-weather allowance. Two 24 V Voc panels make 48 V, and with a 25% allowance the controller needs a rating above 60 V.
Is a 12V charge controller okay for a 24V system?
Only if the controller says it is rated for both voltages. Most MPPT controllers auto-detect 12 and 24 V (and some 48 V), but a controller made for 12 V only will be damaged.
Should I use MPPT or PWM?
MPPT controllers get more energy from the panels (often 10 to 30% more), and allow higher-voltage strings and thinner wire. PWM controllers cost less and suit small 12 V systems where the panel and battery voltages match.
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