What Size Solar Charge Controller Do I Need? Amps and Volts Chart
Part of the project planner: Planning backup power, batteries and solar.
800 W of solar charging a 24 V battery bank is 33.3 A. Add a 25% margin and you need a 50 A MPPT charge controller. The controller also has to take the voltage of the panels in series: two panels of 24 V open-circuit voltage make 48 V, and in the cold they make more, so the controller's input must be rated for at least 60 V. The solar charge controller calculator works out both for your panels and battery.
Controller amps by array size
The charge current into the battery (the array watts divided by the battery voltage) and the standard controller size with a 25% margin:
| Array | 12 V battery | 24 V battery | 48 V battery |
|---|---|---|---|
| 100 W | 8.3 A: 15 A | 4.2 A: 10 A | 2.1 A: 10 A |
| 200 W | 16.7 A: 30 A | 8.3 A: 15 A | 4.2 A: 10 A |
| 400 W | 33.3 A: 50 A | 16.7 A: 30 A | 8.3 A: 15 A |
| 600 W | 50.0 A: 70 A | 25.0 A: 40 A | 12.5 A: 20 A |
| 800 W | 66.7 A: 100 A | 33.3 A: 50 A | 16.7 A: 30 A |
| 1,000 W | 83.3 A: over 100 A | 41.7 A: 60 A | 20.8 A: 30 A |
| 1,200 W | 100.0 A: over 100 A | 50.0 A: 70 A | 25.0 A: 40 A |
| 1,600 W | 133.3 A: over 100 A | 66.7 A: 100 A | 33.3 A: 50 A |
| 2,000 W | 166.7 A: over 100 A | 83.3 A: over 100 A | 41.7 A: 60 A |
| 3,000 W | 250.0 A: over 100 A | 125.0 A: over 100 A | 62.5 A: 80 A |
A higher battery voltage means a lower current for the same array, which is why large systems use 24 V or 48 V: the controller, the cables and the fuses are smaller.
Maximum input voltage by panel and string
The open-circuit voltage (Voc) of panels in series adds up, and grows when the panels are cold. The table shows the controller input rating needed, with a 25% cold-weather allowance:
| Panel | Voc each | 1 in series | 2 in series | 3 in series | 4 in series |
|---|---|---|---|---|---|
| 100 W "12 V" panel | 22 V | 28 V | 55 V | 83 V | 110 V |
| 200 W "12 V" panel | 24 V | 30 V | 60 V | 90 V | 120 V |
| 400 W 72-cell panel | 49 V | 61 V | 123 V | 184 V | 245 V |
| 60-cell home panel (300 W) | 40 V | 50 V | 100 V | 150 V | 200 V |
Take the Voc from the label on the back of the panel, not the Vmp. Use the lowest temperature your site sees: the allowance is about 12% in mild climates, 25% in cold ones and 40% in very cold ones.
Worked example
Six 400 W panels in two strings of three, on a 48 V battery bank, make 2,400 W. The charge current is 50 A, 62.5 A with the margin, so a 70 A controller. Each string of three 49 V panels makes 147 V, so the controller input must be rated for 184 V or more. If you had five panels instead of six, they would not divide evenly into strings of two (2.5 strings): parallel strings should have the same number of panels.
Common mistakes
- Sizing only on amps. A controller with enough amps but too low a PV voltage rating will fail the first cold sunny morning.
- Using Vmp instead of Voc. The open-circuit voltage is the higher figure, and the one that matters at start-up.
- Forgetting the battery voltage. The same panels need twice the amps on 12 V as on 24 V.
- Mixing different panels in one string. The weakest panel limits the string.
- No fuse or breaker. Fuse the controller output to the battery and follow the manual for the panel side.
MPPT or PWM?
An MPPT controller converts the extra panel voltage into extra current, so it gets more energy from the panels (often 10 to 30%), and allows long strings and thinner wire. A PWM controller simply connects the panel to the battery, so the panel and battery voltages must match (a 12 V panel for a 12 V battery), and its size is set by the short-circuit current of the array. PWM is cheaper and fine for small 12 V systems. See the solar panel calculator for how many panels you need, and the inverter size calculator for the other half of the system. These are estimates: follow the manufacturer's manuals and local electrical codes.
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, and 41.7 A with a 25% margin. On 12 V you need 100 A, and on 48 V 30 A.
How do I calculate the charge controller size?
Divide the array watts by the battery voltage and add a 25% margin. For 800 W on 24 V that is 33.3 A × 1.25 = 41.7 A, so the next standard size, 50 A.
Do I size a controller on volts or amps?
Both. The amp rating must cover the charge current, and the maximum input voltage must be above the string voltage when it is cold. Two panels of 24 V Voc in series make 48 V, so the controller must take at least 60 V.
Can I put more solar on a controller than its rating?
Many MPPT controllers allow more panel watts than the amps suggest and just clip the extra, but each has a maximum input power and a maximum input voltage. Never exceed the voltage: that can destroy it.
Is MPPT worth it?
For arrays above about 200 W, usually yes. MPPT gets 10 to 30% more energy from the same panels, and it allows high-voltage strings and thinner wire. PWM suits small 12 V systems where the panel and battery voltages match.
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.