Solar Sizing
Charge Controller Sizing Calculator
Size controller current and compare MPPT vs PWM
Updated July 12, 2026 · Live
What this tool does
Sizes the charge-controller current from array watts and battery voltage with a 1.25 safety factor, and explains the MPPT-versus-PWM trade-off.
Enter array watts, battery voltage and controller type; the calculator returns the minimum controller current with a safety factor applied.
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Formula Used
How the charge controller sizing works
Dividing array watts by battery voltage gives the working current, and a 1.25 factor adds the headroom standards expect for sustained bright conditions. The result is the minimum controller amperage. An MPPT controller tracks the panel’s maximum-power point and typically harvests more energy in cool or low-light conditions than a simpler PWM unit, which pulls the array down to battery voltage.
What you enter
Enter array watts, battery voltage and controller type; the calculator returns the minimum controller current with a safety factor applied. Every field is editable, and the result recalculates the moment you change a value.
Reading the result
Buy a controller rated at or above the amperage shown. Choose MPPT for higher-voltage strings and cool climates; PWM suits small, voltage-matched systems on a budget.
Safe by design
This calculator does the arithmetic of sizing — quantities, geometry and cost. It deliberately stops at the line a certified installer must sign off. It will never tell you a cable or fuse size, an earthing arrangement, a battery charge voltage, or a roof structural load: those are regulated calculations where a wrong number causes fire, electrocution or collapse. All electrical work must be carried out by a qualified, certified installer, and this page is not an installation guide.
Assumptions and limits
This sizes controller current only. It does not select fusing, cable size or disconnects between array, controller and battery — those are regulated and belong to a certified installer. String voltage in cold weather must also stay under the controller’s maximum input voltage; check that with the Array Configuration tool.
Using it with the rest of BuildMetricLab
This calculator is one step in planning a system. Pair it with the other solar sizing, geometry and cost tools to go from an energy figure to a panel count, a layout and a costed bill of materials. Every BuildMetricLab tool runs entirely in your browser — no sign-up, nothing sent to a server, and the formula is shown on the page so the maths can be audited.
Sources & methodology
Sizes the charge-controller current from array watts and battery voltage with a 1.25 safety factor, and explains the MPPT-versus-PWM trade-off. Every result is calculated from the values you enter, all inputs are editable, and no time-sensitive prices or tariffs are embedded.
Frequently asked questions
Why the 1.25 safety factor?
Bright, cool conditions can push panel output above its rating for sustained periods. The 1.25 factor gives the controller headroom so it is not run continuously at its limit.
MPPT or PWM — which should I choose?
MPPT extracts more from arrays whose voltage is well above the battery, and performs better in cool or low light. PWM is cheaper and fine for small, voltage-matched systems.
Can one controller run my whole array?
Only up to its current and voltage limits. Large arrays are often split across several controllers or higher-voltage strings — check the amperage and the maximum input voltage.
Does this replace the installer’s design?
No. It sizes the controller current; the protective devices, cabling and earthing around it are regulated work for a certified installer.
Spotted something off?
Calculations or display — let us know.