Solar Charge Controller Calculator
What size controller, and whether PWM is quietly throwing away a third of your array.
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In short: Size MPPT on power (array watts divided by battery voltage) and PWM on current (array short-circuit current), both plus 25% per NEC 690.8. An 800W array on a 24V battery wastes about 34% through PWM, and 67% on a 12V battery.
MPPT controller
50A
41.7A after the 125% rule
PWM controller
30A
27.5A after the 125% rule
PWM actually delivers
528 W
PWM throws away
34%
Isc is on the label on the back of the panel. Both figures include the 125% continuous duty margin from NEC 690.8.
The difference, in watts
A PWM controller is close to a switch. It connects the array to the battery and holds it there, so the panels are dragged down to battery voltage and every volt above that is simply lost as heat. The current gets through; the surplus voltage does not.
An MPPT controller is a converter. It runs the array at its own best voltage and trades the surplus for extra current on the battery side. That is why the same array on the same battery delivers meaningfully more through MPPT — the energy that PWM discards is energy you already paid for when you bought the panels.
The gap widens as the mismatch grows. A 24V nominal panel on a 12V battery loses close to half. The controller is usually the cheapest line in the bill of materials, which is exactly why saving money there is such an expensive decision.
Sizing, properly
For MPPT, divide array watts by battery voltage, then add 25%. Power is the limit, because the controller converts.
For PWM, add up the short-circuit current of the parallel strings, then add 25%. Current is the limit, because the controller does not convert.
Use the wrong formula and the answer is wrong in an expensive direction: sizing MPPT on panel current buys far more controller than necessary, and sizing PWM on watts buys one that cannot carry the array.
Raise the voltage before you buy anything
If the system is still on paper, build it at 48V. The same array needs a quarter of the controller current it would at 12V, much thinner and cheaper cable, and loses far less in resistance along the way. It is the single cheapest decision in an off-grid design and it can only be made once.
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Questions
MPPT or PWM — which do I need?
MPPT, in almost every case worth calculating. PWM pulls the array down to battery voltage and loses everything above it. With the 800W array above on a 24V battery, PWM delivers about 528W and throws away roughly 34%. On a 12V battery the same array wastes about 67%. PWM only makes sense when the panel is genuinely matched to the battery voltage and the array is small enough that the saving on the controller exceeds the power lost.
Why are MPPT and PWM sized differently?
Because they work differently. PWM is essentially a switch: it passes the array current through to the battery, so you size it on current — the short-circuit current of the array. MPPT is a converter: it trades surplus voltage for extra current, so the limit is power, and you size it on watts divided by battery voltage. Sizing an MPPT controller on panel current is the usual mistake and it buys a controller far larger than needed.
Where does the 1.25 multiplier come from?
NEC 690.8. Solar output counts as a continuous load, so the conductors and the equipment are rated at 125% of it. It is not padding — it is what the code requires and what an inspector checks, and it is the difference between a controller that runs at its limit all afternoon and one that has headroom.
Can I use a controller bigger than I need?
Yes, and it is often sensible. A larger controller runs cooler, lasts longer, and leaves room to add panels later without replacing it. The reverse is not true: an undersized controller caps the whole array at its rating, so the panels you paid for sit there unable to deliver.
What about higher battery voltages?
Raising the battery voltage is the cheapest upgrade in off-grid. The same array on 48V rather than 12V needs a quarter of the controller current, far thinner cable, and wastes much less in resistance. If the system is being designed now rather than extended, design it at 48V.