SMA String Sizing: The Cold-Weather Rule 2023's GW Sales Numbers Don't Tell You

The short version

If you are doing SMA string sizing, don't start with the module nameplate. Start with the coldest temperature your site can reach. A solar module's open-circuit voltage rises when the temperature drops. If the combined string voltage crosses the inverter's max input, the inverter will shut down on a clear winter morning. In my experience, that is the most common emergency callback in commercial solar—and it's rarely a product defect.

SMA inverter sales in 2023 were roughly 12 GW worldwide, according to SMA's published annual results. That tells you how many systems use SMA hardware. It doesn't tell you whether each string was calculated for cold weather. So here's the full article in one breath: recalculate Voc at the record low for the site, leave at least 2% margin, and keep the string under the inverter's absolute maximum PV voltage. If you only remember one step, make it that one.

Why I have a bias

I'm a solar design and commissioning engineer. For the past eight years, I've been brought into late-stage projects when someone realizes the deadline is closer than the design review. I don't write long proposals; I check voltages, confirm the MPPT range, inspect the DC wiring, and figure out why a string isn't producing. More than 50 of those calls involved SMA string inverters, usually with the same root cause: someone sized the strings at 25°C while the site sits somewhere much colder.

In March 2024, I got a call from a contractor with 36 hours before the final utility inspection. Their system had thrown overvoltage warnings on two cold mornings. The module count was inside the datasheet maximum, but the datasheet assumption was 25°C. When I re-ran the calculation at -16°C using the module's actual temperature coefficient, the string was over the inverter limit by about 20V. That's not a big number. It is the difference between reliable operation and an expensive service visit.

What SMA's 2023 GW sales don't tell you

When a supplier quotes SMA's 2023 GW record to justify a proposal, I take it as a market signal, not an engineering answer. According to SMA, inverter sales in 2023 were in the region of 12 GW. But every GW is a mix of small string inverters, large commercial central inverters, and storage inverters. The thing they all share is that the DC input voltage range must match the PV array. If the voltage range doesn't match, the inverter won't make power when it's cold, and it may not protect itself as intended.

Some of the smartest conversations I have with clients start when I say: show me the cold-temperature calc. If nobody can point to it, don't assume it exists.

The string sizing math I use under deadline pressure

1. Get Voc at STC from the module datasheet.
2. Get the temperature coefficient of Voc. It is usually written as a negative percentage, like -0.30%/°C.
3. Choose the lowest expected air temperature for the site. Do not use an average January number unless you want to learn the meaning of the word record low.
4. Calculate the cold Voc per module using this formula: Voc_cold = Voc_STC × (1 + (T_low − 25°C) × TC), where TC is the temperature coefficient expressed as a decimal, such as -0.0030 per °C.
5. Multiply by the number of modules in series.
6. Keep that final number at least 2% below the SMA inverter's absolute max PV input voltage. If it is above, reduce the string length or redesign the combiner.

If you're short on time, SMA's Sunny Design tool handles most of this automatically. But it is only as good as the data you give it. I still manually check the module temperature coefficient and the site temperature file. To be fair, Sunny Design catches many mistakes. It cannot catch a microclimate that the weather data doesn't know about.

Here's a typical case: a module with Voc_STC = 49.8V and a Voc coefficient of -0.30%/°C. At -20°C, the module voltage becomes 49.8 × (1 + (-45 × -0.003)) = 56.5V. Eighteen modules in series gives 1,017V. On a 1,000V SMA inverter, that's over the limit. Drop to 17 modules in series, and the number is about 961V. That margin is not waste. It's the difference between a system that starts on a cold morning and one that doesn't.

Cold voltage gets most of the attention, but don't ignore the hot side. At high cell temperatures, Vmp falls. If the string voltage at the site's expected maximum temperature drops below the inverter's lower MPPT limit, energy production in hot months will be poor. I check both ends before releasing any design.

How Do Wind Turbines Transfer Energy? Same Pattern, Different Input

People who search for how do wind turbines transfer energy are usually asking about something that feels different from solar. The short answer: the wind transfers kinetic energy to the rotor blades, the blades spin a shaft, the shaft drives a generator directly or through a gearbox, and the generator transfers the mechanical energy into electrical energy. In a modern variable-speed turbine, that electrical energy passes through a power converter before it can go onto the grid at a stable frequency and voltage.

Once you see that pattern, it lines up with a PV system. In a PV system, the modules are the energy catcher and the SMA inverter is the power converter. In a wind turbine, the rotor is the energy catcher and the converter is the part that makes the electricity usable. In both cases, the converter has limits: input voltage, current, frequency, temperature. The concept of string sizing is solar-specific, but the habit of respecting converter limits is universal.

What About 300W Car Power Inverters and the Jackery 1000 Plus?

Different problem. If you're reading 300W car power inverter reviews, you're shopping for a device that takes 12V DC from a car and outputs AC. There isn't a string of PV modules to calculate. The ratings to look for are continuous power, surge power, and actual DC input range. I'd also check whether 300W is the continuous rating or just peak power; many inexpensive units advertise peak. And a cigarette lighter socket is not an unlimited copper bus.

Jackery 1000 Plus solar generator reviews are useful in a completely different way. That product is an all-in-one battery, charge controller, MPPT, and inverter in one box. You don't size SMA strings for it; you size the load. I get why people compare it with an inverter like a Sunny Boy, but one is a grid-tie component and the other is a portable energy system. The Jackery reviews tell you if the capacity matches your fridge or CPAP; they won't tell you whether your permanent solar array is safe at -20°C.

The Unspoken Cost in Every Quote

I have learned to ask what's NOT included before asking about the price. The vendor that lists all fees upfront can look expensive. The vendor that leaves the cold-temperature calc, string-sizing review, and commissioning assumptions out will cost more later. That extra cost usually appears as an emergency change order when the inspector asks a question or the inverter faults at sunrise.

This is why I support transparent pricing. Not because I'm trying to make every quote look the same, but because hidden assumptions are where projects die. The same transparency should extend to environmental claims in marketing. Per the FTC Green Guides (ftc.gov, 16 CFR Part 260), claims like renewable or wind-powered have to be substantiated. If you can't prove it in the design, don't put it in the brochure.

Where I Admit the Limits

None of this makes sense for a 12V car inverter or a portable generator. Those products are not tied to a utility grid, they are not usually exposed to -20°C winter mornings, and they do not require the same voltage math. Their equivalent is simpler: match the load to the output and the input to a charging source. And a wind turbine has a drivetrain that solar installers do not need to calculate—until it reaches the converter, where voltage and frequency limits become just as real.

Even with grid-tied solar, I do not know every site's weather extreme. That uncertainty is why I keep a voltage safety margin instead of designing right up to the red line. If someone tells you a system is fine because it's within the datasheet, ask which datasheet temperature they used. If they hesitate, treat it as a red flag and not a sales objection.