Solar String Sizing
Optimize series and parallel panel configuration.
Watch: How to size a solar string without frying your inverter
[VIDEO PLACEHOLDER — embed a short walkthrough here]
String sizing is a math problem until it isn't, and the failure mode is expensive. A video showing a real string voltage reading on a cold morning next to the datasheet's temperature coefficient makes the risk tangible in a way a formula alone doesn't.
Suggested video topics to film or source for this page:
- Measuring open-circuit string voltage on a cold morning and comparing it to the calculated worst case
- Walking through a real inverter datasheet to find max input voltage and the MPPT window
- A side-by-side of series vs. parallel wiring on a partially shaded array
- Diagnosing a charge controller that shut down from undervoltage on a hot afternoon
What is solar string sizing?
Solar string sizing is the process of working out how many photovoltaic panels can be wired in series into a single inverter input. It's arguably the most critical step in designing a solar array, because getting it wrong doesn't just mean lost efficiency, it can mean destroyed equipment.
Connect too many panels, and the voltage on a cold winter morning can exceed the inverter's maximum input rating, destroying the electronics almost instantly. Connect too few, and the voltage on a hot summer day can drop below the inverter's minimum operating range (its MPPT window), shutting the system down right when the sun is out and generating the most power.
How to calculate max string size
The calculation hinges on the temperature coefficient of Voc. Panel ratings are measured at Standard Test Conditions (STC, 25°C), but open-circuit voltage rises as temperature drops, sometimes by a meaningful margin on a cold morning.
1. Calculate max voltage (coldest day)
This sets the maximum safe string length:
Voc_max = Voc_stc × [1 + (Tempmin - 25) × (Coeff / 100)]
Worked example: a panel rated at 40V Voc with a -0.3%/°C coefficient, at -10°C:
| Step | Calculation | Result |
|---|---|---|
| Adjusted Voc | 40V × [1 + (-10 - 25) × (-0.003)] | 40V × 1.105 = 44.2V per panel |
| Max series count | floor(500V / 44.2V) | 11 panels max |
2. Calculate min voltage (hottest day)
This ensures the string stays above the inverter's turn-on threshold, so the system doesn't shut down on a hot day when panel voltage sags:
Vmp_min = Vmp_stc × [1 + (Tempmax - 25) × (Coeff / 100)]
Compare this result against the inverter's minimum MPPT voltage to find the shortest string length that still keeps the system running.
Practical applications
| Use case | Typical voltage limits |
|---|---|
| Grid-tie inverters (SMA, Fronius, and similar) | High voltage limits, often 600V in the US or 1000V in the EU, which allows longer series strings |
| Off-grid charge controllers (Victron, Outback, and similar) | Much stricter limits, often 150V or 250V. Exceeding this is the most common cause of controller failure |
Frequently asked questions
Series vs. parallel, which is better?
Series wiring raises voltage while keeping current low, which reduces the wire gauge needed and cuts voltage drop over long cable runs, making it the more efficient choice for most installations. Parallel wiring raises current but keeps voltage low, and it holds up better if panels are partially shaded, since one shaded panel in a parallel group won't drag down the output of the whole array the way it can in a series string.
Does cloud cover affect voltage?
Not much. Voltage stays relatively steady even in low light, it's current that drops, and it drops roughly in proportion to sunlight intensity. That's why string sizing is done against temperature extremes rather than cloud cover: temperature is what pushes voltage to its limits, not light levels.
What is the temperature coefficient?
It's a datasheet value showing how much a panel's voltage shifts per degree Celsius of temperature change. It's usually negative, something like -0.3%/°C, which means voltage rises as the panel gets colder and falls as it gets hotter.
What happens if I exceed the inverter's max voltage rating?
The inverter's input circuitry can be damaged instantly, and in some cases this isn't covered under warranty since it's considered an installation error rather than a defect. This is why the coldest expected temperature at the install site, not just the STC rating, has to drive the max string length calculation.
Do I need to size strings differently for a hybrid or off-grid system versus grid-tie?
The underlying math is the same, but the safety margins tend to matter more off-grid. Grid-tie inverters generally have wider voltage windows and more forgiving MPPT ranges, while many off-grid charge controllers have tighter limits and less tolerance for a string that drifts outside the MPPT window on a hot or cold day.