How to Size a Solar Inverter: DC/AC Ratio and Strings
How to size an inverter against a real array: the DC/AC ratio and why oversizing is standard, what clipping actually costs, and how to check cold-morning Voc, hot-afternoon Vmp and per-MPPT current, with a worked example.
Sizing starts with the array
Sizing an inverter means finding a device whose four DC-side limits all accept the array you have drawn, while its AC rating matches what you are allowed to export. Those limits are the MPPT voltage range, the absolute maximum DC input voltage, the number of MPPT channels, and the maximum input current per channel. Get one wrong and the system either fails to commission or quietly under-produces for twenty years. The right answer also depends on latitude, orientation, and whether you optimise for annual kWh or peak export.
The DC/AC ratio, and why oversizing is normal
The DC/AC ratio is the array's rated DC power divided by the inverter's nominal AC power. A 1:1 ratio sounds intuitive and is almost always wrong.
An array reaches its rated output only at 1,000 W/m² with a 25 °C cell temperature — conditions that occur a handful of hours a year, and never on the hot afternoons when irradiance peaks but cell temperature is 60 °C or more. For most of the year the array delivers well under its rating, and an inverter sized to the nameplate spends its life at part load, where conversion efficiency is lowest. Oversizing pushes it into its efficient band for more hours and raises morning, evening and winter production, which make up most of the annual total. A ratio of 1.1 to 1.3 is standard practice.
Datasheets acknowledge this with a second AC figure. The Huawei SUN2000-3-10KTL-M1 is 10,000 W nominal against an 11,000 W maximum; the Huawei SUN2000-50KTL-M3 is 50,000 W against 55,000 W; the GoodWe UT SERIES is 320,000 W against 352,000 W. Each reserves 10% above the continuous rating. That is not licence to oversize further — it is what allows the nominal rating to be met when the unit is hot.
What clipping actually costs
When DC output exceeds what the inverter can convert, the inverter moves off the maximum power point and holds its output at the AC limit. The excess is simply not converted — but clipping is bounded by how rarely the array reaches full output. At a ratio of 1.2 in a temperate climate it typically removes under 1% of annual yield, because only the top few hundred hours of the year trigger it. At 1.3 the figure rises but stays small in most locations; past about 1.4 the losses start to outweigh the part-load gains. The crossover depends on the site and is worth modelling rather than guessing.
Clipping also flattens the production curve. On an export limit, or where grid capacity is billed by peak kW, a ratio that clips slightly is not a loss at all — it is what you want.
Latitude and orientation move the target
| Situation | Sensible DC/AC ratio | Reason |
|---|---|---|
| High latitude, low winter sun | 1.25–1.35 | The array rarely approaches its rating |
| Mid latitude, south-facing, fixed tilt | 1.15–1.25 | The standard case; a broad midday peak |
| Low latitude, high irradiance | 1.05–1.15 | Full output is reached often; clipping bites early |
| East–west split array | 1.3–1.45 | Two peaks that never coincide; output is flattened already |
| Hard export limit at the meter | Higher, deliberately | Clipping enforces the limit; off-peak production rises |
The east–west array is the clearest case. Neither half peaks at the same time, so the combined curve never reaches the sum of the two ratings, and a ratio that would clip badly on a south-facing roof barely clips at all.
Matching string voltage to the MPPT window
This is where sizing most often goes wrong, and the failure is seasonal, so it may not appear until months after commissioning.
Two limits apply. The MPPT voltage range is the band in which the inverter can track; below the lower bound it cannot operate. The maximum DC input voltage is an absolute limit, and it is checked against open-circuit voltage on the coldest morning of the year, not against operating voltage.
Module voltage has a negative temperature coefficient, typically −0.25 to −0.30 %/°C. Cold raises voltage; heat lowers it.
- Cold-morning Voc sets your maximum string length. At a −10 °C cell temperature, a module with a Voc of 41.5 V and a coefficient of −0.27 %/°C rises to about 45.4 V — some 9.5% above rated, multiplied by every module in the string.
- Hot-afternoon Vmp sets your minimum string length. At 65 °C the same module's maximum-power voltage falls by around 11%, and a short string can drop below the window and stop tracking during exactly the hours you wanted production.
The windows vary widely. The Fronius Symo GEN24 3.0 Plus tracks 125–800 V with a 1,000 V ceiling and tolerates almost any string length. The Fronius Primo GEN24 5.0 Plus tracks only 230–530 V with a 600 V ceiling. The Deye Sun7 7 5 8 9 10 10 5Kg tracks 70–500 V with a 550 V ceiling — a low floor, but only 50 V of margin above the tracking range, so the cold check needs care. The Victron Inverter Rs Smart Solar tracks 80–450 V against a 450 V maximum.
Per-MPPT current limits
Each MPPT channel has a maximum input current, and paralleling strings on one channel adds their currents together. Check short-circuit current, not operating current; it rises slightly above 1,000 W/m².
Channel count and current limit together decide how you can split an array. The Deye Sun40 45 50Kg04 has four MPPT channels at 40 A each; the Huawei SUN2000-50KTL-M3 has four at 30 A; the GoodWe DNS G4 has two at 26 A; the Solis S6-GR1P is limited to 16 A. A 30 A channel takes two typical strings comfortably and three not at all.
A worked example
A three-phase site with a 22 kWp roof, using the Deye Sun18 20 22 23 25Kg05 — 20,000 W nominal AC, 22,000 W maximum, MPPT range 200–1000 V, 1,100 V maximum DC input, 2 MPPT channels, 26 A per channel.
Assume 48 modules of 460 W, each with Voc 41.5 V, Vmp 34.5 V, Isc 11.6 A and a voltage coefficient of −0.27 %/°C, arranged as four strings of twelve, two strings per MPPT channel.
| Check | Calculation | Result | Limit | Pass? |
|---|---|---|---|---|
| DC/AC ratio | 48 × 460 ÷ 20,000 | 1.10 | 1.1–1.3 target | Yes |
| Voc, coldest morning (−10 °C) | 41.5 × 1.095 × 12 | 545 V | 1,100 V max DC | Yes |
| Vmp, standard conditions | 34.5 × 12 | 414 V | 200–1000 V window | Yes |
| Vmp, hottest afternoon (65 °C) | 34.5 × 0.892 × 12 | 369 V | 200 V floor | Yes |
| Current per MPPT | 11.6 × 2 strings | 23.2 A | 26 A per channel | Yes |
| AC output when clipping | — | 20,000 W | 22,000 W max | Yes |
Every check passes with margin. Now break it. Three strings on one channel gives 34.8 A against a 26 A limit — not permitted, whatever the total power works out to. Fourteen modules per string raises cold Voc to 636 V, still safe here, but on a 600 V inverter such as the Fronius Primo GEN24 5.0 Plus it would exceed the DC maximum. Eight modules drops hot Vmp to 246 V, uncomfortably close to the 200 V floor. Note also the 1,100 V DC ceiling against a 1,000 V tracking top: that 100 V gap exists to absorb cold-morning Voc rise, so never plan strings that use it as working voltage.
Before you order
- Recalculate Voc at the site's record low temperature, not the seasonal average.
- Recalculate Vmp at a cell temperature of 65–70 °C, not at ambient.
- Check current per channel against Isc, with strings paralleled as actually wired.
- Confirm the DC/AC ratio against your export limit as well as your yield target.
- Confirm the MPPT channel count covers every orientation and shading zone.
All of these limits are listed on every model page. Start from how to choose a solar inverter if you have not settled on a topology, or MPPT explained for the tracking mechanism.