Sizing a solar plant is an energy problem before it is an equipment problem. You start from the energy a site consumes, decide how much of it solar should supply, apply a realistic view of losses, and only then convert that into kilowatt-peak, modules and inverters.
This guide sets out the full chain — load, solar resource, performance ratio, required DC capacity, module count, inverter capacity and DC/AC ratio — with the equations and worked numbers behind each step.
Run the whole chain automatically with the PV System Size Calculator.
What PV System Sizing Actually Means
"System size" is ambiguous unless you say which of these you mean:
| Quantity | Symbol | Meaning |
|---|---|---|
| Required DC capacity | P_req | Array capacity the energy balance demands |
| Installed DC capacity | P_dc | Modules actually bought: N × module wattage |
| Inverter AC capacity | P_ac | Sum of the inverter nameplate AC ratings |
| DC/AC ratio | R | P_dc ÷ P_ac |
| Expected generation | E | kWh produced per day, month or year |
A quoted "100 kW system" may mean 100 kWp of modules, or 100 kW of inverters, or both. Every serious design states DC and AC separately.
Step 1 – Establish the Energy Requirement
A household using 20 kWh/day that wants an 80% offset needs 16 kWh/day from PV, or 5,840 kWh/year.
Where the load number comes from. Twelve months of utility bills is the only reliable source. An appliance-by-appliance estimate is acceptable for off-grid design where no meter history exists, but expect it to be 10–30% optimistic.
Step 2 – Quantify the Solar Resource
A site receiving 5.4 kWh/m²/day in the plane of array has 5.4 PSH. It is not the number of daylight hours, and it is not the same number as horizontal irradiation (GHI) unless the array is flat.
Two things matter here:
- PSH must be for the tilt and azimuth you will actually build. A 15° tilt facing 30° west of south does not receive the same energy as an optimally tilted array.
- Regional averages are not site data. Use TMY files, NASA POWER, Meteonorm or Solargis for the project coordinates.
If you have no site-specific data, enter a manual PSH value and treat the result as conceptual. Actual PV generation depends on location, orientation, tilt, weather, temperature, shading, system losses and other site conditions.
Specific yield already contains the losses, so you must not also apply a performance ratio in this mode. Use one method or the other, never both.
Step 3 – Performance Ratio and the Loss Chain
Performance Ratio (PR) is the fraction of the theoretical STC-rated energy that reaches the meter. It is defined in IEC 61724-1 and is the single most abused number in solar sizing.
Worked example with typical values:
| Loss | Value | Factor |
|---|---|---|
| Temperature | 8.0% | 0.920 |
| Soiling | 3.0% | 0.970 |
| Mismatch | 2.0% | 0.980 |
| DC cable | 1.5% | 0.985 |
| Inverter | 2.2% | 0.978 |
| AC cable | 1.0% | 0.990 |
| Availability | 1.0% | 0.990 |
| Shading | 2.0% | 0.980 |
| LID / IAM / spectral | 2.5% | 0.975 |
The arithmetic sum of the same losses is 23.2%, which would suggest PR = 0.768. The multiplicative result, 78.4%, is the technically correct one; the sum is a rough approximation that always overstates the loss.
Do not double-count. If inverter loss is inside PR, do not divide the result by inverter efficiency again. The same applies to cable losses — count DC cable loss once, AC cable loss once.
Default assumption. When no detailed loss data exists, 75–80% is a defensible engineering assumption for a well-designed grid-tied plant in a warm climate, and 80–84% in a cool one. Label it as an assumption, not a guarantee.
The full breakdown of every loss term is covered in the PV performance ratio guide.
Step 4 – Required DC Capacity
For off-grid and hybrid systems, energy that cycles through the battery pays a round-trip penalty. Divide by the round-trip and battery-inverter efficiencies for the fraction of energy that is stored — the calculator applies a system factor for this and shows it explicitly.
A hidden margin baked into a "conservative" PR is bad practice, because it cannot be audited later.
Step 5 – Module Count and Installed Capacity
For 4.08 kWp with 550 Wp modules: 4,080 ÷ 550 = 7.42 modules → 8 modules → 4.40 kWp installed.
Rounding always overshoots. On small systems that overshoot can be 10–30%, which is why a lower module wattage sometimes fits a small roof requirement more precisely. Work through more examples in How Many Solar Panels Do I Need?.
Step 6 – Inverter Sizing and DC/AC Ratio
At 4.40 kWp and a 1.20 ratio: 4.40 ÷ 1.20 = 3.67 kW AC.
Then convert the theoretical figure into a real configuration. For 2,521 kW of AC requirement, 8 × 320 kW and 10 × 250 kW are both valid; no single configuration is universally correct, and the choice depends on MPPT count, block layout and O&M strategy.
| Ratio | Interpretation |
|---|---|
| Below 1.00 | DC undersizing relative to AC capacity; inverters rarely reach rated output |
| 1.00–1.40 | Typical conceptual range; final selection depends on clipping analysis and economics |
| Above 1.40 | High — verify inverter loading, clipping, MPPT and thermal limits |
| Above 1.50 | Deliberate, analysed decision only; expect significant clipping in sunny climates |
Oversizing DC against AC harvests more morning and evening energy at the cost of midday clipping. It is a project economics decision, not a fixed rule. Size individual inverters in detail with the Inverter Sizing Calculator.
Step 7 – Expected Generation
For 4.40 kWp, 5 PSH, PR 0.784: 17.25 kWh/day, or 6,295 kWh/year.
If monthly PSH data exists, calculate month by month — an annual average hides the winter deficit that determines battery sizing and off-grid feasibility. If it does not exist, do not invent monthly values.
Step 8 – Energy Coverage Check
Because of module rounding and any design margin, real coverage is usually slightly above the target. On small systems it can be far above it — that is a physical consequence of whole modules, not an error.
On-grid, Off-grid and Hybrid Sizing
On-grid. Size on annual energy. The grid absorbs the daily mismatch, so average PSH is adequate. Net-metering caps and sanctioned load often limit exportable AC capacity more than the roof does.
Off-grid. Size on the worst irradiation month, not the annual average, and add the battery round-trip penalty. An array that meets the annual average will fail in December. Pair the sizing with the Battery Bank Calculator.
Hybrid. Somewhere between the two: the grid covers shortfalls, the battery covers outages, and only the stored fraction of energy pays the round-trip penalty.
Common PV Sizing Mistakes
- Using GHI instead of plane-of-array irradiation, which understates a tilted array's yield.
- Adding losses arithmetically instead of multiplying loss factors.
- Counting inverter efficiency twice — once inside PR and once as a separate divisor.
- Sizing an off-grid array on annual average sun hours.
- Choosing an inverter from a catalogue first, then reporting whatever DC/AC ratio falls out.
- Reporting generation from the required capacity rather than the installed capacity.
- Treating a PR of 85%+ as achievable in a hot, dusty climate without tracking and frequent cleaning.
Why kWh ÷ PSH Alone Is Misleading
The one-line formula kWh ÷ PSH gives the array size a *lossless* plant would need. Real plants lose a fifth of that energy to heat, soiling, wiring, conversion and downtime, so the one-line answer is roughly 20–25% too small. It also ignores rounding to whole modules, inverter capacity, clipping and the seasonal profile of both resource and load.
When You Need a Full Simulation
This calculator produces a preliminary, energy-based conceptual size. For financing, EPC contracts or generation guarantees you need a site-specific hourly simulation (PVsyst, SAM or Solargis) using measured meteorological data, a 3D shading scene, the exact module and inverter models, and the real cable layout. Preliminary sizing tells you what to price; simulation tells you what to guarantee.
Related Calculators
- PV System Size Calculator — the full chain described above
- Number of Panels Calculator — module count and string layout
- Inverter Sizing Calculator — AC capacity and MPPT checks
- Annual Energy Yield Calculator — generation from an existing capacity
- Performance Ratio Calculator — PR from measured data
- DC Cable Sizing and AC Cable Sizing
- Voltage Drop Calculator — check drop on any run
- Solar ROI & Payback Calculator