Introduction #

This guide is for electrical engineers, facility managers, and EPC contractors who need to size transformers for solar photovoltaic (PV) systems. It solves the problem of selecting transformer kVA when load includes PV inverters with harmonic content, bidirectional power flow (grid-tied systems), and variable generation profiles. Use this knowledge when specifying transformers for solar farms, commercial rooftop PV, or industrial facilities integrating renewable energy.

For the overall sizing process and formulas, see the Transformer Sizing Guide. After sizing, verify with the Transformer Size Calculator.

Solar System Load Characteristics #

Solar PV systems differ from conventional loads in several ways that affect transformer sizing:

Inverter Output Characteristics #

PV inverters convert DC from solar panels to AC for grid connection or local consumption. Modern inverters use power electronics (IGBTs, MOSFETs) that produce non-sinusoidal current waveforms with harmonic distortion. Total harmonic distortion (THD) typically ranges from 3–5% for high-quality inverters to 8–15% for older or lower-cost units. The harmonic spectrum is dominated by odd-order harmonics (3rd, 5th, 7th, 11th, 13th), which increase RMS current and transformer heating.

Power Variability #

Solar generation varies with irradiance, weather, and time of day. Peak generation occurs during midday under clear conditions; output drops to near zero at night and during heavy cloud cover. Transformer sizing must account for peak generation capacity, not average output. For grid-tied systems, the transformer must handle both maximum PV output and maximum facility load (which may occur at different times).

Bidirectional Power Flow #

In grid-tied systems, power flows from PV to grid during generation periods and from grid to load during consumption periods. The transformer must be sized for the maximum power flow in either direction. For systems with net metering, the transformer may need to handle reverse power flow equal to or exceeding the facility's peak load.

Power Factor Characteristics #

Modern PV inverters typically operate near unity power factor (0.98–1.0) when exporting to grid, but may operate at lower power factor (0.85–0.95) when supplying local load with reactive power requirements. The effective power factor for transformer sizing depends on the mix of PV output and facility load.

Inverter Harmonic Considerations #

PV inverters produce harmonics that increase transformer losses and reduce effective capacity. The harmonic content depends on inverter topology, filter design, and operating conditions.

Harmonic Spectrum #

Typical PV inverter harmonic characteristics:

Harmonic Order Typical Magnitude (% of fundamental) Notes
3rd 0.5–2% Usually filtered by delta-wye transformer
5th 2–5% Most significant after fundamental
7th 1–3%
11th 0.5–2%
13th 0.3–1.5%
THD (total) 3–8% (high-quality), 8–15% (standard) Depends on inverter quality

For transformer sizing, use measured THD when available, or assume 5–8% THD for modern string inverters and 8–12% for central inverters without active filtering.

K-Factor vs Harmonic Derating #

Two approaches are used for harmonic-rich solar loads:

1. Standard transformer with harmonic derating
Apply derating factor k_harmonic = 0.90–0.95 for moderate THD (5–8%), or 0.85–0.90 for higher THD (8–12%). Minimum nameplate kVA = Required kVA ÷ k_harmonic.

2. K-factor rated transformer
Select K-4 or K-13 transformer sized for required kVA. K-4 is typically sufficient for PV systems with modern inverters; K-13 may be needed for older inverters or systems with additional non-linear load. When K-factor transformer is correctly selected, no additional harmonic derating is applied.

For detailed harmonic sizing methodology, see Transformer Sizing for Harmonic Loads.

Transformer Sizing Methodology for Solar Systems #

Capacity Selection: Peak vs Average #

Transformer sizing must account for peak conditions, not average generation:

  • Grid-tied systems: Size for maximum of (peak PV output, peak facility load, or peak reverse power flow if net metering allows significant export).
  • Off-grid systems: Size for peak load plus any battery charging requirements.
  • Hybrid systems: Size for peak combined load (PV + grid + battery discharge).

Do not size based on average daily generation or capacity factor; transformers must handle peak power without overload.

Grid-Tied System Sizing Formula #

For grid-tied solar systems:

Required kVA = max(PV_kVA, Load_kVA, Reverse_kVA) × (1 + Safety margin)
Minimum nameplate kVA = Required kVA ÷ (k_harmonic × k_temp × k_altitude)

Where:

  • PV_kVA = Peak PV output (kW) ÷ inverter power factor (typically 0.98–1.0)
  • Load_kVA = Peak facility load (kW) ÷ load power factor
  • Reverse_kVA = Maximum reverse power flow (if net metering allows export exceeding load)
  • Safety margin = 1.15–1.25 (15–25%) for growth and contingencies
  • k_harmonic = Harmonic derating factor (0.90–0.95 typical for PV)
  • k_temp, k_altitude = Temperature and altitude derating (if applicable)

Then round up to next standard transformer size.

Off-Grid and Hybrid Systems #

For off-grid systems, transformer sizing must account for:

  • Peak load demand
  • Battery charging requirements (if PV charges batteries)
  • Inverter efficiency losses
  • Harmonic content from inverters

Formula:

Required kVA = (Peak_load_kW ÷ PF_load) + (Battery_charging_kW ÷ PF_inverter) × (1 + Safety margin)

For hybrid systems with grid connection, size for peak combined load (PV + grid + battery discharge) and ensure transformer can handle bidirectional power flow.

Example: 500 kW Solar Farm Transformer Sizing #

Given:
500 kW DC solar array, 480 V AC output. String inverters with 5% THD. Grid-tied system with net metering allowing export up to 500 kW. Facility peak load 200 kW at 0.90 PF. Safety margin 20%. Standard ambient temperature and altitude.

Step 1 – PV output kVA:
PV AC output ≈ 500 kW (assuming inverter efficiency 98% and unity PF)
PV_kVA = 500 ÷ 1.0 = 500 kVA

Step 2 – Facility load kVA:
Load_kVA = 200 ÷ 0.90 = 222.2 kVA

Step 3 – Reverse power flow (net export):
Maximum export = 500 kW (PV output exceeds load)
Reverse_kVA = 500 ÷ 1.0 = 500 kVA

Step 4 – Required kVA (maximum of the three):
Required kVA = max(500, 222.2, 500) × 1.20 = 500 × 1.20 = 600 kVA

Step 5 – Harmonic derating:
THD = 5%, k_harmonic = 0.93 (moderate harmonics)
Minimum nameplate kVA = 600 ÷ 0.93 ≈ 645.2 kVA

Selection: 750 kVA transformer (next standard size above 645.2 kVA)

At site, usable capacity = 750 × 0.93 = 697.5 kVA, which exceeds the 600 kVA required. The transformer can handle peak PV export (500 kVA) plus margin, or peak facility load (222.2 kVA) plus margin, with harmonic derating applied.

Example: Commercial Rooftop PV with Load #

Given:
300 kW DC rooftop PV, 208 V AC. Facility peak load 400 kW at 0.85 PF. Central inverter with 8% THD. Grid-tied, no significant export (load exceeds PV). Safety margin 25%. Ambient temperature 45°C.

Step 1 – PV output kVA:
PV AC output ≈ 300 kW (inverter efficiency 98%, PF 0.99)
PV_kVA = 300 ÷ 0.99 ≈ 303 kVA

Step 2 – Facility load kVA:
Load_kVA = 400 ÷ 0.85 = 470.6 kVA

Step 3 – Required kVA (load exceeds PV, no reverse flow):
Required kVA = 470.6 × 1.25 = 588.3 kVA

Step 4 – Harmonic derating:
THD = 8%, k_harmonic = 0.90
After harmonic: 588.3 ÷ 0.90 = 653.7 kVA

Step 5 – Temperature derating:
45°C ambient, k_temp = 0.92 (dry-type, 1.5% per °C above 40°C)
Minimum nameplate kVA = 653.7 ÷ 0.92 ≈ 710.5 kVA

Selection: 750 kVA transformer

Usable capacity at site = 750 × 0.90 × 0.92 = 621 kVA, which exceeds the 588.3 kVA required.

Common Mistakes in Solar Transformer Sizing #

Mistake 1: Sizing Based on Average Generation #

Error: Using average daily or annual generation (capacity factor) instead of peak output for transformer sizing.

Example:
500 kW array with 20% capacity factor → average 100 kW
Incorrect: Size transformer for 100 kW
Correct: Size transformer for 500 kW peak

Impact: Transformer severely undersized; overloads during peak generation periods, overheating, premature failure.

Solution: Always size for peak PV output, not average. Use peak irradiance conditions (typically 1000 W/m² STC) and account for inverter nameplate capacity.

Mistake 2: Ignoring Harmonic Content #

Error: Sizing transformer on kW only, without harmonic derating or K-factor selection.

Example:
500 kW PV at unity PF → 500 kVA
Incorrect: Select 500 kVA transformer
Correct: Apply harmonic derating (e.g. 0.93) → 500 ÷ 0.93 ≈ 538 kVA → select 600 kVA

Impact: Transformer operates at higher temperature due to harmonic losses, reduced life, potential overheating.

Solution: Always apply harmonic derating (0.90–0.95 for PV) or select K-factor transformer. Measure THD when possible; use conservative values (5–8% THD) when unknown.

Mistake 3: Not Accounting for Bidirectional Power Flow #

Error: Sizing transformer only for facility load, ignoring reverse power flow from PV to grid.

Example:
Facility load: 200 kW
PV output: 500 kW
Incorrect: Size for 200 kW load only
Correct: Size for maximum of (200 kW load, 500 kW PV export) = 500 kW

Impact: Transformer overloads during peak PV generation when exporting to grid, especially with net metering.

Solution: Size transformer for maximum power flow in either direction. For grid-tied systems, use: Required kVA = max(PV_kVA, Load_kVA, Reverse_kVA) × margin.

Mistake 4: Ignoring Voltage Fluctuations #

Error: Not considering voltage rise from PV export or voltage drop from high load, especially in weak grids.

Impact: Voltage outside acceptable limits (±5% per ANSI C84.1), equipment malfunction, potential transformer tap adjustment needed.

Solution: Verify voltage regulation under both peak PV export and peak load conditions. Consider transformer tap settings or voltage regulation equipment if voltage fluctuations exceed limits.

Engineering Recommendations #

Capacity Selection Strategy #

Grid-tied systems: Size transformer for the maximum of:

  1. Peak PV output (accounting for inverter efficiency and power factor)
  2. Peak facility load
  3. Maximum reverse power flow (if net metering allows significant export)

Add 15–25% safety margin for growth and contingencies. Do not use capacity factor or average generation for sizing.

Off-grid systems: Size for peak load plus battery charging requirements (if applicable). Account for inverter efficiency and harmonic content.

Hybrid systems: Size for peak combined load (PV + grid + battery discharge). Ensure transformer can handle bidirectional power flow and variable generation profiles.

Harmonic Management #

For modern PV inverters (THD < 8%):
Apply harmonic derating factor 0.90–0.95, or select K-4 rated transformer. Standard transformers with derating are typically cost-effective.

For older inverters or high THD (> 8%):
Apply derating factor 0.85–0.90, or select K-13 rated transformer. Consider active harmonic filters if THD exceeds 10% and affects other equipment.

Measurement: Measure actual THD at transformer location during peak generation. Use measured values for final design; use conservative estimates (5–8% THD) for preliminary sizing.

Voltage Management #

Voltage rise from PV export:
In weak grids or long feeders, PV export can cause voltage rise. Verify transformer secondary voltage remains within limits (±5% per ANSI) during peak export. Consider:

  • Transformer tap settings (raise no-load voltage to compensate for rise)
  • Voltage regulation equipment
  • Conductor sizing (larger conductors reduce voltage rise)

Voltage drop from high load:
During peak facility load (especially when PV generation is low), verify voltage drop does not exceed limits. Transformer regulation plus conductor drop must keep load voltage within ±10% utilization limits.

Documentation and Future Expansion #

Document the following for each solar transformer installation:

  • Peak PV capacity (DC and AC)
  • Inverter specifications (efficiency, power factor, THD)
  • Peak facility load and power factor
  • Maximum reverse power flow (if applicable)
  • Harmonic derating factors applied
  • Temperature and altitude derating (if applicable)
  • Safety margin used
  • Selected transformer nameplate kVA and K-factor (if applicable)

This documentation supports future capacity reviews, expansion planning, and maintenance. When PV capacity is expanded, recalculate transformer sizing with updated peak output and harmonic content.

Frequently Asked Questions #

Q1: How do I size a transformer for a grid-tied solar system? #

A: Calculate peak PV output kVA, peak facility load kVA, and maximum reverse power flow kVA (if net metering allows export). Required kVA = max(PV_kVA, Load_kVA, Reverse_kVA) × (1 + margin). Apply harmonic derating (0.90–0.95 for PV) and temperature/altitude derating if applicable. Round up to next standard size. See the Transformer Sizing Guide for base methodology.

Q2: Do I need a K-factor transformer for solar systems? #

A: Not always. Modern PV inverters typically have THD 5–8%, which can be handled with standard transformer and harmonic derating (0.90–0.95). K-4 rated transformers may be used if preferred, but are not required for most PV systems. K-13 may be needed for older inverters or systems with additional non-linear load. See Transformer Sizing for Harmonic Loads for detailed guidance.

Q3: Should I size the transformer for peak PV output or average generation? #

A: Always size for peak PV output, not average. Transformers must handle maximum power flow without overload. Average generation (capacity factor) is useful for energy production estimates but not for transformer sizing. Use inverter nameplate capacity and peak irradiance conditions (1000 W/m² STC) for sizing.

Q4: How do I handle bidirectional power flow in transformer sizing? #

A: Size transformer for maximum power flow in either direction. For grid-tied systems: Required kVA = max(PV_kVA, Load_kVA, Reverse_kVA) × margin. If PV can export more than facility load (net metering), size for peak export. If load exceeds PV, size for peak load. The transformer must handle both conditions.

Conclusion #

Transformer sizing for solar systems requires accounting for PV inverter harmonics, bidirectional power flow, and peak generation capacity (not average). Size for maximum power flow in either direction, apply harmonic derating (0.90–0.95 for modern inverters), and include 15–25% safety margin. Document assumptions for future expansion and verify voltage regulation under both peak export and peak load conditions.

About the Author: Michael Rodriguez, P.E. is a senior power systems engineer with 12+ years of experience in factory electrical design and renewable energy integration. Specializes in transformer sizing, harmonic analysis, and grid-tied solar system design. Has designed transformer and distribution systems for solar farms, commercial rooftop PV, and industrial facilities with renewable energy. All content in this guide has been reviewed and validated by licensed engineers.