Summary

System sizing for solar PV involves two interconnected calculations: (1) how large an array (in kWp) the roof can accommodate and what yield (kWh) it will generate, and (2) how to configure that array into strings that work within the inverter's electrical specifications.

Both calculations are required for MCS compliance. Undersizing means the customer generates less than optimal. Oversizing the inverter (array much smaller than the inverter rating) reduces efficiency. Incorrect string design (voltage outside the MPPT window) can prevent the inverter from operating at all.

This article covers the system sizing process for a typical UK domestic installation.

Key Facts

  • kWp (kilowatt-peak) — the rated output of the solar panel array at Standard Test Conditions (STC: 1,000W/m², 25°C, AM1.5); the total kWp is the sum of all individual panel wattages
  • kWh (kilowatt-hours) — the actual energy generated per year; depends on kWp, orientation, tilt, location, and shading
  • Specific yield (kWh/kWp) — annual generation per kWp of array; typical UK values: 900–1,100 kWh/kWp (SE England); 700–900 kWh/kWp (Scotland, NW England)
  • PVGIS (Photovoltaic Geographical Information System) — EU/JRC free online tool for estimating annual solar PV yield for any European location, orientation, and tilt; the most widely used UK yield estimation tool
  • Performance ratio — the ratio of actual system output to theoretical maximum output (no losses); typically 80–85% for a modern well-designed system; accounts for temperature losses, cable losses, inverter efficiency, and shading
  • MPPT (Maximum Power Point Tracking) — the inverter algorithm that finds the optimal operating point of the DC array; the array must operate within the inverter's MPPT voltage window for MPPT to function
  • Voc (open-circuit voltage) — the maximum string voltage when disconnected; must not exceed the inverter's maximum input voltage (damage risk)
  • Vmpp (voltage at maximum power point) — the string operating voltage under normal MPPT conditions; must be within the inverter's MPPT voltage range
  • Isc (short-circuit current) — the maximum panel current; relevant for fuse and cable sizing per string
  • Impp (current at maximum power point) — the operating current under normal conditions; relevant for MPPT input current rating
  • Array oversizing (DC:AC ratio) — it is common practice to oversize the array slightly relative to the inverter (DC:AC ratio 1.1–1.3); this "clips" peak summer generation but captures more morning/afternoon and winter generation; standard practice for UK installations
  • String sizing — the number of panels in series per string; determined by: Voc_string < inverter Vmax; Vmpp_string within inverter MPPT range at operating temperatures
  • Temperature correction — panel Voc increases as temperature falls (winter cold mornings can produce Voc 10–20% higher than STC; must not exceed inverter maximum input voltage)

Quick Reference Table: UK Annual Yield by Location and Orientation

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Location Orientation Tilt Specific Yield (kWh/kWp) 4kWp Yield (kWh/yr)
SE England (London) South 35° ~1,000 ~4,000
SE England (London) Southwest 30° ~970 ~3,880
Midlands South 35° ~940 ~3,760
NW England (Manchester) South 35° ~880 ~3,520
Scotland (Edinburgh) South 35° ~840 ~3,360
SE England East 35° ~730 ~2,920

Values are indicative. Use PVGIS for site-specific estimates.

Detailed Guidance

Step 1: Determine Available Roof Area and Panel Count

From the roof survey (see solar pv roof survey):

  • Measure the usable roof area (exclude chimneys, skylights, aerials, and setback areas)
  • Calculate how many panels fit in the available area (account for panel dimensions and any required clearances)
  • Select the target panel size (e.g., 420Wp panels measuring 1,722mm × 1,134mm)
  • Number of panels × panel Wp = array kWp

Example: Roof area 25m². Panel: 420Wp, 1.72m × 1.13m = 1.95m² per panel. Allow for 100mm gaps and 300mm clearance from roof edges: effectively 22m² usable. 22/1.95 = ~11 panels. Array kWp = 11 × 420Wp = 4.62kWp.

Step 2: Yield Calculation Using PVGIS

PVGIS (JRC tool, available at re.jrc.ec.europa.eu/pvg_tools):

  1. Enter the installation location (postcode or coordinates)
  2. Enter array size (kWp), orientation (azimuth in degrees from south), tilt angle
  3. Enter system losses (typically 14% for standard systems: cable losses 3%, mismatch 2%, inverter losses 4%, shading 4%, soiling 1%)
  4. PVGIS outputs: monthly and annual generation (kWh), maximum/minimum monthly output, and the specific yield (kWh/kWp)

PV*SOL, HelioScope, SolarEdge Designer: Alternative tools with more detailed shading simulation. Used for complex installations or where high accuracy is required.

MCS design documentation: MCS 001/012 requires the system design to include a yield estimate (calculated using an appropriate tool) and the system design parameters. Record the PVGIS output in the customer proposal and the MCS documentation file.

Step 3: String Sizing and Inverter Matching

String sizing ensures the panels connected in series operate within the inverter's electrical limits.

Parameters needed:

  • Panel datasheet: Voc, Vmpp, Isc, Impp, temperature coefficients (typically -0.28 to -0.35%/°C for Voc; +0.06%/°C for Isc)
  • Inverter datasheet: maximum input voltage (typically 600Vdc or 1000Vdc); MPPT voltage range (typically 100–600Vdc or 150–800Vdc); maximum input current per MPPT input; number of MPPT inputs

Temperature-corrected Voc: In cold UK conditions (January, early morning at -10°C — unusual but possible), Voc increases:

  • Voc_cold = Voc_STC × (1 + temperature_coefficient × (T_min - 25))
  • For a panel with Voc = 40V and Tcoeff = -0.29%/°C: Voc_cold = 40 × (1 + (-0.0029) × (-10 - 25)) = 40 × 1.1015 = 44.06V
  • For 10 panels in series: Voc_cold_string = 440.6Vdc

This must be less than the inverter's maximum input voltage. For an inverter rated at 600Vdc maximum: 440.6Vdc < 600Vdc ✓

MPPT voltage check: At operating temperature (summer, 50°C cell temperature), Vmpp falls:

  • Vmpp_hot = Vmpp_STC × (1 + temperature_coefficient × (50 - 25))
  • Vmpp_STC = 33V; Vmpp_hot = 33 × (1 + (-0.0029 × 25)) = 33 × 0.9275 = 30.6V
  • For 10 panels: Vmpp_hot_string = 306Vdc

This must be within the inverter's MPPT range. For an MPPT range of 100–480Vdc: 306Vdc is within range ✓

Typical domestic string calculation for 10 × 420Wp panels: Most 420–450Wp panels have Voc ~40–42V, Vmpp ~33–35V. For a standard 600Vdc inverter:

  • Maximum string length (cold Voc check): 600Vdc / 44V (cold Voc) = ~13 panels maximum
  • Minimum string length (hot Vmpp check — must be above MPPT lower limit of ~100V): 100Vdc / 30V = ~4 panels minimum
  • For a 10-panel string: fully within the typical inverter MPPT range

Step 4: DC:AC Ratio and Inverter Selection

The DC:AC ratio compares the array's kWp to the inverter's AC output rating:

  • DC:AC ratio = array kWp / inverter kW AC output
  • Typical UK domestic target: 1.1–1.3

Why oversize the DC array: In the UK, maximum irradiance (1,000W/m² STC) is rarely reached outside of a few peak summer hours. Oversizing the array (more panels than the inverter's nominal rating) means:

  • The inverter operates at or near capacity for more hours per day (morning, afternoon, winter)
  • In summer peak hours, the inverter "clips" the excess: it cannot output more than its AC rating, so some potential generation is lost. But in the UK climate, this clipped generation is modest.
  • Net result: more annual kWh generation from the same inverter

Example: 4.62kWp array with a 3.68kW inverter: DC:AC ratio = 4.62/3.68 = 1.26. Appropriate for UK conditions.

Self-Consumption and Battery Sizing

Once the yield is estimated, align it with the customer's consumption profile:

Daily consumption profile:

  • Average UK household: 8–12 kWh/day
  • Summer: may consume only 5–7 kWh/day (no heating, more lighting from longer days)
  • Winter: may consume 12–20 kWh/day (heating, appliances, lighting)

Estimating self-consumption: A 4kWp system generates ~10–12 kWh on a summer day. A household consuming 8 kWh/day will self-consume all 8 kWh (100% if battery fitted) or will export the rest. Without a battery, self-consumption on a summer day might be 30–50% — the rest is exported.

Battery sizing: A battery sized to the daily import reduction target:

  • Target: capture the evening demand that would otherwise be imported (typically 3–5 kWh)
  • Battery size: 5–10 kWh for most domestic applications
  • Matching to annual solar: a 10 kWh battery is beneficial if the system generates at least 10 kWh surplus on most summer days

See solar battery storage installation for battery sizing and integration.

Frequently Asked Questions

How do I explain kWp vs kWh to a customer?

kWp is the size of the solar system — like the engine size of a car. kWh is the energy it produces — like the miles driven per year. A 4kWp system is a "4kW engine"; in the UK it produces approximately 3,500–4,000kWh of electricity per year — roughly equivalent to 40–50% of a typical household's annual consumption.

Can I oversize the array significantly to get more generation?

Yes, within limits. The inverter's maximum input voltage is the key constraint — the cold-condition Voc of the string must not exceed this. On the current side, the array's Isc must not exceed the inverter's maximum input current per MPPT input. Beyond these electrical limits, oversizing increases generation cost (more panels) without proportional yield increase, as the inverter clips an increasing proportion of peak summer output.

Does PVGIS account for UK real-world performance?

PVGIS uses historical satellite irradiance data for the specific location and applies standard loss factors. It is a good estimate but not a guarantee. Real-world performance can vary by ±10–15% depending on actual weather, actual installation angle and orientation, cleaning, and shading. Present PVGIS estimates to customers as estimates with this caveat.

Regulations & Standards