Quick Answer: How Many Panels Does a Factory Need?
Panel Count Summary
A typical UK factory needs between 200 and 4,000 solar panels depending on electricity consumption and available roof space. As a rough guide: a small factory using 200,000 kWh/year needs around 280–320 panels (a 100kW system), while a large manufacturing site using 2,000,000 kWh/year would benefit from 2,800–3,200 panels (a 1MW system). Each 400W monocrystalline panel generates around 320–380 kWh/year in the UK, depending on location and orientation.
These figures assume a 40% offset of daytime electricity consumption — the typical sweet spot for UK factories with daytime operations. The calculation and reasoning behind this target are explained in the sections below.
Step-by-Step Calculation Guide
You can size a factory solar system yourself in four steps using your electricity bills. Here is the method used by professional solar designers for initial system sizing.
Find Your Annual kWh Electricity Consumption
Look at your electricity bills for the past 12 months and total up the kWh consumed. Most commercial bills show monthly consumption in kWh. If you have a half-hourly (HH) meter, your energy supplier or energy manager can provide annual consumption data. For accuracy, use the most recent full year — energy consumption can vary significantly year to year based on production volumes.
Decide What Percentage of Consumption to Offset
Solar only generates during daylight hours (roughly 8am–6pm in summer, 9am–4pm in winter). For a factory running a standard day shift, you can realistically offset 35–55% of annual consumption. For a factory running 24/7, solar offset is naturally lower (20–35%) without battery storage. Trying to offset more than 50–60% typically leads to significant surplus export, reducing the financial return.
Calculate the Target System Size in kWp
Use the formula: (Annual kWh × Offset%) ÷ 950 = kWp required. The figure 950 represents the approximate annual output per kWp in the UK (in kWh/kWp/year), which averages 900–1,050 across the country depending on location and orientation. Use 950 for the Midlands, 1,050 for the South East, and 900 for Scotland.
Convert kWp to Number of Panels
Divide your required kWp by the wattage of the panel you intend to use. Standard industrial panels in 2026 are typically 400W–430W. Using 400W panels: kWp ÷ 0.400 = number of panels. Using 420W panels: kWp ÷ 0.420 = number of panels.
Panel Count Table by UK Factory Type
The table below gives indicative panel counts for common UK industrial building types, based on typical electricity consumption ranges and realistic offset targets.
| Factory Type | Annual kWh | Typical Offset | System Size | Panels Needed (400W) |
|---|---|---|---|---|
| Light assembly / workshop | 100,000–300,000 | 40–60% | 50–200kW | 125–500 |
| Food and drink manufacturer | 500,000–2,000,000 | 30–50% | 200–1,000kW | 500–2,500 |
| Automotive Tier 1 supplier | 1,000,000–5,000,000 | 25–45% | 300–2,000kW | 750–5,000 |
| Distribution centre / warehouse | 500,000–3,000,000 | 35–55% | 200–2,000kW | 500–5,000 |
| Cold storage facility | 1,000,000–4,000,000 | 20–35% | 250–1,500kW | 625–3,750 |
| Plastics / injection moulding | 300,000–1,500,000 | 35–55% | 150–900kW | 375–2,250 |
| Pharmaceutical / cleanroom | 500,000–3,000,000 | 25–40% | 150–1,200kW | 375–3,000 |
| Metal fabrication / engineering | 200,000–1,000,000 | 35–55% | 100–600kW | 250–1,500 |
Note: Offset targets are lower for 24/7 operations (cold storage, some food manufacturing) where significant night-time consumption cannot be offset without battery storage. Offset targets are higher for daytime-only operations.
How Much Roof Space Do You Need?
A standard 400W monocrystalline solar panel measures approximately 2.0m × 1.0m = 2m² of panel area. However, the total roof space required per panel is considerably more once you account for:
- Row spacing — to prevent panels shading those behind them (particularly important in winter), rows must be spaced apart. On a flat roof, typical spacing adds 50–80% to the panel area.
- Perimeter setback — a 1–2 metre clear zone around the roof edge is required for both wind loading and safe access.
- Maintenance walkways — 600mm access paths are required between panel rows for cleaning and maintenance.
- Obstacle exclusion zones — rooflights, HVAC units, vents, and access hatches require clear zones around them.
Roof Space Rule of Thumb
Allow 8–10m² of roof area per kW of solar capacity when planning a flat-roof factory installation. This accounts for panel area, row spacing, perimeter setbacks and walkways. For a 200kW system, you need approximately 1,600–2,000m² of usable roof area.
| System Size | Number of 400W Panels | Minimum Roof Area (8m²/kW) | Comfortable Roof Area (10m²/kW) |
|---|---|---|---|
| 50kW | 125 panels | 400m² | 500m² |
| 100kW | 250 panels | 800m² | 1,000m² |
| 250kW | 625 panels | 2,000m² | 2,500m² |
| 500kW | 1,250 panels | 4,000m² | 5,000m² |
| 1MW | 2,500 panels | 8,000m² | 10,000m² |
Why You Should Not Try to Offset 100%
It might seem logical to install enough panels to generate 100% of your annual electricity consumption. In practice, this is almost never the right approach for a factory, for the following reasons:
Demand Profile Mismatch
Even if annual generation matches annual consumption, solar generates all its energy during the day and none at night. Unless you have battery storage, electricity consumed at night still comes from the grid. A system sized to produce 100% of annual consumption would produce huge surpluses during sunny summer days and fall short in winter and at night.
Export Constraints and G99
Many DNOs limit the amount of power you can export back to the grid under G99 rules, particularly in constrained network areas. An oversized system may be forced to curtail generation (switch panels off) during periods of high output and low consumption — wasting the energy you have paid to generate.
Diminishing Financial Returns
The first 40–50% of consumption offset by solar delivers the best returns — you are replacing grid electricity at 28–32p/kWh. The marginal panels in an oversized system generate electricity that mostly gets exported at Smart Export Guarantee rates of 3–6p/kWh, drastically reducing the value per kWh generated. The economics rarely justify going beyond 60% offset.
The Sweet Spot: 40–60% Offset
For a factory operating standard daytime hours, targeting a 40–60% offset of total annual consumption typically delivers the best financial return — maximising self-consumption of generated solar electricity and minimising low-value export. This is the range where most professional solar designers will target your system.
Worked Examples: Three UK Factory Types
Example 1: Small Engineering Workshop, Birmingham
CNC machining facility, 5-day week, 7am–6pm operations
Inputs
- Annual consumption: 180,000 kWh/year
- Operations: Weekdays, daytime only
- Target offset: 50%
- Location: Midlands (950 kWh/kWp/yr)
Calculation
- Target generation: 180,000 × 50% = 90,000 kWh/yr
- System size: 90,000 ÷ 950 = 94.7kWp → 95kW
- Panels needed: 95,000W ÷ 400W = 238 panels
- Roof area needed: 95 × 9m² = 855m²
Estimated Cost
£68,000–£85,000
Annual Saving
£27,000
Payback
~3.3 years
Example 2: Medium Food Manufacturer, Yorkshire
Ready meals production, two-shift operation, 6am–10pm
Inputs
- Annual consumption: 950,000 kWh/year
- Operations: Two shifts, moderate night consumption
- Target offset: 35%
- Location: Yorkshire (920 kWh/kWp/yr)
Calculation
- Target generation: 950,000 × 35% = 332,500 kWh/yr
- System size: 332,500 ÷ 920 = 361kWp → 360kW
- Panels needed: 360,000W ÷ 400W = 900 panels
- Roof area needed: 360 × 9m² = 3,240m²
Estimated Cost
£228,000–£306,000
Annual Saving
£99,750
Payback
~2.7 years
Example 3: Large Distribution Centre, Northampton
Multi-temp logistics facility, 24/7 operations, large flat roof
Inputs
- Annual consumption: 2,400,000 kWh/year
- Operations: 24/7 with high daytime demand
- Target offset: 30%
- Location: East Midlands (960 kWh/kWp/yr)
Calculation
- Target generation: 2,400,000 × 30% = 720,000 kWh/yr
- System size: 720,000 ÷ 960 = 750kWp
- Panels needed: 750,000W ÷ 400W = 1,875 panels
- Roof area needed: 750 × 9m² = 6,750m²
Estimated Cost
£450,000–£622,500
Annual Saving
£216,000
Payback
~2.5 years