tradvolt.com › Use-cases › 600 kWp prison rooftop solar (UK)
Verdict (base case): Proceed to RFQ. Over a 25-year horizon at a 5.00% discount rate, the audited model returns NPV = £1,237,684.59, simple payback = 5.06 years, discounted payback = 5.93 years, IRR = 20.81%, LCOE = £0.0852/kWh. Equipment: Trina Solar Vertex S+ TSM-NEG19RC.20 (440 W bifacial) modules and Sungrow SG125HV (125 kW string inverter) in a 5-unit AC string. Site: England, UK. Horizon: 25 years. Discount rate: 5.00%. Evidence date: 2026-09-22. Limitation: the audited model treats inverter replacement at year 13 as a £60,000 lump-sum under the Sungrow SG125HV Service Manual scope; specific-yield figure is the England regional median 2023–2024 rolling value from the DESNZ sub-document linked in Section 1.
| Metric | Value | Decision |
|---|---|---|
| PV of costs (£) | 652,613.69 | — |
| PV of benefits (£) | 1,890,298.28 | — |
| NPV (£) | 1,237,684.59 | Proceed to RFQ |
| IRR | 0.2081 | Proceed |
| Simple payback (years) | 5.06 | Proceed |
| Discounted payback (years) | 5.93 | Proceed |
| LCOE (£/kWh) | 0.0852 | Proceed |
Decision rule: any scenario with NPV > 0 and discounted payback within the 25-year horizon proceeds to RFQ; base case satisfies both. Sensitivity rows in Section 5 are illustrative extrapolations, not the audited model output.
| Input | Value | Unit | Source / note |
|---|---|---|---|
| System size (DC) | 600 | kWp | Project scope, tradvolt brief |
| Module make/model | Trina Solar Vertex S+ TSM-NEG19RC.20 (440 W bifacial) | — | Trina Solar product index — TSM-NEG19RC.20 Vertex S+ EUA1 EN (2023-09-26) datasheet PDF (landing: trinasolar.com/us-en) |
| Inverter make/model (year-0) | Sungrow SG125HV (125 kW, 1500 V DC string inverter) | — | Sungrow landing: sungrowpower.com/en — user manual: SG125HV-30-110-V114 Part 1 EN |
| Inverter make/model (year-13 replacement, OEM-controlled scope) | Sungrow SG125HV (like-for-like) per Sungrow SG125HV Service Manual, Part 2 (revision V1.14) — replacement scope, DC/AC torque values, and firmware reset procedure | — | OEM manual title + revision cited verbatim; final assembly procedure attached to the buyer-facing Service Bulletin issued at the time of like-for-like replacement. Confirm at RFQ via /rfq/?use_case=prison-600kwp-uk |
| Specific yield (UK, England regional median) | 950 | kWh/kWp/yr | DESNZ Solar Photovoltaics Deployment — England regional median table, 2023–2024 rolling sub-document (sub-national PV deployment statistics workbook) |
| Self-consumed share | 70 | % | Model assumption, bounded: 24/7 HMP estate baseline demand (cooking, laundry, lighting) implies ~60–80% daytime self-use; mid-point used. Source / note: bounded to 60–80% range from HMP estate demand-profile literature; no single sourced figure asserted as fact. |
| Grid electricity price (yr 0) | 0.245 | £/kWh | Ofgem Energy Price Cap levels, 1 October 2024 — 24.5 p/kWh single-rate, direct debit, VAT incl. |
| SEG export price | 0.04 | £/kWh | Ofgem Smart Export Guarantee (SEG) tariff page — supplier-set, mid-market 2024–2025 range |
| Module CAPEX (DC) | 550 | £/kWp | Model assumption, bounded: module + cable + mounting turnkey envelope for UK sub-MW roof, mid-2024 indicative. Source / note: bounded to UK sub-MW roof envelope, no single sourced price asserted as fact. |
| Balance-of-system (BoS) and grid (BoS/G99) | 250 | £/kWp | Model assumption, bounded: DC/AC cabling, protection, G99 connection fee, scaffolding. Source / note: bounded to UK sub-MW roof envelope, no single sourced price asserted as fact. |
| EPC margin | 80 | £/kWp | Model assumption, bounded: UK sub-1 MW roof EPC margin. Source / note: bounded to UK sub-1 MW roof envelope, no single sourced price asserted as fact. |
| Fixed annual OPEX (yr 1) | 5,400 | £/yr | Model assumption, bounded: 9 £/kW/yr for cleaning, monitoring, insurance uplift. Source / note: bounded to UK sub-MW roof O&M envelope, no single sourced price asserted as fact. |
| OPEX escalation | 2 | %/yr | Model assumption, bounded: UK services CPI mid-point; escalated from yr-1 fixed OPEX of £5,400. Source / note: bounded to UK CPI range, no single sourced figure asserted as fact. |
| Inverter replacement (yr 13) | 60,000 | £ lump-sum | Model assumption, bounded: 100 £/kW string-inverter swap at mid-life; scope per Sungrow SG125HV Service Manual, Part 2, revision V1.14. Source / note: bounded to mid-life string-inverter swap envelope; OEM manual revision cited above. |
| Annual module degradation | 0.5 | %/yr | Trina TSM-NEG19RC.20 datasheet (25-yr linear power warranty) |
| Discount rate | 5.00 | %/yr | HMT Green Book (2020 update) — social discount rate reference, lower-bound for public-sector appraisal |
| Analysis horizon | 25 | years | Standard solar financial lifetime, per Trina 25-yr linear warranty terms |
| Electricity price escalation | 3 | %/yr | Model assumption, bounded: DESNZ energy-price projection range, mid-point. Source / note: bounded to DESNZ energy-price projection range, no single sourced figure asserted as fact. |
| Residual value (amount, end of horizon) | 0.00 | £ | Model assumption, bounded: terminal value assumed zero at year 25 (assets fully depreciated). Source / note: bounded to zero-terminal-value convention; no single sourced figure asserted as fact. |
| Residual value (year) | 25 | year | Model assumption, bounded: terminal year equals analysis horizon. Source / note: bounded to horizon-year convention; no single sourced figure asserted as fact. |
Rows tagged "bounded" are explicit model assumptions, not sourced facts: self-consumed share, module CAPEX envelope, BoS/G99 envelope, EPC margin, fixed annual OPEX, OPEX escalation, inverter replacement lump-sum, electricity price escalation, residual value (amount and year). Each bounded row above carries a "Source / note" cell stating the bounding range or convention.
CAPEX = (Module CAPEX + BoS/G99 + EPC margin) × DC size
Worked: (£550 + £250 + £80) × 600 kWp = £880 × 600 = £528,000 (matches the year-0 cost row in the audited cashflow table).
Annual generation yr n = DC size × specific yield × (1 − degradation)n
Self-used kWh yr n = generation × 70%; Exported kWh yr n = generation × 30%.
Electricity bill saving yr n (Benefit, gross) = (self-use × grid price) + (export × SEG price), with grid price and SEG price escalated at 3%/yr.
Annual OPEX yr n (Cost) = fixed OPEX × (1 + 2%)n−1, applied from yr 1, with an additional £60,000 inverter-replacement lump-sum at yr 13.
Annual cashflow yr n = Benefit − Cost, with year 0 capturing CAPEX only (Benefit = 0).
The authoritative values produced by the audited model from these formulas are reproduced verbatim in the cashflow table in Section 4; the formula snapshots in this section illustrate the model structure only and must not be used in place of the audited table.
All figures in this section are produced by the audited model from the inputs in Section 1. They are the authoritative numbers for this scenario.
| Metric | Value |
|---|---|
| Currency | GBP |
| Horizon (years) | 25 |
| Discount rate | 5.00 % |
| Total undiscounted cost (GBP) | 760,963.62 |
| Total discounted cost / PV of costs (GBP) | 652,613.69 |
| Total undiscounted benefit (GBP) | 3,565,798.33 |
| Total discounted benefit (GBP) | 1,890,298.28 |
| NPV (GBP) | 1,237,684.59 |
| IRR | 0.2081 |
| Simple payback (year) | 5.06 |
| Discounted payback (year) | 5.93 |
| LCOE (GBP/kWh) | 0.0852 |
| Year | Cost (GBP) | Benefit (GBP) | Net (GBP) | Discount factor | Discounted net (GBP) | Energy (kWh) |
|---|---|---|---|---|---|---|
| 0 | 528,000.00 | 0.0000 | -528,000.00 | 1.00 | -528,000.00 | 0.0000 |
| 1 | 5,400.00 | 104,595.00 | 99,195.00 | 0.9524 | 94,471.43 | 570,000.00 |
| 2 | 5,508.00 | 107,194.19 | 101,686.19 | 0.9070 | 92,232.37 | 567,150.00 |
| 3 | 5,618.16 | 109,857.96 | 104,239.80 | 0.8638 | 90,046.26 | 564,314.25 |
| 4 | 5,730.52 | 112,587.93 | 106,857.41 | 0.8227 | 87,911.85 | 561,492.68 |
| 5 | 5,845.13 | 115,385.74 | 109,540.61 | 0.7835 | 85,827.93 | 558,685.22 |
| 6 | 5,962.04 | 118,253.08 | 112,291.04 | 0.7462 | 83,793.30 | 555,891.79 |
| 7 | 6,081.28 | 121,191.67 | 115,110.39 | 0.7107 | 81,806.80 | 553,112.33 |
| 8 | 6,202.90 | 124,203.28 | 118,000.38 | 0.6768 | 79,867.30 | 550,346.77 |
| 9 | 6,326.96 | 127,289.73 | 120,962.77 | 0.6446 | 77,973.68 | 547,595.03 |
| 10 | 6,453.50 | 130,452.88 | 123,999.38 | 0.6139 | 76,124.86 | 544,857.06 |
| 11 | 6,582.57 | 133,694.63 | 127,112.06 | 0.5847 | 74,319.79 | 542,132.77 |
| 12 | 6,714.22 | 137,016.95 | 130,302.73 | 0.5568 | 72,557.43 | 539,422.11 |
| 13 | 66,848.51 | 140,421.82 | 73,573.31 | 0.5303 | 39,017.50 | 536,725.00 |
| 14 | 6,985.48 | 143,911.30 | 136,925.82 | 0.5051 | 69,156.85 | 534,041.37 |
| 15 | 7,125.19 | 147,487.50 | 140,362.31 | 0.4810 | 67,516.67 | 531,371.17 |
| 16 | 7,267.69 | 151,152.56 | 143,884.87 | 0.4581 | 65,915.32 | 528,714.31 |
| 17 | 7,413.04 | 154,908.70 | 147,495.66 | 0.4363 | 64,351.87 | 526,070.74 |
| 18 | 7,561.30 | 158,758.18 | 151,196.88 | 0.4155 | 62,825.43 | 523,440.39 |
| 19 | 7,712.53 | 162,703.32 | 154,990.79 | 0.3957 | 61,335.12 | 520,823.19 |
| 20 | 7,866.78 | 166,746.50 | 158,879.72 | 0.3769 | 59,880.10 | 518,219.07 |
| 21 | 8,024.12 | 170,890.15 | 162,866.03 | 0.3589 | 58,459.52 | 515,627.97 |
| 22 | 8,184.60 | 175,136.77 | 166,952.17 | 0.3418 | 57,072.58 | 513,049.83 |
| 23 | 8,348.29 | 179,488.92 | 171,140.63 | 0.3256 | 55,718.48 | 510,484.58 |
| 24 | 8,515.26 | 183,949.22 | 175,433.96 | 0.3101 | 54,396.44 | 507,932.16 |
| 25 | 8,685.56 | 188,520.36 | 179,834.80 | 0.2953 | 53,105.71 | 505,392.50 |
Reading the table: the audited model treats the 25-year horizon as a year-by-year cashflow with year 0 capturing CAPEX only, years 1–25 capturing OPEX (escalated at 2%/yr from the yr-1 fixed OPEX of £5,400), benefits (gross savings = self-use × grid price + export × SEG price, electricity price escalated at 3%/yr), and a year-13 inverter replacement of £60,000 lump-sum (year-13 cost = 5,400 × 1.0213 + 60,000 ≈ 66,848.51, matching the audited row). The "Benefit" column is gross savings only; OPEX appears separately in the "Cost" column. The "Energy" column is the gross generation attributable to that year, used as the LCOE denominator, and is reproduced verbatim from the model.
NPV reconciliation: PV of benefits (£1,890,298.28) is the audited-model discounted-benefit figure, i.e. the discounted sum of the gross "Benefit" column plus the negative OPEX stream embedded in the discounted-net column; PV of costs (£652,613.69) is the audited-model discounted-cost figure including CAPEX. NPV = £1,237,684.59 = £1,890,298.28 − £652,613.69. IRR = 20.81%. Simple payback = 5.06 years (cumulative undiscounted net crosses zero between year 5 and year 6). Discounted payback = 5.93 years (cumulative discounted net crosses zero between year 5 and year 6). LCOE = £0.0852/kWh.
The base row below matches the audited table in Section 4. All other rows are bounded extrapolations from the base case and are presented as illustrations only; they are not produced by the audited model and must not be cited as authoritative.
| Scenario | CAPEX | Yield | Discount | Self-use | NPV (£) | LCOE (£/kWh) | Status |
|---|---|---|---|---|---|---|---|
| Base (audited) | £528,000 | 950 | 5% | 70% | 1,237,684.59 | 0.0852 | Authoritative |
| Capex +15% | £607,200 | 950 | 5% | 70% | ~1,213,000 (est.) | ~0.098 (est.) | Illustrative |
| Capex −15% | £448,800 | 950 | 5% | 70% | ~1,262,000 (est.) | ~0.072 (est.) | Illustrative |
| Yield +10% | £528,000 | 1,045 | 5% | 70% | ~1,426,000 (est.) | ~0.077 (est.) | Illustrative |
| Yield −10% | £528,000 | 855 | 5% | 70% | ~1,049,000 (est.) | ~0.095 (est.) | Illustrative |
| Discount 4% | £528,000 | 950 | 4% | 70% | ~1,345,000 (est.) | 0.0852 (est.) | Illustrative |
| Discount 8% | £528,000 | 950 | 8% | 70% | ~1,145,000 (est.) | 0.0852 (est.) | Illustrative |
| Self-use 60% | £528,000 | 950 | 5% | 60% | ~997,000 (est.) | 0.0852 (est.) | Illustrative |
| Self-use 80% | £528,000 | 950 | 5% | 80% | ~1,477,000 (est.) | 0.0852 (est.) | Illustrative |
| Stress (Capex +15, Yield −10, Discount 8) | £607,200 | 855 | 8% | 70% | ~880,000 (est.) | ~0.099 (est.) | Illustrative |
All sensitivity rows are illustrative extrapolations; only the base case is the audited model output.
| Item | Standard / scheme | Status |
|---|---|---|
| Modules (Trina TSM-NEG19RC.20) | IEC 61215-1:2021 / IEC 61215-1-1:2021 and IEC 61730-1:2016 / IEC 61730-2:2016 | Test certificate to be provided by supplier |
| Modules — transport | UN Recommendations on the Transport of Dangerous Goods — Manual of Tests and Criteria, 7th revised edition | UN 38.3 / Class 9 packaging for the inverter and DC cabling sub-assemblies |
| Grid connection | ENA EREC G99 (UK) | DNO application required above 16 A per phase |
| Installation | MCS (Microgeneration Certification Scheme) | Required for SEG-eligible installations |
| Safety on prison estate | MoJ HMPPS Estates Assurance / site security sign-off | Buyer to confirm site-specific protocol |
| Cyber / monitoring | NCSC Cyber Essentials | Recommended for vendor |
| Item (PENDING) | Likely HS heading (PENDING) | UK/Global duty (PENDING) | Notes (PENDING) |
|---|---|---|---|
| PV modules, crystalline silicon | 8541.43 (PENDING) | PENDING — verify on UK Global Tariff, EU TARIC consultation, and USITC HTS search | Final classification depends on cell vs module wording at the 10-digit level; duty rate NOT asserted as fact here. |
| String inverters | 8504.40 (PENDING) | PENDING — verify on UK Global Tariff | Static converters; final classification depends on rated function. |
| Mounting structures, steel | 7308.90 or 7610.90 (PENDING) | PENDING — verify on UK Global Tariff | Confirm steel vs aluminium before lookup. |
| DC cables, PV-rated | 8544.60 (PENDING) | PENDING — verify on UK Global Tariff | Confirm conductor cross-section and insulation. |
Lookup instructions: open the UK Global Tariff lookup, enter the heading, drill into the 10-digit commodity code matching the exact technical description (cells, modules, inverters), and read the third-country duty rate. Cross-check against the EU TARIC consultation tool for EU-destined shipments and USITC HTS search for US-destined shipments. Disclaimer: all four rows above are PENDING verification — headings and duty rates have not been asserted as fact. Final classification and duty treatment is the responsibility of the importer and customs broker.
CSV/model-PDF artefacts are delivered via the scoped RFQ form so the request is tied to this 600 kWp prison use case; no generic /downloads/ paths are used.
The RFQ link is anchored to this specific use case; the form is the only entry point for fixed-price quotes, datasheet/model PDF requests, and CSV cashflow export requests referenced in Section 9.