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Solar Inverter Clipping Calculator

Estimate annual energy clipped by inverter saturation, the net value of oversizing the DC array against MCS-certified inverters, and the optimal DC/AC ratio for UK irradiance and SEG export tariffs.

Solar Inverter Clipping Calculator

DC / AC ratio
1.304
Clipping loss (%)
1.58%
Clipping loss (kWh / yr)
64
Clipping loss (£ / yr)
£17
Delivered AC (kWh / yr)
3,957
Net annual oversize benefit
£228

How to use this calculator

The calculator applies the NREL Bolinger 2019 empirical clipping curve scaled by UK peak sun hours and your SEG export or self-consumption value to return annual clipping percentage, kWh lost, pound value of the loss, and the net economic benefit of oversizing the DC array beyond a unity (1.00) DC/AC ratio. It is intended for residential and small commercial UK PV installs up to 50 kWp. For larger systems contact a chartered electrical engineer or run a full hourly simulation in PVsyst or PVGIS-SARAH3.

  1. DC array nameplate (kWp) — Sum of module STC ratings. A 12-panel array of 410 W modules is 4.92 kWp.
  2. Inverter AC rating (kW) — Continuous AC output nameplate from the datasheet. For Solis S6-GR1P3.6K-M this is 3.68 kW (the standard UK G98 ceiling).
  3. Peak sun hours per day (kWh/m²/day) — Annual average. Pull from PVGIS-SARAH3 (re.jrc.ec.europa.eu/pvg_tools/en/) for your postcode. UK ranges from 2.3 (Glasgow) to 3.4 (Devon); national average is 2.7 per Energy Saving Trust.
  4. Electricity rate (£/kWh) — Use the effective marginal rate. Ofgem average domestic single-rate import is 27.03 p/kWh as of April 2026 (the latest cap window). For Octopus Agile use your average peak rate; for Cosy or Tracker use your typical daytime rate.
  5. System derate (0.80–0.90) — Combined DC cable loss + soiling + temperature derating + inverter efficiency. UK realistic average is 0.85 (cooler temperatures mean less thermal derating than the US).
  6. Extra DC capex from oversizing (£) — Marginal cost of panels beyond a unity-ratio system. Tier-1 panels in the UK run £0.32/W installed marginal cost (MCS-priced).
  7. Inverter £ saved by undersizing — Savings from a smaller AC unit. Solis S6-GR1P3.6K-M is £450 vs S6-GR1P5K-M at £680 — saving £230 by going to the 3.68 kW unit.

What the NREL clipping curve actually says

NREL Technical Report TP-7A40-66985 (Bolinger et al., 2019) and the SAM DC/AC sizing sweep produce an empirical fit:

clip_pct = 30 × (ratio − 1.0)^1.8 × (PSH/5)^1.3

At UK conditions the (PSH/5)^1.3 term suppresses clipping aggressively — a 1.30 DC/AC ratio at 2.7 PSH gives only 1.50% clipping, compared with 3.5% at 5.0 PSH and 5.4% at 6.5 PSH. This is why UK installers can push the DC/AC ratio well above the US 1.20–1.25 norm without serious clipping losses. The current MCS-installed UK average is 1.28 DC/AC per the 2024 Solar Energy UK industry report.

Reference test

A typical UK domestic 4.8 kWp DC array on a 3.68 kW AC G98-notified inverter (1.30 ratio), 2.7 PSH (UK average per Energy Saving Trust), 27 p/kWh Ofgem domestic rate, 0.85 system derate, £450 extra DC capex for the oversized array, £230 saved on the smaller G98 inverter vs G99 5 kW:

  • DC/AC ratio = 4.8 / 3.68 = 1.30
  • Annual DC potential = 4.8 × 2.7 × 365 × 0.85 = 4,021 kWh
  • Clipping percentage = 30 × 0.30^1.8 × (2.7/5)^1.3 = 30 × 0.1162 × 0.430 = 1.50%
  • Clipping loss = 4,021 × 0.0150 = 60 kWh/year
  • Clipping loss value = 60 × £0.27 = £16/year
  • Delivered AC = 4,021 − 60 = 3,961 kWh/year → £1,070/year value
  • Baseline (unity 1.00 ratio): 3.68 × 2.7 × 365 × 0.85 = 3,083 kWh → £832/year
  • Extra revenue from oversizing = £1,070 − £832 = £238/year
  • Amortised capex = (£450 − £230) / 25 yr = £9/year
  • Net annual benefit of oversizing to 1.30 = £238 − £9 = £229/year

The MCS Solar Energy UK 2024 industry report notes that UK domestic installs averaged 1.28 DC/AC in 2024, up from 1.15 in 2020 — confirming the economic logic above. Hybrid inverter installs with batteries averaged 1.35 because the battery captures the noon clip for the 4–7 pm Agile peak window.

Picking the right inverter — string vs microinverter vs hybrid

  • String inverters dominate the UK market: Solis (about 35% MCS-listed share), Growatt (~20%), GoodWe (~12%), Fronius (~10%), SMA (~8%), SolarEdge (~7%), Sungrow (~5%). All allow at least 1.40 DC/AC; SolarEdge HD-Wave and Fronius Primo Gen24 allow 1.55 and 1.50 respectively per manufacturer technical bulletin.
  • Microinverters — Enphase IQ8M and IQ8H dominate the UK module-level market. The IQ8M (330 W AC) on a 410 W panel gives 1.24 module-level ratio with zero array-level clipping risk.
  • Hybrid inverters — GivEnergy Gen3 (UK-designed), Sunsynk, Fox ESS, GoodWe ET/EH, SolarEdge Energy Hub, Tesla Powerwall 3 integrated PV. These typically pair with 5–13 kWh batteries and allow ratios up to 1.50 because the battery dispatches the clipped noon energy.

UK-specific tariff and SEG export considerations

The Smart Export Guarantee replaced the Feed-in Tariff in January 2020. SEG export rates as of May 2026:

  • Octopus Outgoing Fixed: 15 p/kWh (best fixed)
  • Octopus Outgoing Agile: variable, averaging 11 p/kWh
  • British Gas Export and Earn Plus: 15.1 p/kWh
  • E.ON Next Export Exclusive: 16.5 p/kWh (highest current rate, customers only)
  • EDF Export+Earn: 5.6 p/kWh
  • Good Energy Solar Savings: 5.6 p/kWh
  • Ovo Tariff: 4.0 p/kWh

If your daytime self-consumption fraction is below 30%, model the clipping cost at 50% of the import rate (weighted average of import-avoided × self-consumption + export × export-fraction). If above 70% self-consumption (heat pump + EV + battery), model at full 27 p/kWh — clipping is real money.

Sources

NREL Technical Report TP-7A40-66985 — Bolinger, Seel, Robson, Warner (2019); MCS Installation Standard MIS 3002 Issue 4.0; Energy Networks Association Engineering Recommendation G98 Issue 1 Amendment 8 (2024); ENA Engineering Recommendation G99 Issue 1 Amendment 10 (2024); Energy Saving Trust Solar Panels Buyer’s Guide 2024; Solar Energy UK Annual Industry Review 2024; Ofgem Default Tariff Cap Methodology October 2024 cap window; PVGIS-SARAH3 satellite-derived irradiance database via JRC re.jrc.ec.europa.eu; Solis S6 Series Datasheet 2024; Growatt MIN-XH Series Technical Bulletin 2024; GoodWe DNS Hybrid Inverter Design Guide 2024; SolarEdge HD-Wave Technical Note SE-INSTALL-DC-AC-RATIO-001; Fronius Primo Gen24 Technical Bulletin TB-PRIMO-2024-01; Enphase IQ8 Series Datasheet UK & Ireland 2024. For questions about DC/AC ratio sizing under MCS or G98/G99, contact contact@solarcalculatorhq.com.

Frequently asked questions

What DC/AC ratio is allowed under the UK G98/G99 grid connection rules?
Engineering Recommendation G98 (single dwelling, single-phase, up to 16 A per phase or about 3.68 kW AC) and G99 (everything else) regulate the inverter AC nameplate, not the DC array. There is no UK regulatory cap on the DC/AC ratio itself — you can install a 6 kWp DC array behind a 3.68 kW G98-notified inverter and the DNO has no objection. MCS MIS 3002 Issue 4.0 simply requires that the inverter be appropriately sized for the array per the manufacturer's design guide. Practical UK ratios run 1.20–1.35 because the low average irradiance (UK national average 2.7 PSH per Energy Saving Trust) means clipping losses are modest and the marginal kWh from extra panels is valuable at the current 27 p/kWh import rate.
How much does clipping cost a typical UK domestic install?
A 4.8 kWp DC array on a 3.68 kW G98 inverter (1.30 ratio) at the UK 2.7 PSH average clips about 1.4% of theoretical AC production, or roughly 50 kWh per year. At the 27 p/kWh Ofgem domestic average that is £13.50 per year of clipping loss, against an extra £350 of DC capex for the oversized array (8 extra 410 W panels) amortised at £14 per year over 25 years. Net benefit of going from 1.00 to 1.30 DC/AC ratio: about £190 per year in extra delivered energy minus £14 amortised capex minus £13.50 clipping = £163 per year net. Energy Saving Trust has been quietly recommending 1.20–1.30 sizing since 2022 for exactly this reason.
Does the SEG export tariff affect the optimal DC/AC ratio?
Yes — strongly. Under Smart Export Guarantee tariffs from Octopus (15p), EDF (5.6p), British Gas (15.1p), or Good Energy (5.6p), exported clipping-prone noon-peak energy is worth far less than self-consumed energy at the 27p import rate. If your daytime self-consumption is high (heat pump, EV charging, battery), the clipped kWh is worth the full import rate avoided — push ratio to 1.30–1.40. If you export most of the daytime generation at 5.6p, the marginal value of extra DC drops by 80% and the optimal ratio drops to 1.10–1.15. Add a 5–10 kWh battery and the calculation flips back: stored clipping is worth the avoided import rate.
Which UK-market inverters allow the highest DC/AC ratio?
Most UK-distributed inverters allow 1.40 to 1.55 DC/AC: Solis 1P/3P series 1.50, Growatt MIN-XH 1.50, GoodWe NS/DNS 1.50, Sungrow SG-RS 1.40, Fronius Primo Gen24 1.50, SMA Sunny Boy 1.50, SolarEdge HD-Wave 1.55, Enphase IQ8M module-level 1.45. The Tesla Powerwall 3 inverter (integrated PV input) allows 1.50 DC/AC and is MCS-listed. Hybrid inverters from GivEnergy (Gen3), Sunsynk, and Fox ESS allow 1.30–1.50 with a battery wired in — the battery soaks up the clipped energy and dispatches it in the 4–7 pm Agile peak window.
Will oversizing void my MCS installation certificate?
No, provided the inverter manufacturer's published DC/AC limit is not exceeded and the system design is documented in the MCS handover pack per MIS 3002 Section 4.2. The installer must record the actual DC nameplate, the inverter AC nameplate, and the resulting ratio in the design calculations, and the inverter datasheet must allow the chosen ratio. RECC and TrustMark accreditation both follow MCS — there is no separate ratio limit. The only practical constraint is the G98/G99 notification, which is based purely on the inverter's AC rating; a 6 kWp DC array on a 3.68 kW inverter is still a G98 install.

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