Hybrid Solar System Calculator

What this calculator does

This calculator sizes a complete grid-tied hybrid solar PV and battery system for a UK home, based on your annual electricity consumption, your local Peak Sun Hours, and how much of your evening peak you want to cover with stored energy. It returns:

• PV array (kWp) — the panel nameplate needed to offset your target share of annual import
• Battery (usable kWh and nameplate kWh) — the storage required to cover your evening peak and any EPS critical-load reserve
• Hybrid inverter (kW continuous) — sized to PV output, EPS surge, and the G98/G99 export threshold
• Annual PV yield (kWh), self-consumption share (%) and annual SEG export (kWh)

The defaults reflect an average UK household: 8 kWh/day consumption (≈2,900 kWh/year per Ofgem TDCV), 2.6 Peak Sun Hours (PVGIS Midlands average), 1.5 kW backed-up critical load (boiler controls, fridge-freezer, broadband and lighting) for 6 hours, and a LiFePO₄ battery at 90% DoD with 95% round-trip efficiency.

How hybrid sizing works (first principles)

A hybrid system has three sizing problems that connect through the inverter.

1. PV array (kWp)

PV_kWp = (daily_kWh × offset) ÷ (PSH × performance_ratio)

The performance ratio bundles inverter loss, soiling, panel temperature derate, cable resistance and shading. PVGIS reports UK rooftop PR between 0.78 and 0.84; this calculator uses 0.80 as a midpoint, which matches the Energy Saving Trust’s published assumption. A hybrid system runs 1–2 points higher than straight grid-tied because the battery absorbs midday clipping during summer that a straight grid-tie wastes when the inverter saturates.

2. Battery

usable_kWh    = backup_hours × backed_up_load_kW
nameplate_kWh = usable_kWh ÷ (DoD × round_trip_efficiency)

LiFePO₄ — the de-facto chemistry in 2026 — can be cycled to 90% of nameplate without warranty issues and returns about 95% of energy in. Older lead-acid AGM gives only 50% DoD and 85% round-trip, so you need around 2.1× the nameplate to deliver the same usable kWh. Anything below 75% round-trip efficiency on a battery datasheet should be treated with suspicion.

3. Hybrid inverter

inverter_kW = max(PV_kWp × 0.95, backed_up_load_kW × 1.25)

Two constraints: AC output during peak sun and motor-surge headroom on the EPS loop (fridge compressors, central-heating circulators, well pumps for rural properties). The greater of the two is the binding value. On single-phase G98, the inverter export must also stay at or below 16 A (≈3.68 kW) unless you are on a G99 agreement.

UK installed costs (2026)

Mid-2026 UK hybrid pricing, drawn from MCS installer quotes and Energy Saving Trust market data:

Battery costs in the UK have fallen roughly 9% year-on-year since 2023. The LiFePO₄ floor in mid-2026 sits at about £400–£480 per usable kWh installed, with Checkatrade and the Energy Saving Trust quoting nearly identical ranges. The variance largely reflects scaffolding cost (Greater London adds £600–£900) and roof type (slate is consistently £1,000–£1,500 more than concrete tile).

When a hybrid pays off in the UK

Three scenarios where the maths works:

1. You’re on a time-of-use import tariff. Octopus Flux, Intelligent Octopus, EDF GoElectric and OVO Charge Anytime offer overnight rates below 10 p/kWh and peak rates near 30–40 p/kWh. The battery arbitrages: store cheap overnight import (or surplus midday PV) and discharge during 16:00–19:00 peak. On Octopus Flux specifically, a 10 kWh battery typically pays back in 6–8 years, even before factoring in PV output.
2. You want SEG protection plus self-consumption uplift. The Smart Export Guarantee export rates ranged from 5p to 15p/kWh in early 2026 — well below the 28–32 p import price. A hybrid pushes self-consumption from around 30% (PV-only) to 75–85%, which is worth roughly £300–£500/year on a 4 kWp array.
3. You’re rural and lose power more than 4 hours a year. EPS-capable hybrid inverters (GivEnergy Gen3, Solis S6 Pro, Sunsynk ECCO) keep selected circuits alive during a DNO outage. Above 10 hours of annual outage, a 5 kWh battery typically beats a petrol genset on lifetime cost.

For the SEG numbers specifically, see the solar feed-in tariff calculator. For a deeper payback model, run the solar battery ROI calculator.

BS 7671 and DNO requirements

• BS 7671 (IEE Wiring Regulations 18th Edition Amendment 2:2022) — governs all fixed installations. Section 712 covers PV; battery systems must meet Section 722 (energy storage). A separate AC isolator on the consumer-unit feed and a labelled DC isolator at the array are mandatory.
• G98 (Engineering Recommendation, Energy Networks Association) — fast-track notification for ≤16 A single-phase or ≤16 A per phase three-phase. Installer must notify the DNO within 28 days; no prior approval. Most 4 kWp PV-only systems and many 5 kWp hybrid systems with export limiting fit here.
• G99 — required for anything above the G98 limits. Prior application to the DNO, 4–10 week response. Some constrained networks (London inner zones, parts of South Wales, Cornwall) will impose export limits down to 3.68 kW even on G99 applications.
• MCS standard MCS 005 / MIS 3002 — installer accreditation. Required to claim SEG payments and to maintain the 0% VAT rate.

The inverter must hold an EN 50549-1 compliance certificate and (since 2022 update) loss-of-mains protection per ENA Type Tested matrix. Anti-islanding disconnects within 0.5 s of grid loss and reconnects after a 60 s observation window per the Distribution Code.

Common UK sizing mistakes

• Oversizing PV past your roof’s south-facing limit. UK irradiance is 30–40% lower than southern Spain. Pushing an east/west split above 6 kWp rarely pays back faster — diminishing returns kick in around 75% offset of annual use. Below 75% offset, every additional kWp is highly value-accretive; above that, battery storage is the better next pound.
• Buying battery storage before PV is sized correctly. A 10 kWh battery on a 2.5 kWp array sits half-empty most days. The MCS rule of thumb: usable battery kWh = around 1.0–1.3 × daily PV generation in shoulder season.
• Ignoring G98 export-limit firmware. If you’re on the G98 fast-track route, the inverter must be export-limited to 3.68 kW. Some installers leave default firmware and create non-compliant exports that risk SEG payment claw-back.
• Forgetting battery temperature derate. LiFePO₄ stops accepting charge below 0 °C. Outhouse and garage installs in Scotland and northern England need a heated enclosure or warranty cover lapses.

Sources

• MCS — Microgeneration Certification Scheme — installer accreditation and product standard
• Energy Saving Trust solar guide — UK consumer cost and savings data
• Ofgem Smart Export Guarantee — SEG licensee tariff list
• Solar Energy UK market reports — UK installed system pricing
• Energy Networks Association G98 and G99 — DNO connection process
• HMRC VAT Notice 708/6 — 0% VAT relief detail
• Checkatrade solar cost guide — installer-sourced quote ranges