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A primer for new owners and investors

How a solar and storage plant earns, from irradiance to the evening peak

Eight steps, term first, then plain language. The battery is what turns a daytime plant into a dispatchable evening one.

8steps, water to wire
4 htypical battery duration
25 yrtypical PPA term
35 yrplant life to plan for

Process flow

Water to wire, in eight steps

  1. 01

    Irradiance

    Sunlight: free fuel, measured site by site and modelled hour by hour.

  2. 02

    PV modules

    On the ground, on trackers or on floats on a lake, modules turn light into DC power.

  3. 03

    Combiners and inverters

    DC is gathered and converted to grid-ready AC at 690 V.

  4. 04

    MV transformers

    Stepped up to medium voltage and collected on one bus, the plant's internal backbone.

  5. 05

    Battery storage

    The battery charges by day and returns full power for four hours after sunset.

  6. 06

    Generator step-up

    One substation raises the voltage; one line carries the plant to the grid.

  7. 07

    Point of interconnection

    Where the plant meets the grid. Metered here, and paid under the PPA.

  8. 08

    Load

    Homes and industry: the evening peak served from the plant.

A day in the life

Day: PV to grid, surplus to the battery. Evening: the battery to the grid.

Demand peaks after sunset, the hours a grid pays most for. A four-hour battery moves midday energy into those hours, and can also hold frequency and voltage, reduce curtailment and help restart the grid.

A day of a solar plant with four hours of battery storageIllustrative profile. Solar output rises after sunrise, peaks at midday and falls to zero at sunset, while part of it charges the battery. Demand peaks in the evening after sunset; the battery discharges at full power for four hours to serve that peak.00:0003:0006:0009:0012:0015:0018:0021:0024:00Solar to gridsurplus charges BESSBESS, 4 hevening peakPOWER (ILLUSTRATIVE)A day of a solar plant with four hours of battery storage: solar peaks at midday, the battery serves the evening peak00h06h12h18h24hSolarBESS 4 h-- -- demand, evening peak
Illustrative profile, not to scale.

How the plant connects

One bus, one step-up, one line to the grid

PV blocks and the battery feed one medium-voltage collection bus, one generator step-up transformer and one gen-tie to the point of interconnection, where the plant meets the grid and is metered.

The final voltage levels follow the utility's standard and the transformer offering. Sizes on any concept diagram are indicative until the studies are done.

Concept single-line diagram of a solar and storage plantPV strings at 1500 volts DC feed inverters that convert to AC at 690 volts. Transformers step up to a 35 kV class collection bus. The battery, through its power conversion system, connects to the same bus. A generator step-up transformer raises the voltage to transmission level, and a gen-tie line runs to the point of interconnection with the utility, where the energy is metered. The plant controller and SCADA send telemetry to the grid operator.PV / FPV1500 VdcInverterDC to ACMV transf.0.69/35 kVBESSPCS + BMSGSU35 kV / HVPOImetered35 KV CLASS BUSGEN-TIEUTILITY GRIDPPC / SCADA: P, Q, V, f to the grid operatorConcept single-line diagram: PV to inverter to MV transformer to a 35 kV class bus shared with the battery, then the step-up transformer, the gen-tie and the point of interconnection with the utility grid35 KV CLASS BUSPV / FPV1500 VdcInverterDC to ACMV transf.0.69/35 kVBESSPCS + BMSGSU35 kV / HVGEN-TIEPOImeteredUTILITY GRIDPPC / SCADAto the gridoperator
Concept single-line diagram (SLD), simplified.

Where to put the panels

Floating, ground-mount or agrivoltaic

Where land is scarce and farmed, water and dual use are reasonable choices. Each brings its own cost and risk, and the choice should come from a study.

Floating PV

Uses water instead of farmland, and the water cools the modules, which adds yield. Floats, mooring and anchoring add cost and risk that a lake survey must bound.

Ground-mount

The established utility-scale form: fixed tilt or trackers, the lowest cost per watt where land, soil and access allow.

Agrivoltaics

Crops or grazing under elevated panels. Row pitch and clearance height trade a little yield for land that stays in production.

The owner's view

A 25-year contract on a 35-year plant

The tariff is won in year 0 and kept in years 8 to 25. We price every component that will wear out before financing closes, so the financial model, the maintenance contract and the warranties tell one story.

  1. Yr 0 to 2

    Build and commission

    Capacity and performance tests set the baseline.

  2. Yr 1 to 10

    Warranty window

    Module, inverter, float and battery warranties enforced by owner-side audits.

  3. Yr 8 to 12

    BESS augmentation

    Cells added or replaced to hold the usable capacity.

  4. Yr 12 to 15

    Inverter and PCS replacement

    Planned and priced in year 0, not a surprise.

  5. Yr 15 to 25

    Second battery life

    Augmentation or full replacement; tariff and dispatch re-optimised.

  6. Yr 25 to 35

    Beyond the PPA

    Modules and floats still working: extension, merchant sale or repowering.

Picture glossary

The terms, explained before they are decided

The words that appear in a notice, a PPA or a design review, in plain language.

  • FPVFloating photovoltaic
    Modules on HDPE floats, moored to the lake bed and the shore.
  • APVAgrivoltaics
    Dual-use solar over crops or grazing.
  • BESSBattery energy storage system
    Stores energy by day and returns it when the grid needs it.
  • PCS and BMSPower conversion, battery management
    The battery's inverter, and the system that keeps its cells healthy and safe.
  • GSUGenerator step-up transformer
    Raises the plant's voltage to transmission level.
  • POIPoint of interconnection
    Where the plant meets the grid and is metered.
  • PPAPower purchase agreement
    The contract under which the utility buys the plant's energy.
  • EPCEngineering, procurement, construction
    The contractor that builds the plant for a fixed scope and price.
  • OEOwner's engineer
    The engineer who works for the owner, not the builder.
  • IFR to IFCIssued for review to issued for construction
    From first drawings to the drawings a plant is built from.
  • FAT and SATFactory and site acceptance tests
    Equipment proven at the factory, then again on site.
  • P50 and P90Expected and bank-case yield
    The energy a plant beats half the time, and nine years in ten.
  • PVsyst 8760Hourly energy model
    A model of every hour of the year: 8,760 of them.
  • CurtailmentTurn-down by the grid
    The grid instructing the plant to reduce output.
  • Deemed energyPaid but not taken
    Energy the plant could have delivered, paid under the PPA when the grid could not take it.
  • SCADA and PPCControl and monitoring
    The layer that runs the plant and talks to the grid operator.
  • ESIAEnvironmental and social impact assessment
    The study of the plant's effect on land, water and communities.
  • CODCommercial operation date
    The day the plant starts earning under the PPA.

New to power?

We walk owners through every term before it is decided.

Ask an engineer Watch the film