Start from a fossil-fueled baseline, then switch on low carbon technologies in any order. Running costs and the assets you would replace anyway are tracked together, so each step is compared fairly.
Schedules the battery to minimise cost against the half-hourly prices (charging when cheap, discharging or exporting when expensive, with perfect foresight) rather than following the simple solar-surplus rule. With flat import and export prices there is nothing to arbitrage, so this makes little difference.
Octopus Cosy, priced from the real half-hourly price feed (region K, inc. VAT, including price-cap changes through the year). Cheap windows 04:00–07:00, 13:00–16:00 and 22:00–00:00, a teatime peak 16:00–19:00, day rate in between. To edit rates by hand, use Custom instead.
Prices (p/kWh) apply from each band's start until the next, wrapping past midnight.
Average across this build: {{ current.avgAgileImport==null ? Number(p.elecRate).toFixed(1) : current.avgAgileImport.toFixed(1) }}p import / {{ current.avgAgileExport==null ? Number(p.segRate).toFixed(1) : current.avgAgileExport.toFixed(1) }}p export.
Each interval's grid import and export is valued at the national grid carbon intensity at that time. Across this build: average import intensity {{ current.avgImportIntensity==null ? '–' : Math.round(current.avgImportIntensity) + ' g/kWh' }} · average export intensity {{ current.avgExportIntensity==null ? '–' : Math.round(current.avgExportIntensity) + ' g/kWh' }}.
Carbon intensity data unavailable in the loaded dataset — using the flat factors.
Electricity import, export & standing charge are set under Build your home · Tariff (flat, Agile or a custom schedule).
Lumpy purchases (car, boiler, heat pump, solar, battery) are turned into an equivalent annual cost with a capital-recovery (annuity) factor at this real (above-inflation) rate, so cash spent today weighs more than cash spent years out; resale values, received at end of life, are discounted back the same way. Set to 0 for plain straight-line spreading. Running costs (energy, fuel, maintenance) are level year-on-year, so a constant annual figure already is their discounted equivalent.
Used by the payback / IRR / ISA-crossover metrics in This build · the numbers: the extra upfront is treated as the investment and the drop in running costs as the annual return over this horizon. The crossover compares putting that same money in a shares ISA growing at the rate above against spending it now and reinvesting each year's bill saving.
Dispatches solar, battery and EV charging across a real year of 15-minute data (from the solar-matching dataset). Self-consumption, import and export emerge from the timing rather than a fixed ratio. The EV charge window is set under Electric vehicle. The Agile tariff switch always uses this simulation, since it prices each interval against the dataset's half-hourly wholesale-linked rates. Optimal battery dispatch is set under Build your home, alongside the home battery.
Simple annualised estimate: a fixed daytime-match plus battery shift, with no time-of-day detail. Useful as a rough comparison against the 15-minute simulation.
Grid import and export carbon follow the national half-hourly intensity dataset — set under Build your home · Grid carbon.
| Running costs | |
| Petrol | {{ gbp(current.petrol) }} |
| Gas{{ cfg.hp && !cfg.disconnectGas ? ' (cooking only)' : '' }} | {{ gbp(current.gas) }} |
| Electricity import{{ cfg.agile ? ' ('+tariffLabel+')' : '' }} | {{ gbp(current.importCost) }} |
| {{ cfg.exportMode==='agile' ? 'Agile export credit' : 'Solar export credit' }} | −{{ gbp(current.exportRevenue) }} |
| Electricity standing charge | {{ gbp(current.elecStandingCost) }} |
| Running subtotal | {{ gbp(current.running) }} |
| Annualised assets | |
| {{ cfg.ev?'EV':'Petrol car' }} | {{ gbp(current.carAsset) }} |
| {{ cfg.hp?'Heat pump':'Gas boiler' }} | {{ gbp(current.heatAsset) }} |
| Solar PV | {{ gbp(current.solarAsset) }} |
| Battery | {{ gbp(current.batteryAsset) }} |
| Assets subtotal | {{ gbp(current.assets) }} |
| All-in total | {{ gbp(current.allIn) }} |
| Operational | |
| Petrol | {{ co2(current.petrolCO2) }} |
| Gas{{ cfg.hp && !cfg.disconnectGas ? ' (cooking)' : '' }} | {{ co2(current.gasCO2) }} |
| Grid electricity | {{ co2(current.gridCO2) }} |
| Exported solar (displaces gas) | −{{ co2(current.exportCO2Credit) }} |
| Operational subtotal | {{ co2(current.opCO2) }} |
| Embodied (annualised) | |
| {{ cfg.ev?'EV (glider + battery)':'Petrol car' }} | {{ co2(current.carCO2) }} |
| {{ cfg.hp?'Heat pump':'Gas boiler' }} | {{ co2(current.heatCO2) }} |
| Solar PV | {{ co2(current.solarCO2) }} |
| Battery | {{ co2(current.batteryCO2) }} |
| Embodied subtotal | {{ co2(current.embCO2) }} |
| Total CO₂e | {{ co2(current.totalCO2) }} |
The household figure is net of the {{ gbp(p.busGrant) }} BUS grant, so it can read as a saving: privately the homeowner may be paid to decarbonise. The societal figure adds the grant back, because it is a transfer from taxpayers, a real resource cost rather than a saving the measure creates, so the true cost of abatement to society is higher. For context, UK government appraisal currently values carbon at roughly £250/tCO₂e.
Switching adds about {{ co2(extraEmbodiedTotal) }} of one-off manufacturing carbon (the carbon debt). Lower running emissions repay it in {{ carbonPayback.toFixed(1) }} years, after which every year is a net saving. Running emissions do not fall in this build, so that debt is never repaid.
| Total electricity demand | {{ kwh(current.demand) }} |
| Household baseload | {{ kwh(p.elecBaseload) }} |
| Cooking (induction) | {{ kwh(current.cookingElec) }} |
| EV charging | {{ kwh(current.evElec) }} |
| Heat pump | {{ kwh(current.hpElec) }} |
| Solar generated | {{ kwh(current.solarGen) }} |
| Self-consumed ({{ current.selfPct }}%) | {{ kwh(current.solarSelf) }} |
| Exported | {{ kwh(current.solarExport) }} |
| Grid import | {{ kwh(current.gridImport) }} |
| Average {{ tariffLabel }} import price | {{ current.avgAgileImport.toFixed(1) }} p/kWh |
{{ current.selfPct }}% of your solar is used at home. Adding load (EV, heat pump) or a battery pushes this up, because every self-consumed kWh is worth the {{ current.avgAgileImport!=null ? current.avgAgileImport.toFixed(1) : Number(p.elecRate).toFixed(1) }}p {{ current.avgAgileImport!=null ? 'average ' : '' }}import rate you avoid, against only {{ current.avgAgileExport!=null ? current.avgAgileExport.toFixed(1) : Number(p.segRate).toFixed(1) }}p {{ current.avgAgileExport!=null ? 'average ' : '' }}if it is exported.
| Use | kWh/yr | cash | all-in |
|---|---|---|---|
| {{ row.label }} | {{ kwh(row.r.kwh) }} | {{ row.r.cashRate.toFixed(1) }}p | {{ row.r.levRate.toFixed(1) }}p |
What each use actually pays per kWh once solar and battery are mixed in. Cash prices home-generated power at what it gives up: self-used solar at the {{ current.elecRates.forgoneExport.toFixed(1) }}p export you forgo, battery at the {{ current.elecRates.batUnitCash.toFixed(1) }}p it cost to charge. All-in prices it at its own capital cost: solar at its {{ current.elecRates.solarLcoe.toFixed(1) }}p levelised cost, battery at {{ current.elecRates.batUnitLev.toFixed(1) }}p including the storage hardware. This is a diagnostic view only; the capital is already in the asset lines, so it is not added again to the all-in total.
The heat pump's electricity effectively costs {{ current.elecRates.hp.cashRate.toFixed(1) }}p–{{ current.elecRates.hp.levRate.toFixed(1) }}p/kWh (grid {{ Math.round(current.elecRates.hp.gridKwh/current.elecRates.hp.kwh*100) }}%, solar {{ Math.round(current.elecRates.hp.solarKwh/current.elecRates.hp.kwh*100) }}%, battery {{ Math.round(current.elecRates.hp.batteryKwh/current.elecRates.hp.kwh*100) }}%), at the flat grid rate, before any solar or battery.
At SCOP {{ p.scop }} that is {{ current.sparkGap.hpHeatCash.toFixed(1) }}p–{{ current.sparkGap.hpHeatLev.toFixed(1) }}p per kWh of heat, versus gas at {{ current.sparkGap.gasHeat.toFixed(1) }}p/kWh ({{ p.gasRate }}p ÷ {{ Math.round(p.boilerEff*100) }}% boiler).
A unit of heat is {{ Math.round((1-current.sparkGap.hpHeatLev/current.sparkGap.gasHeat)*100) }}–{{ Math.round((1-current.sparkGap.hpHeatCash/current.sparkGap.gasHeat)*100) }}% cheaper from the heat pump than from gas. Solar and battery widen that gap by cutting the heat pump's unit rate. Heat is cheaper from the heat pump on a cash basis, but once the solar and battery capital is priced in it is roughly level with gas. More solar or battery capacity, or a cheaper tariff, tips it further in the heat pump's favour. At these rates a unit of heat costs more from the heat pump than from gas. A higher SCOP, a cheaper tariff (Agile), or self-supply from solar and battery would close the gap.
How to read this. Lumpy purchases (a car, a boiler) are spread over their life as an equivalent annual cost, using an annuity at a real {{ p.discountRate }}% discount rate (editable under Assumptions · Discounting; set it to 0 for plain straight-line spreading). This lets options compare like-for-like, and means cash spent today weighs more than costs and resale values years away. The fossil status quo already carries a car and a boiler you would replace anyway, so switching to an EV or heat pump swaps one asset cost for another rather than adding to it. ‘Extra upfront’ is the cash needed today beyond a like-for-like petrol car or boiler replacement.
The model. A real year of 15-minute data is simulated interval by interval (model.js): solar, battery and EV charging are dispatched through the year, so self-consumption and the import / export split emerge from the timing rather than a fixed ratio. The Agile tariff switch re-costs the same dispatch against the dataset's half-hourly wholesale-linked prices; with it off, flat import and export rates apply. The EV charges within the window set under Electric vehicle; a simple annualised estimate is available under Advanced · solar matching for comparison. v2 · 15-minute
Simplifications. Insurance is treated as roughly neutral between petrol and EV. All prices include VAT: flat rates default to the price cap, and Agile import comes from the region-D dataset grossed up by 5%. Prices are a mid-2026 snapshot and unusually high.
Carbon basis. Petrol ~2.9 kgCO₂e/L well-to-wheel; gas 0.183 kgCO₂e/kWh burned plus an upstream and methane uplift (default +20%, GWP100, and likely a floor rather than a ceiling). Grid electricity defaults to the measured national half-hourly carbon intensity dataset: each interval's import and export is valued at the grid intensity at that time, so overnight EV charging, battery arbitrage and midday solar export each carry their true carbon weight. Note this is an average-intensity basis on both sides — export is credited with the grid mix it displaces at that moment, not the marginal plant. Switching Build your home · Grid carbon to Flat uses two fixed factors instead: imports at ~75 gCO₂/kWh (a deliberately conservative forward average — 2024-25 actual is ~125, falling towards ~50 by 2030) and exported solar credited with the marginal gas (CCGT) generation it backs out, at ~400 gCO₂/kWh — a much larger export credit than the average basis gives. Expect both the price and the carbon value of export to fall as more solar joins the grid (daytime prices decline, curtailment rises). Both flat factors are editable under Carbon · operational factors. Embodied carbon follows the Hoekstra framing: the car glider is roughly equal for petrol and EV, the battery is the main difference (~75 kgCO₂e/kWh of cell), and ‘carbon payback’ is that one-off extra manufacturing carbon divided by the annual operational saving. factors editable
Abatement cost. The change in all-in annual cost divided by the tonnes of CO₂e cut per year (whole build vs status quo, or per step given the rest of the build). A negative figure means the measure cuts carbon and saves money, so the household is effectively paid to decarbonise. A positive figure is the cost per tonne avoided (for context, UK government appraisal values carbon at roughly £250/tCO₂e). The household figure is net of any grant; the societal figure adds the BUS grant back, since a grant is a transfer from taxpayers rather than a saving the measure creates. Steps that do not cut carbon show ‘n/a’.