United Kingdom version
How it works
Estimate the financial effect of a specified rooftop solar scenario, with exported electricity valued separately from electricity used in the home.
This is the United Kingdom planning view, with monetary inputs and outputs in GBP. A currency label does not establish that a tax rate, construction price, coverage rule or legal limit has been verified locally. The inputs and limitations below describe the actual scope.
Input query strings and outputs
Use an input name in this page’s URL as ?name=value, and join additional inputs with &. Portable shared links may instead use the compact ?ac= state parameter.
Input query strings
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?annualUse=Annual household electricity use, kWhNumber · Required · Default: 10,000
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?offset=Target annual generation / usePercentage · Required · Default: 80
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?specificYield=Site-specific annual yield, kWh per kWNumber · Required · Default: 1,200
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?selfConsumption=Generation used directly on sitePercentage · Required · Default: 60
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?importRate=Avoided electricity price per kWhMoney · Required · Default: 0.2
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?exportRate=Confirmed export payment per kWhMoney · Required · Default: 0.05
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?installedCostPerKw=Installed cost per kWMoney · Required · Default: 2,500
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?confirmedIncentive=Confirmed eligible incentiveMoney · Required · Default: 0
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?maintenance=Annual maintenance allowanceMoney · Required · Default: 100
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?years=Planning horizon, yearsNumber · Required · Default: 25
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?discountRate=Annual discount ratePercentage · Required · Default: 4
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?degradation=Annual output degradationPercentage · Required · Default: 0.5
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?energyEscalation=Annual energy-price escalationPercentage · Required · Default: 2
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?replacementYear=Replacement year (0 means none)Number · Required · Default: 0
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?replacementCost=Replacement costMoney · Required · Default: 0
Outputs
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sizeKwTarget DC system size, kWNumber
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generationFirst-year generation, kWhNumber
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selfConsumedOn-site use of generation, kWhNumber
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exportedExported generation, kWhNumber
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capitalNet capital costCurrency · Currency: GBP
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annualSavingFirst-year net savingCurrency · Currency: GBP
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paybackSimple payback, yearsNumber · Primary output
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npvDiscounted net valueCurrency · Currency: GBP
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breakevenCash-flow break-even yearNumber
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cumulativeUndiscounted lifetime net benefitCurrency · Currency: GBP
Formula
kW = annual use × target offset ÷ specific yield; net benefit = self-used kWh × avoided rate + exported kWh × export rate − maintenance.
Calculations retain numeric precision internally. Displayed values are rounded for readability. Quantities that must be purchased as whole units are rounded up only where the formula explicitly requires it.
Read the inputs before calculating
- Annual household electricity use, kWh: Enter the quantity described by the label.
- Target annual generation / use: Enter the quantity described by the label.
- Site-specific annual yield, kWh per kW: Enter the quantity described by the label.
- Generation used directly on site: Enter the quantity described by the label.
- Avoided electricity price per kWh: Enter the quantity described by the label.
- Confirmed export payment per kWh: Enter the quantity described by the label.
- Installed cost per kW: Enter the quantity described by the label.
- Confirmed eligible incentive: Enter the quantity described by the label.
- Annual maintenance allowance: Enter the quantity described by the label.
- Planning horizon, years: Enter the quantity described by the label.
- Annual discount rate: Enter the quantity described by the label.
- Annual output degradation: Enter the quantity described by the label.
- Annual energy-price escalation: Enter the quantity described by the label.
- Replacement year (0 means none): Enter the quantity described by the label.
- Replacement cost: Enter the quantity described by the label.
Reproducible worked example
The following is a fictional arithmetic example, not a local quote, a verified claimant or a market forecast. Its values are included so the formula can be checked. The form does not automatically confirm any verification checkbox.
| Input | Example value |
|---|---|
| Annual household electricity use, kWh | 10,000 |
| Target annual generation / use | 80 |
| Site-specific annual yield, kWh per kW | 1,200 |
| Generation used directly on site | 60 |
| Avoided electricity price per kWh | 0.2 |
| Confirmed export payment per kWh | 0.05 |
| Installed cost per kW | 2,500 |
| Confirmed eligible incentive | 0 |
| Annual maintenance allowance | 100 |
| Planning horizon, years | 25 |
| Annual discount rate | 4 |
| Annual output degradation | 0.5 |
| Annual energy-price escalation | 2 |
| Replacement year (0 means none) | 0 |
| Replacement cost | 0 |
| Output | Calculated result |
|---|---|
| Target DC system size, kW | 6.6667 |
| First-year generation, kWh | 8,000 |
| On-site use of generation, kWh | 4,800 |
| Exported generation, kWh | 3,200 |
| Net capital cost | GBP 16,666.6667 |
| First-year net saving | GBP 1,020 |
| Simple payback, years | 16.3399 |
| Discounted net value | GBP 2,166.0581 |
| Cash-flow break-even year | 14.6056 |
| Undiscounted lifetime net benefit | GBP 14,461.5828 |
Interpreting a changed assumption
Start with one complete scenario and change only one input when diagnosing a difference. Recheck units, the period of every rate, whether an amount is recurring or one-off, and whether a cost is already included elsewhere. A larger calculated benefit is not evidence that an assumption is more likely to occur.
Limitations
- Solar yield, export compensation and self-consumption are explicit assumptions, not a site survey or utility eligibility determination.
- Incentives must be confirmed for the installation date. No automatic US residential federal solar credit is applied to a 2026 installation.
- A positive simple payback is not a financing offer; degradation, replacement and discounting appear separately in the cash-flow table.
Frequently asked questions
What does this Solar Savings result represent?
Estimate the financial effect of a specified rooftop solar scenario, with exported electricity valued separately from electricity used in the home. It applies the displayed formula to your inputs; it does not infer omitted facts.
Are the starting amounts verified United Kingdom prices or legal rules?
No. Numeric defaults are illustrative unless an input or dated reference explicitly identifies a researched component. Currency is GBP; local quotes, eligibility and unsupported rules must be entered and verified separately.
What must I check before relying on the result?
Check the input units, supplier or authority evidence, the stated model exclusions and the period used for costs. This arithmetic does not supply missing site-specific design or local legal rules.
Can I compare or share different assumptions?
Change one assumption at a time and use the browser-local project or share controls. A shared URL contains the encoded input values; do not include information you do not wish to disclose.
Sources and reference scope
- DOE energy-use arithmetic — retrieved 2026-09-24. Energy consumption versus power; not a local tariff or an approval of this implementation.
- PVWatts: site-specific solar estimation — retrieved 2026-09-24. External site-yield model. AskCalculators uses your entered yield; it does not call or reproduce PVWatts.
- DOE insulation overview — retrieved 2026-09-24. Insulation and thermal resistance concepts; not structural or code approval.
These references support only their stated scope. They are not evidence that every local rule or price needed by the master expansion has been loaded.