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Find out what it actually saves you.
Answer a handful of everyday questions — what you pay, how far you drive, what you spend on heating — and get a straight answer. We show the full working, so you can check every number yourself.
Every home runs on more than one price a day. This is yours.
What do you want to work out?
Plain questions only
No jargon, no efficiency coefficients. If you can read your energy bill, you can use this. Anything technical sits behind a "fine-tune" link with a sensible default already filled in.
The maths is on the page
Every result comes with a line-by-line breakdown showing exactly how we got there — the same sums you'd do on paper, just faster.
Honest about the guesswork
Where a result is too close to call, the page says so — a small saving that a slightly different price could wipe out isn't shown as a solid win. You'll never see a confident number where the underlying gap is this thin.
Before you rely on any of this
The legal bit, in plain English
Estimates, not a quote
Every figure on this site is an illustration, not a promise. Your actual bill depends on your property, your usage, market prices on the day and the quality of the installation — all of which will differ from what's shown here. Get proper quotes before spending money.
Not advice
Nothing here is financial, legal, tax or engineering advice, and using this site doesn't create an advisory relationship of any kind. Always get independent professional advice, and at least two or three proper quotes, before signing anything.
No warranty on accuracy
Figures, calculations and third-party rates are believed correct when published, but may be out of date, incomplete or simply wrong. We don't guarantee the accuracy, completeness or currency of anything on this page. Check anything material against a primary source before acting on it.
Third parties named here
Tariff names, suppliers, government schemes and product names are referenced for identification only. We aren't affiliated with, endorsed by, or speaking for any of them, and all trade marks belong to their owners. Rates and scheme terms change frequently — always confirm current details directly with the provider.
No commercial relationships
This site carries no adverts, affiliate links or paid placements, and no one pays to appear in these results. Nothing here is a recommendation to buy from any particular supplier. If that ever changes, it will be disclosed clearly on this page first.
Your decision, your risk
To the fullest extent the law allows, WattWise and its operators exclude all liability for any loss — financial or otherwise, direct or indirect — arising from use of, or reliance on, this site or its calculators. Nothing here limits liability the law doesn't allow us to limit, such as liability for death or personal injury caused by negligence, or for fraud.
Where each figure comes from
The calculators start with default values so they work before you type anything. Some come from published sources; others are reasonable industry estimates that have not been verified. Anything marked estimate should be replaced with your own figure or a real quote before you rely on it.
| Figure | Default | Basis |
|---|---|---|
| Electricity unit rate | 26.11p/kWh | Sourced — Ofgem price cap, 1 Jul–30 Sep 2026. Resets quarterly. |
| Gas unit rate | 7.33p/kWh | Sourced — Ofgem price cap, same period. |
| Typical gas use | 6,000 / 9,500 / 14,000 kWh | Sourced — Ofgem Typical Domestic Consumption Values, revised 1 Jul 2026. |
| Boiler Upgrade Scheme | £7,500 / £9,000 | Sourced — £9,000 uplift for off-gas-grid oil and LPG homes, 21 Jul 2026 to 31 Mar 2027. |
| Energy in a litre of kerosene | 10.35 kWh | Sourced — gross calorific value, standard across UK oil suppliers. |
| Heating oil price | 86p/litre | Sourced range — 77p to 132p during 2026. The most volatile figure here. |
| Octopus Go / Intelligent Go windows | 00:30–05:30 / 23:30–05:30 | Sourced — off-peak rates approximate and regional. Day rates unverified. |
| Grid carbon intensity | 0.131 kg CO₂e/kWh | Sourced — DESNZ 2026 conversion factor. Combustion factors not re-verified this year. |
| Solar yield per kW | 900 kWh | Estimate — common rule of thumb. Varies widely with orientation, pitch, shading and region. |
| Solar install cost | £1,400/kW | Estimate — replace with an actual quote. |
| Export payment | 15p/kWh | Estimate — SEG rates are set per supplier and vary widely. |
| Boiler efficiencies | 85 / 72 / 82% | Estimate — real performance is often below the badge figure. |
| Heat pump SCOP / hot water COP | 3.5 / 2.5 | Estimate — depends heavily on design flow temperature and emitter sizing. |
| Heat pump install cost | £5,500 | Estimate — varies widely with property and pipework. |
| Fuel price and mpg | £1.42/litre, 45 mpg | Estimate — not checked against published statistics. |
| Public rapid charging | 79p/kWh | Estimate — varies by network and connector speed. |
| EV efficiency bands | 2.9–4.2 mi/kWh | Estimate — varies with speed, temperature and load. |
| Load shifted to off-peak | 25% heat / 95% EV | Estimate — a judgement, not a derived figure. Set it yourself. |
| All US figures | — | Estimate — the US mode has not been researched. Every value is a placeholder. |
What the model leaves out: payback is simple and undiscounted, with no financing cost. Prices are held flat with no inflation. Solar excludes inverter replacement and maintenance. Heat pump excludes servicing and radiator upgrades. The electric car tab compares running costs only — not purchase price, depreciation, insurance or VED, which normally dominate.
This site and these terms are governed by the law of England and Wales.
Solar panels
Four questions. We'll work out what the panels generate, what that's worth to you, and how many years it takes to get your money back.
Most homes fit somewhere between 8 and 16 panels.
The full price you'd pay, including fitting. We've put in a typical price for that size — change it once you have a quote.
Pick the closest match and adjust from there. Changing any rate or time switches it to Custom. This is shared across all three calculators.
It's on your bill as a unit rate. The UK average is about 27p per kWh.
This matters more than anything else. Electricity you use as it's generated is worth full price. Anything you don't use gets sold back for much less.
Fine-tune the assumptions
Under the Smart Export Guarantee, typical offers are 12–16p per kWh.
A south-facing UK roof gets roughly 900 kWh a year for every 1 kW installed. Shaded or east/west roofs get less.
Panels are usually warrantied for 25 years and lose about 0.5% of their output each year.
Assumes prices stay flat in real terms and the panels keep working as expected. It doesn't include the cost of an inverter replacement (typically once, around year 12–15) or any battery.
Heat pump
Space heating and hot water are worked out separately, because a heat pump doesn't make them at the same efficiency. If you have a heat loss survey, use it — it beats every estimate on this page.
A survey gives you the heat your home actually needs. Bills get there indirectly, through the fuel you burn.
Pick electric heaters only for panel, fan or storage heaters — not an existing heat pump.
It's on your annual statement, and it matters more than anything else here.
Ofgem puts a typical household at 9,500 kWh a year.
The unit rate on your bill, not the standing charge. The Ofgem cap is 7.33p per kWh for July–September 2026.
The SCOP: units of heat out per unit of electricity in. A system designed for low flow temperatures reaches 4.0–4.5. An average install lands near 3.2, a poor one 2.5.
Always lower, because a cylinder needs 50–55°C rather than 35–45°C. 2.2–2.8 is realistic. Quoted SCOPs usually don't include this.
Pick the closest match and adjust from there. Changing any rate or time switches it to Custom. This is shared across all three calculators.
The rate the heat pump would run on. This moves the answer more than the SCOP does.
Enter the price after any grant. The Boiler Upgrade Scheme pays £7,500 in England and Wales — or £9,000 for off-gas-grid oil and LPG homes between 21 July 2026 and 31 March 2027.
Fine-tune the assumptions
The share of fuel that becomes useful heat. An old boiler run hot does worse than its badge figure.
Taken off before the heat calculation. Set to 0 if you cook on electric.
Only if you disconnect the supply completely, cooking included. The UK gas standing charge is 29.04p a day, so about £106 a year. Otherwise leave at 0.
A heat pump only reaches its rated SCOP in a home set up for it — emitters sized for low flow temperatures, and a fabric that holds heat. Ask any installer what design flow temperature their SCOP assumes; the same unit will not deliver 4.2 at 50°C that it delivers at 40°C.
Electric car
Charging cost against fuel cost, on your real mileage. This compares running costs only — not the price of the car itself.
If you're not sure, the UK average is about 7,400 miles.
A starting point only — put your own figure in below if you know it.
This is the number that decides everything on this page. Your car shows it on the trip computer — use whichever unit yours displays.
Pick the closest match and adjust from there. Changing any rate or time switches it to Custom. This is shared across all three calculators.
The Ofgem cap is 26.11p. On an overnight EV tariff it can drop to 7–10p, which changes this result dramatically.
Public rapid chargers cost several times more than charging on your drive, so a small slice of public charging carries a large slice of the cost.
Charging is the most shiftable load in the house — if the cheap window is long enough to fill the car overnight, this is 100%. On Intelligent Octopus Go, smart-scheduled charging is billed at the off-peak rate even when it runs outside the window, so 100% is realistic.
The car you'd be replacing. Around 45 mpg for petrol, 55 for diesel.
Petrol or diesel, whichever your current car takes. Price per litre at your usual filling station.
Fine-tune the assumptions
Rapid chargers in the UK typically run 70–85p per kWh.
Fuel and charging only. Servicing is usually cheaper on an electric car and insurance is often dearer; the purchase price, depreciation and any company-car tax difference will normally outweigh both.
Everything together
Your answers from the other three tabs, added up. Nothing to fill in here — change anything on the other tabs and this updates.
One caution on adding these up: if you get solar and a heat pump and an electric car, they compete for the same sunshine. Charging the car during the day uses electricity the panels would otherwise have sold, and a heat pump does most of its work in winter when the panels generate least. Treat the combined figure as the optimistic end.