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To estimate what an energy price increase will do to your budget, recalculate your old and new costs with the same household usage and the same number of days. For each fuel, multiply every usage band (in kWh) by its unit rate, add the daily standing or supply charge, then include applicable fees, taxes, discounts and credits. Subtract the old scenario from the new one. That difference is the price shock; it is not the same as the change caused by using more or less energy.
Headline “average bill” figures are illustrations. Your result depends on your meter readings, tariff, payment method, region, billing dates and consumption pattern.
The calculation in one line
For a bill or budget period, use this formula for each fuel and rate band:
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Estimated cost = Σ (usage in each rate band × that band’s unit price) + (daily standing or supply charge × number of days) + applicable fees and taxes − credits or discounts.
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Run the formula twice:
- Old-price scenario: your existing unit rates and fixed charges.
- New-price scenario: the proposed or current replacement rates and fixed charges.
Then calculate:
Estimated shock = new-price scenario cost − old-price scenario cost.
Use the same consumption and days in both scenarios. A full-year comparison needs 365 days (or 366 in a leap year) on both sides; a bill comparison needs the exact bill-period days. This holds usage constant so the result isolates the effect of prices rather than weather, appliance use or a different meter-reading interval.
Single-rate shortcut
For one fuel on a single-rate tariff, the change can be written compactly as:
Change = usage × (new unit rate − old unit rate) + days × (new daily charge − old daily charge).
Add any difference in applicable fees, taxes, rebates, discounts, debt repayments or credits separately. Keep units consistent: kWh with a price per kWh, and days with a price per day.
What an energy price shock actually measures
A price shock is the extra amount your household would pay if rates changed but its consumption stayed the same. It answers “What would this period cost at the new prices?” It does not predict your exact future bill if your usage changes.
Ofgem, the Great Britain energy regulator, puts the consumption relationship plainly: “The more energy you use, the higher your bill will be.” A cold winter, a new electric vehicle, a vacant home or improved insulation can therefore move the actual bill away from a price-only estimate.
Report both the currency change and, when useful, the percentage change:
Percentage change = (new scenario cost − old scenario cost) ÷ old scenario cost × 100.
Use the old scenario as the denominator. A percentage can look large on a small bill or modest on a large one, so show the currency amount as well.
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Collect the figures before you calculate
Your latest bill, tariff confirmation or meter statement should supply most inputs. Record the figures for electricity and gas separately.
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- Unit rates: the current price per kWh for every band and the proposed or changed price for that same band.
- Standing or supply charges: the old and new amount charged per day.
- Period: the bill’s start and end dates, or the exact number of days.
- Reading status: mark whether usage is actual or estimated. An estimated reading is an assumption, not a measured consumption figure.
- Adjustments: taxes, supplier fees, rebates, discounts, credits and debt repayments that apply to this household and period.
- Tariff details: fixed or variable plan, payment method, meter type and region.
Keep the units and time basis visible in your worksheet. A rate quoted in pence per kWh must be converted to pounds before adding it to a pound-denominated standing charge, or the final result will be 100 times too high.
Actual versus estimated usage
If the bill says usage is estimated, label the result as a scenario. Replace it with an actual meter reading where possible, and use readings taken on comparable dates when testing an annual change. Do not treat an estimated winter reading as a precise measure of a full year.
Rates from a published table
A published rate table is useful for an illustration or a household that has not yet received its bill, but it cannot replace the tariff details on the account. Check the table’s geography, fuel, meter type, payment method and effective dates before copying a rate. Averages are not guarantees for an individual customer.
Step-by-step method for a single-rate bill
- Count the days. Subtract the bill start date from the end date according to the supplier’s convention, and use that same count in both scenarios. If the bill states the number of days, use it.
- Enter consumption. Use the kWh billed for the fuel. If you are making an annual budget, use a full-year kWh estimate and document how you obtained it.
- Calculate old variable cost. Multiply kWh by the old unit rate.
- Calculate old fixed cost. Multiply the old daily standing charge by the bill days.
- Add old adjustments. Include only taxes, fees, credits or discounts that apply during that period.
- Repeat with new rates. Keep kWh, days and all non-price assumptions unchanged unless the charge itself is changing.
- Subtract. New total minus old total is the estimated shock. Divide by the old total and multiply by 100 for the percentage change.
Illustrative annual example
Suppose a household uses 3,000 kWh of electricity in a year. The unit rate rises by 5 pence per kWh and the standing charge rises by 10 pence per day. The unit-rate effect is:
3,000 × £0.05 = £150.
The fixed-charge effect for 365 days is:
365 × £0.10 = £36.50.
The estimated additional annual cost is therefore £186.50 before other adjustments. This is arithmetic from the stated hypothetical inputs, not a forecast or a reported household statistic. If the comparison covers a leap year, use 366 days, which would make the fixed-charge effect £36.60.
Multi-rate and time-of-use tariffs
Economy 7 and other time-of-use plans have a separate price for each band. Do not multiply all kWh by one average rate unless you are deliberately using a clearly documented weighted average.
- List each band, such as peak, shoulder and off-peak.
- Enter the kWh used in that band for the comparison period.
- Multiply each band’s usage by its old rate and add the results.
- Repeat with the new rate for the corresponding band.
- Add the standing charge and any applicable adjustments once for the period.
| Band | Usage | Old rate | New rate | Old cost | New cost |
|---|---|---|---|---|---|
| Peak | Use the meter’s peak kWh | Old peak price per kWh | New peak price per kWh | Peak kWh × old peak rate | Peak kWh × new peak rate |
| Off-peak | Use the meter’s off-peak kWh | Old off-peak price per kWh | New off-peak price per kWh | Off-peak kWh × old off-peak rate | Off-peak kWh × new off-peak rate |
The table’s cells are calculation slots rather than universal prices: fill them with the rates and readings on your account. If a price change affects only the peak band, the shock is driven by peak kWh, not total kWh. A household able to move laundry, vehicle charging or water heating into a cheaper period may have a different future bill, but that behavioral change belongs in a separate scenario rather than in the constant-usage price-shock calculation.
Standing charges can dominate a small user’s increase
Unit rates attract attention because they are quoted per kWh, but a daily standing or supply charge applies even when consumption is low. Calculate its change independently:
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For a household using little energy, this fixed component can represent a substantial share of the increase. For a high-use household, the per-kWh component may be larger. Showing both components makes the result auditable and helps explain why two homes with the same percentage rate increase can see different cash amounts.
Great Britain price-cap caveats
In Great Britain, Ofgem’s cap is a maximum unit rate and standing charge for covered standard variable tariffs. It is not a cap on the total amount a household can pay: total cost rises with consumption. Rates and charges can vary by region, payment method, fuel and meter type.
Ofgem’s described protection does not apply in the same way to fixed tariffs, business contracts, heat networks or heating oil. Do not apply a Great Britain cap figure to another country or assume it governs every British household. Confirm that your tariff is covered and use the rate applicable to your region and payment method.
Dated Q4 2026 illustration
Ofgem’s direct-debit average cap table for Great Britain lists these rates for 1 October–31 December 2026:
| Fuel | Average unit rate | Average daily standing charge |
|---|---|---|
| Electricity | 26.32 pence per kWh | 54.83 pence per day |
| Gas | 7.97 pence per kWh | 29.68 pence per day |
These are period-specific averages, not guaranteed rates for a household. Verify the current Ofgem table immediately before using any published example: rates, VAT treatment and relief programs can change. Comparisons with an earlier period may be affected by a VAT change, so make sure both scenarios use comparable tax assumptions.
Australia and other jurisdictions
Australian bills commonly distinguish variable consumption charges from fixed supply charges, with additional fees and credits shown separately. Australian Government guidance recommends comparing offers against actual total usage and the household’s time-of-use pattern. The arithmetic is transferable, but the labels, market rules, taxes and protections are not.
Outside Great Britain and Australia, use the definitions printed on your bill and the rates published by your local regulator or supplier. Check whether prices are expressed per kWh, megajoule or another unit; convert only when the bill or regulator provides a valid conversion. Confirm local rules for taxes, concessions, rebates, network charges, payment methods and contract eligibility.
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A future rate is a scenario until the supplier or regulator publishes a binding figure. Create separate scenarios instead of presenting one number as certain.
Base scenario
Use the latest confirmed rate, your best annual usage estimate and the current standing charge. State the period and whether consumption is actual, estimated or projected.
High-use scenario
Keep the new prices but increase kWh to represent a colder period, additional occupants or a new appliance. This answers “What might I pay if both prices and use rise?” It is not the pure price shock, so label it separately.
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Time-of-use scenario
Keep total kWh constant but move a documented share between bands. This tests whether a household can reduce cost through timing, subject to the tariff’s rules and realistic behavior.
Policy-adjusted scenario
Apply a rebate, tax, credit or relief measure only when the household qualifies and the measure covers the modeled dates. Show the adjustment as its own line so a policy ending or changing can be seen immediately.
Round only the displayed final result. Keep full-precision rates in the calculation, especially when a daily charge is multiplied by hundreds of days.
Comparing tariffs fairly
To compare two offers, hold the household’s consumption, billing days and meter readings constant, then replace the rates and fixed charges for each offer. Compare electricity and gas separately before adding them.
- Use the same annual kWh in both offers.
- For time-of-use plans, use the same kWh in each band unless the comparison explicitly models a behavior change.
- Include payment-method conditions, regional eligibility, meter requirements and recurring fees.
- Apply discounts and credits only when the household qualifies and the offer’s period matches the comparison.
- Show total annual cost, absolute difference and percentage difference.
- Check whether the quoted price is a temporary promotion or a recurring rate.
A tariff that appears cheaper at one usage level may not be cheaper at another because fixed charges and band prices interact. The relevant question is the total cost for this household’s actual pattern, not the advertised unit rate alone.
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Common mistakes and how to correct them
Treating an average cap bill as a personal forecast
Why it fails: an average combines assumptions about usage and may cover only a particular region, payment method or meter type.
Correction: replace the average consumption with your own kWh and use the rate table only after checking its scope and dates.
Using the percentage increase on the whole bill
Why it fails: the unit rate and standing charge may change by different amounts, and credits or taxes may not move proportionally.
Correction: calculate each component separately, then add them.
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Why it fails: a 28-day bill and a 92-day bill have different fixed-charge totals even at identical daily rates.
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Correction: use the actual bill-period dates or a documented annual day count.
Applying one rate to a multi-rate meter
Why it fails: peak and off-peak kWh have different prices.
Correction: calculate every band independently.
Mixing estimated and actual readings
Why it fails: a correction to an estimated reading can look like a price shock.
Correction: compare like-for-like readings and flag estimates.
Forgetting credits, taxes or debt repayment
Why it fails: the underlying energy price may be unchanged while the amount collected on the bill changes.
Correction: list each adjustment, identify whether it recurs, and include it only for the relevant household and dates.
Comparing different seasons
Why it fails: winter consumption is often higher than summer consumption.
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A reusable worksheet
| Input | Old scenario | New scenario |
|---|---|---|
| Bill start and end dates | Enter dates | Use the same dates |
| Days | Count days | Use the same count |
| Electricity kWh by band | Enter meter or bill values | Keep usage constant for a price-only shock |
| Gas kWh | Enter meter or bill value | Keep usage constant for a price-only shock |
| Unit rate for each band | Enter current rate | Enter changed rate |
| Daily standing or supply charge | Enter current charge | Enter changed charge |
| Fees and taxes | Enter applicable amount | Enter applicable amount |
| Credits and discounts | Enter applicable credit | Enter applicable credit |
| Total | Sum variable cost, fixed cost and adjustments | Sum variable cost, fixed cost and adjustments |
| Difference | New total minus old total | |
Keep a note beside every figure: source, effective date, unit, and whether it is actual, estimated, one-time or recurring. That note is often more valuable than an extra decimal place.
How to present the result in a household budget
Convert an annual shock into a monthly planning amount only after calculating the annual total. Divide an annual increase by 12 for a simple budgeting envelope, while recognizing that real bills are seasonal and standing charges accrue daily. If the shock is for a single bill, budget the actual bill-period difference instead of averaging it across months.
Show a small sensitivity table when uncertainty matters:
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| Scenario | Usage assumption | Price assumption | What it answers |
|---|---|---|---|
| Price-only | Same kWh as old period | New rates and charges | Effect of the tariff change alone |
| Higher-use | Higher kWh | New rates and charges | Effect of price and increased consumption |
| Lower-use | Lower kWh | New rates and charges | Potential cost with conservation or milder weather |
| Time-shifted | Same total kWh, different bands | New time-of-use rates | Effect of changing when electricity is used |
Do not combine these scenarios into one “expected” number unless you have a defensible probability or planning assumption. Label each result so a reader can tell which part is measured and which part is hypothetical.
Final checks before relying on the estimate
- Are electricity and gas calculated separately?
- Does every kWh use the correct band rate?
- Are standing or supply charges multiplied by the correct number of days?
- Are old and new scenarios using identical usage and dates for the price-only result?
- Are actual and estimated readings clearly marked?
- Have taxes, fees, credits, rebates and debt repayments been included only where applicable?
- Does the rate’s region, payment method, meter type and effective period match the household?
- Have volatile future rates and policy changes been labeled as scenarios?
- Are the currency difference and percentage difference both shown?
The resulting figure is a transparent estimate: it shows exactly which consumption, rates, days and adjustments produced the number. Update it when a confirmed rate, meter reading or policy change replaces an assumption.
Quick Recap
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