HOUSEHOLD ENERGY PLANNING

Daily shifting or days without the grid?

A battery can recharge every day to shift cheap electricity. Covering several days of household and heat-pump demand during an outage needs much more stored energy.

Compare the two below. All starting values are examples, not a battery specification or an installation quote.

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Read the worked examples and planning guide →

Both estimates are calculated together: daily shifting recharges every day; outage storage must bridge the selected days without grid recharge. Use these links to view either result.

View daily shifting result ↓View outage result ↓
Your selected day

Use a winter day for winter resilience. Enter electricity excluding the heat pump, then heat delivered to rooms and hot water. Include auxiliary and immersion electricity in household demand if it is not included in your period COP.

AC electricity for appliances, lighting, cooking and other household loads. Exclude the heat pump if you enter its useful heat separately.
Heat delivered to rooms on your chosen day, not electrical input. Divide by period COP to estimate heat-pump electricity.
Useful heat delivered to hot water on the chosen day. Include immersion electricity in household demand if it is not represented by period COP.
Heat delivered divided by heat-pump electricity over this period. Winter COP can be lower than annual SCOP. Use a consistent scope for hot water and auxiliaries.Heat delivered ÷ electricity used over this period. Use winter performance for a winter day, not annual SCOP.
Capacity and losses: separate assumptions

90% usable depth of discharge is a capacity limit, not 90% conversion efficiency. Each input below has a different job.

Maximum share of nameplate energy you can use in a daily cycle. A 90% DoD means at least 10% SOC stays in the battery. This is not an energy-conversion loss.
95% is an illustrative conversion assumption, not a measured whole-system figure. Use a relevant manufacturer or measured value; fixed DC self-draw is entered separately.
95% is an illustrative conversion assumption. Battery DC energy is larger than delivered AC energy by the conversion loss. Avoid counting fixed self-draw here if entered below.
80W is the user-scenario default; try 60–100W or set 0 to exclude it. This range is not universal: some manufacturer no-load values are much lower. DC self-draw = watts × 24 ÷ 1000 kWh/day, added directly without another discharge loss.

At 80W continuously, self-draw is 1.92 kWh/day. Use your equipment’s measured value. Manufacturer examples of efficiency and zero-load power.

Cheap-rate grid shifting

Capacity covers the demand you move out of the expensive period; it is not multiplied by outage days.

Hours available for cheap charging each day. Battery charging power shown is an average minimum; simultaneous house load needs extra supply headroom.
Share of daily AC demand occurring outside the cheap window. Enter your own profile rather than assuming constant demand through the day.Editable because household and heating demand are not uniform through the day.
7.48p is a user-supplied import scenario, not a verified supplier offer. Battery losses are applied by the model; do not enter an already loss-adjusted delivered price.7.48p is a user-supplied scenario, not a verified supplier offer. Change it to your tariff.
26.32p is the dated Ofgem electricity reference for October to December 2026. Enter your actual avoided peak import price.26.32p: Ofgem average Direct Debit electricity rate, 1 October–31 December 2026; checked 1 November 2026. Your regional and payment rates vary.

Ofgem dated rate reference. This is a snapshot, not a live tariff feed; check your actual cheap and peak rates.

Grid outage
Whole outage days from 1 to 14. Delivered AC demand grows by daily demand times days. Nominal capacity also includes conversion loss, self-draw and available SOC.
Actual charge when the outage begins, as a percentage of nominal battery capacity. This is not assumed to stay full; edit it for your expected starting charge.
The battery stops discharging at this SOC. It must be at least 100% minus usable DoD. Net available fraction = starting SOC minus minimum SOC; usable DoD is not multiplied again.

No solar replenishment or grid recharge is credited. Battery-only results assume the same demand every day and continuous inverter self-draw.

Optional generator backup — LPG, petrol or diesel

Off by default. This models energy across a fixed schedule; it does not show that a generator or inverter can run your heat pump or start motors.

All engine generators need scheduled servicing. Petrol and diesel add liquid-fuel storage and fuel-system upkeep: petrol deteriorates in storage; diesel needs checks for water, contamination and filter condition. Stored propane avoids liquid-fuel ageing, but an LPG engine still needs maintenance. Follow the equipment manual and local storage rules. Petrol storage guidance · Oil/diesel storage guidance.

Use a generator outdoors, away from openings: exhaust can cause fatal carbon-monoxide poisoning. House connection requires suitable transfer/isolation equipment and a qualified installer; never backfeed a socket.

What these figures mean

Energy is not power. Capacity in kWh does not establish whether an inverter can start or run your heat pump, or supply simultaneous loads. Outage supply needs compatible island/backup equipment, safe isolation and a defined set of backed-up circuits. A grid-connected battery does not automatically provide backup.

The charging kW shown is the minimum average battery charging power during the cheap window. Simultaneous household demand needs additional supply headroom. Peak loads, charge limits and scheduling need interval data and an electrical assessment.

Daily gross saving compares avoided peak-rate energy with cheap-rate recharge, including both AC loss directions. It excludes standing charges, battery wear, capital cost and installation. Negative savings are shown. There is no assumed annual solar credit, return-on-investment forecast or product recommendation.

See the energy balance and generator assumptions

Daily delivered AC electricity = household electricity + (useful space heat + useful hot water) ÷ period COP. DC self-draw = inverter watts × 24 ÷ 1000.

Daily shifting DC withdrawal = shifted AC energy ÷ discharge efficiency + full-day DC self-draw. Nominal daily shifting capacity = DC withdrawal ÷ usable DoD; recharge AC = DC withdrawal ÷ charging efficiency. This conservatively allocates all continuous inverter self-draw to battery charging. Delivered variable energy price excludes fixed self-draw; the gross saving includes it.

Battery-only outage: delivered AC energy ÷ discharge efficiency + DC self-draw = total DC withdrawal. Nominal capacity = DC withdrawal ÷ (starting SOC − minimum SOC). Charging efficiency does not increase an already charged battery’s outage capacity. Starting SOC and reserve already account for usable capacity.

Optional generator: flat AC load = daily delivered energy ÷ 24. The generator runs in the final chosen hours of each day, serving concurrent AC demand directly. Surplus AC charges the battery through charging efficiency, while DC self-draw continues. Refill is capped at starting SOC; surplus beyond that is curtailed. Storage is the maximum DC depletion before refill, divided by net available SOC. Remaining DC shortfall is the battery energy still used at the end, not proof of unmet load.

Petrol/diesel: scheduled fuel litres = running hours × entered litres/hour at the selected AC output. Electricity per litre = AC kW ÷ litres/hour. This uses actual fuel consumption rather than applying the LPG efficiency. The starting 1.5 litres/hour is illustrative for either fuel, not a product claim.

47kg bottle AC yield = 47 × propane LHV × fuel-to-electric efficiency. Bottles = all scheduled AC production ÷ bottle AC yield. Battery-routed energy has additional conversion losses; direct AC output does not. Whole bottles round up the calculated fuel requirement and do not include a contingency stock.

Sources checked 1 November 2026: Sibelga’s Energuide: net propane energy content; Champion: fuel consumption varies by operating load. Honda: petrol storage and maintenance; Cummins: diesel storage and filters. The 20% LPG efficiency, 1.5 litres/hour liquid consumption and 3kW output are illustrative inputs, not extracted performance claims.

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