How we calculate your kit

We want you to be able to check our working. Every number the calculator uses is listed below with where it comes from. Figures labelled official guidance come straight from the government’s Be Ready campaign. Planning assumptions are sensible round numbers we have chosen. Typical figures are averages – your own devices may differ, so check their labels.

Power sizing

Daily energy = phones (one per adult, one charge a day) + each ticked device (watts × hours a day, or a fixed daily figure for fridges).
Total energy = daily energy × number of days.
Phones only: we pick the smallest power bank that covers the total after charging losses.
Otherwise: we pick the smallest power station whose usable capacity covers the total and whose continuous output can run everything you ticked at once.
If that would mean an impractically large battery, we suggest a station that covers at least a day plus a solar panel, and tell you how many days it covers on its own.

Constants

Figure	Value	Type	Source
`DEFAULT_DAYS` days	3	Official guidance	GOV.UK Be Ready checklist (Oct 2026) – https://beready.campaign.gov.uk/ – "Being ready for 3 days helps you manage while support reaches those most in need."
`DAY_OPTIONS` days	0: 3, 1: 7, 2: 14	Planning assumption	Owner decision: 3 (official baseline), 7 and 14 as extended options.
`MAX_CUSTOM_DAYS` days	30	Planning assumption	Owner decision: cap custom input to keep results sensible.
`WATER_DRINKING_L_PER_PERSON_DAY` litres / person / day	3	Official guidance	GOV.UK Be Ready checklist (Oct 2026) – https://beready.campaign.gov.uk/ – "3 litres per person per day for drinking." Applied to adults and children. Babies are covered by ready-to-feed formula instead.
`PURIFIER_DEVICE_FROM_DAYS` days	7	Planning assumption	Owner decision: suggest a reusable filter (not just tablets) for plans of 7+ days.
`STORED_WATER_BOTTLE_L` litres / bottle	5	Typical figure	Common UK supermarket multi-litre bottle size, used only to express litres as a bottle count. Check against the products you list (2 L and 5 L are both common).
`KCAL_ADULT_PER_DAY` kcal / adult / day	2500	Planning assumption	Owner decision. Cross-check: NHS reference intake 2,500 kcal (men) / 2,000 kcal (women); SACN Dietary Reference Values for Energy (2011) – https://www.gov.uk/government/publications/sacn-dietary-reference-values-for-energy Deliberately at the upper end so a mixed household is not under-supplied.
`KCAL_CHILD_PER_DAY` kcal / child (under 12) / day	1600	Planning assumption	Owner decision. Cross-check: SACN 2011 EARs run from roughly 1,000 kcal (age 1–3) to ~2,000 kcal (age 11).
`MEALS_PER_DAY` meals / day	3	Planning assumption	Owner decision: used only to express calories as a rough meal count.
`PHONES_PER_ADULT` phones / adult	1	Planning assumption	Owner decision.
`PHONE_FULL_CHARGE_WH` Wh / full charge	15	Typical figure	Typical modern smartphone battery ≈ 4,000 mAh × 3.85 V ≈ 15 Wh. Verify against current best-selling handsets.
`PHONE_CHARGES_PER_DAY` charges / phone / day	1	Planning assumption	Owner decision: assumes careful use during an outage.
`POWER_BANK_NOMINAL_V` volts	3.7	Official guidance	Standard lithium-ion cell nominal voltage used to convert mAh ratings to Wh (Wh = mAh × 3.7 / 1000). Industry convention printed on power bank labels.
`POWER_BANK_USABLE_FRACTION` fraction of rated capacity delivered to the phone	0.65	Typical figure	Conversion losses (voltage step-up, cable, phone charging circuitry). Commonly quoted as 60–70%. Verify.
`POWER_BANK_MAX_WH` Wh	100	Planning assumption	Practical upper limit for a pocket power bank (also the common airline carry-on limit). Above this we recommend a portable power station instead.
`STATION_USABLE_FRACTION` fraction of rated Wh usable via AC outlet	0.85	Typical figure	Inverter and battery losses. Commonly quoted as 80–90%. Verify against manufacturer specs.
`STATION_MAX_PRACTICAL_WH` Wh	3000	Planning assumption	Owner decision: above this, recommend a smaller station + solar recharge rather than a bigger battery.
`SOLAR_PEAK_SUN_HOURS_UK_WINTER` equivalent full-sun hours / day	1	Typical figure	UK winter solar yield is low; used only to warn that solar is a top-up, not a replacement. Verify against PVGIS (EU JRC) data for a central UK location.
`TORCHES_PER_ADULT` torches / adult	1	Planning assumption	GOV.UK Be Ready checklist (Oct 2026) – https://beready.campaign.gov.uk/ lists "Torch (and spare batteries)". Quantity is an owner decision.
`BLANKETS_PER_PERSON` blanket or sleeping bag / person	1	Planning assumption	GOV.UK Be Ready checklist (Oct 2026) – https://beready.campaign.gov.uk/ lists "Blankets, sleeping bags and warm clothing". Quantity is an owner decision.
`RADIOS_PER_HOUSEHOLD` radios / household	1	Official guidance	GOV.UK Be Ready checklist (Oct 2026) – https://beready.campaign.gov.uk/ – "FM radio (battery/wind-up)".
`PET_WATER_ML_PER_KG_DAY` ml / kg bodyweight / day	50	Typical figure	Royal Canin Vetfocus (cats, ~50 ml/kg/day) – https://vetfocus.royalcanin.com/en/doc-10.html ; dog sources give 40–60 ml/kg/day. Not UK-official. Ideally replace with a PDSA / BVA figure. More needed in heat or on dry food.
`DOG_WEIGHT_KG` kg (representative weight per size band)	small: 8, medium: 18, large: 35	Typical figure	Owner decision: representative weights for small / medium / large dogs.
`CAT_WEIGHT_KG` kg	4.5	Typical figure	Typical adult domestic cat (Royal Canin Vetfocus uses 4–5 kg).

Devices

Device	Typical power	Default use	Energy a day
Fridge-freezer	150 W	All day (cycles on and off)	1,100 Wh
Wi-Fi router	12 W	24 h/day	288 Wh
Laptop	60 W	3 h/day	180 Wh
LED lamp	8 W	5 h/day	40 Wh
TV	80 W	3 h/day	240 Wh
CPAP machine	40 W	8 h/day	320 Wh
Other powered medical device	50 W	8 h/day	400 Wh
Electric blanket	60 W	4 h/day	240 Wh
Electric heater	1000 W	2 h/day	2,000 Wh

Device figures are typical values. Check the label or manual of your own appliance – especially for medical equipment.