What size portable power station do you actually need? A UK sizing guide

Watt-hours, watts and the 85% rule

Two numbers decide everything. Watts (W) are what a device draws moment to moment — printed on its label, its plug or its power brick. Watt-hours (Wh) are what a battery stores. Multiply a device's watts by the hours you need it and you have the watt-hours it will consume: a 15W broadband router for 12 hours is 180Wh. Portable power stations are sold by Wh capacity, and that one number tells you more than any marketing tier.

Two corrections before you add anything up. First, the three-pin sockets on a power station run through an inverter, which loses roughly 10–15% converting battery power to 230V mains — so treat a nominal 1,000Wh station as about 850Wh delivered. Second, size against a realistic outage, not a fantasy one. Under Ofgem's guaranteed standards, network operators must restore supply within 12 hours of a normal-weather fault (24 hours where 5,000 or more premises are affected) or pay you compensation, and most cuts are far shorter. '12 hours of essentials' is a sensible planning ceiling for most UK homes.

Worked example: the fridge-freezer over 12 hours

A fridge-freezer’s rating plate might say 100–240W, but the compressor cycles on and off, so over a day it averages far less: about 12–51W on its energy label (the kWh a year divided by 8,760 hours), with older and American-style models at the top. Over 12 hours that is roughly 150–610Wh at the plug. A power station also uses about 10W itself while its socket is on, and delivers around 85% of what it stores, so a nominal 1,000Wh (1kWh) station covers a half-day cut for any fridge-freezer, and an efficient one uses well under half of it.

Capacity is only half the check. When the compressor starts it briefly draws several times its running figure, so the station's surge (peak) rating needs to cover that moment, not just its continuous output. Most 1kWh-class stations manage this comfortably; very small units may not.

The honest arithmetic, though, is that you often don't need to power the fridge at all. Food Standards Agency guidance is that an unopened fridge keeps food safe for up to four hours, and a full, unopened freezer for up to 48 hours (around 24 if it's half full). For the typical cut of an hour or three, keeping the doors shut costs nothing and works. The fridge case for a power station is really about the long tail — storm damage and rural overhead lines. Our fridge freezer guide works the same sum for 1kWh and 2kWh units, allowing for the power station’s own consumption.

The combi boiler: small load, big caveat

A gas combi boiler heats your home with gas; electricity only runs the pump, fan and controls — about 35–123W while firing on Vaillant, Worcester Bosch and Ideal manuals (check your model's own). An evening of heating, say five hours, is roughly 175–615Wh at most, because the boiler only draws its running watts while the burner is on. That sits comfortably inside a mid-size station's budget, which is why central heating in a power cut is more achievable than most people assume; can a power station run a gas boiler? has the manual figures, the wiring and worked runtimes.

The catch is the connection, not the consumption. Boilers are hard-wired into a fused spur — there is no plug to move. The safe arrangement is electrician-fitted: a manual changeover switch (or, only where the boiler maker's instructions allow it, converting the boiler's supply to a plug and socket), so the boiler is completely isolated from the mains before the power station feeds it. Never improvise a lead that pushes power back into your house wiring through a socket — backfeeding can energise the local network and endanger the engineers working to restore it. In England and Wales this work falls under Part P of the Building Regulations, and Scotland and Northern Ireland have their own equivalent rules: use a registered electrician, and budget for the visit as part of the cost of 'heating backup'.

CPAP, router and phones: the small loads

A CPAP machine’s blower is a small load: ResMed’s own battery guide puts it at about 4–20W depending on pressure, with the heated humidifier and tube switched off. With them on it is several times that, about 21–67W on the same guide. An eight-hour night is therefore roughly 30–160Wh without the humidifier and 170–540Wh with it, so a small (about 290Wh) station covers roughly one to two nights without humidification through its mains socket, more on the DC lead, and a 1kWh unit covers roughly one to three humidified nights. Using the machine’s DC lead, where it has one, skips the inverter loss. If anyone in your home relies on powered medical equipment, join the free Priority Services Register before sizing any battery; our CPAP guide sizes it night by night.

The rest of the comms load is small. A router and fibre terminal draw 10–20W, so 12 hours is 120–240Wh — worth having, because home broadband (and Wi-Fi calling) dies with the power even when the wider network stays up. A smartphone takes roughly 15–20Wh per full charge including losses, so charging four phones once barely dents even a small station. Add LED lighting at 5–12W a bulb and the whole phones-router-lights-CPAP bundle comes in under about 500Wh across 12 hours.

Three tiers, mapped to real outages

Tier 1 — around 250–500Wh, roughly £150–£300. Runs phones, the router, LED lights and a CPAP through a night. This matches the great majority of UK power cuts: a local fault fixed within hours. If your area's cuts are rare and short, this tier is the sensible ceiling.

Tier 2 — around 1,000Wh, roughly £400–£800. Adds the fridge-freezer for 12 hours or so, plus the boiler's pump and controls once the changeover wiring is in place. This is the tier that covers Ofgem's 12-hour normal-weather restoration standard with everything that genuinely matters running.

Tier 3 — 2,000Wh and up, £1,000-plus. For rural homes on overhead lines that see multi-day storm outages. At this scale the smarter spend is usually a Tier 2 station plus portable solar panels to recharge it in daylight, rather than ever-more battery — capacity you can refill beats capacity you carry. Once you know your tier, and if you can wait until November, our Black Friday power stations page checks whether each brand-store price is genuinely lower than we’ve recorded it.

What a power station won't do — and when not to buy one

Skip anything that makes heat. Kettles, toasters, ovens and electric heaters draw 2,000–3,000W: many stations cannot supply that at all, and even a big one is drained in minutes — a 2kW heater flattens 1,000Wh in about half an hour. Plan around those loads (flask, blankets, a gas hob you can light safely) rather than trying to power them.

And if your realistic need is phones and a lamp, don't buy a station at all. A 20,000mAh power bank stores about 74Wh — three to four full phone charges — for £25–£35, and battery lanterns and head torches run on their own cells. One genuine extra consideration: if you've moved to a digital landline, the handset stops working in a power cut, and Ofcom requires providers to offer a backup solution (such as battery back-up) so customers who depend on the landline can reach 999 for at least an hour — ask your provider if that's you. Otherwise, a charged mobile plus a power bank is the resilient answer.