GuardarSecção 03
Bateria externa 20 000 mAh
Why: cerca de 74 Wh: carrega um telemóvel várias vezes. Uma por pessoa, na mala de emergência.
Essential guide · Base level · 11 min
Essential guideBase level · open to all11 min read
Backup power at home: batteries, solar panels and generators
How much energy to store for a blackout, in what, and how to top it up with the sun — with the numbers worked out for Portugal, the rules for small solar systems and safety for batteries and generators.
Reviewed 1 Oct 2026
08·ABackup power at home
In 30 seconds
The essentials, in four points.
Wh
Energy stored in a battery is measured in watt-hours. Device power (W) × hours of use = the Wh you need. Add it all up and allow a margin.
74 Wh
is what a 20,000 mAh power bank holds (20,000 × 3.7 V ÷ 1000). Enough to charge a phone several times — fewer than the maths says, because energy is lost in conversion.
≈ 250
watt-hours a day is what a 100 W panel facing south produces, on average, on a December day in Portugal's north and interior. In July, more than double. On cloudy days, far less.
6 m
is the minimum distance between a generator and doors, windows and garages. Never use one indoors, in a garage or on an enclosed balcony — carbon monoxide kills within minutes.
01Why prepare
Without the grid, you only have the energy you stored.
In the blackout of 28 April 2025, most of Portugal was without electricity for hours. Those with torches, a battery radio and charged power banks got through the day with far less anxiety. Those who depended on medical equipment plugged into the wall had a hard time.
This guide helps you decide how much energy to store, in what, and how to replenish it, without spending money on what you don't need and without creating new risks — battery fires and carbon monoxide poisoning. What to do during the blackout is in the Long blackout guide, in the Emergencies module.
A blackout of a few hours
Light, a charged phone and a radio. Torches, batteries and one or two power banks are enough.
I have rooftop solar
Most grid-connected solar systems switch off when the grid fails, even in full sun. Check whether yours has an emergency output.
Medical equipment
Oxygen, a ventilator, a powered wheelchair, refrigerated medicines. You need your own plan and to tell the network operator (section 05).
02How much energy you need
List. Add up. Allow a margin.
Step 1
List
Write down the essential devices and the power of each, in watts (W). It's on the label, the charger or the manual.
Step 2
Add up
Multiply the power by the hours of use per day: that gives watt-hours (Wh). Add everything and multiply by the days you want to cover.
Step 3
Allow a margin
Some energy is lost as heat in conversion, and batteries don't discharge all the way. Choose a capacity well above the total.
Example: a family of three, one day without power
| Device | Power (W) | Hours per day | Energy (Wh) |
|---|---|---|---|
| 3 phones (one charge each) | — | — | ≈ 45 |
| 2 LED camping lamps | 5 | 5 | 50 |
| Radio | 2 | 6 | 12 |
| Internet router | 10 | 4 | 40 |
| Total per day | ≈ 150 |
Example values: check your own devices. A phone's energy comes from its battery capacity — a 4,000 mAh battery at 3.85 V holds about 15 Wh (4,000 × 3.85 ÷ 1000). For three days, this example needs about 450 Wh, plus the margin.
03Where to store energy
From the simplest to the most expensive.
Batteries
For light and radio
Cheap, last for years in storage and need no grid. Choose torches and a radio that use the same battery size. Rechargeables are good for everyday use; for your reserve, make sure they're charged.
Power bank
For phones
One 20,000 mAh bank (≈ 74 Wh) per person or per small household. They charge over USB and fit in the grab bag. Check and recharge them whenever you check the bag.
Power station
For several devices
A large battery (from 300 to over 2,000 Wh) with sockets and USB. Look at two things: capacity (Wh) and output power (W), which must exceed that of your most demanding device.
Generator
For a lot of power
Plenty of energy, but it's noisy, needs fuel and produces carbon monoxide. Outdoors only, well away from the house (section 06). For most families, not the first choice.
Which option is for what
| Option | Phones | Light and radio | 230 V appliance | Indoors | Recharges from |
|---|---|---|---|---|---|
| Batteries | ○ No | ● Yes | ○ No | ● Yes | shop |
| Power bank | ● Yes | ◐ via USB | ○ No | ● Yes | socket, panel |
| Power station | ● Yes | ● Yes | ● up to output power | ● Yes | socket, panel, car |
| Generator | ● Yes | ● Yes | ● Yes | ○ never | fuel |
04Recharging with the sun
A portable panel replaces energy — less in winter.
Plate 08·B — Portable solar panelFacing south, tilted, no shade
What a 100 W panel produces on an average day
| City | December (Wh/day) | July (Wh/day) |
|---|---|---|
| Porto | ≈ 280 | ≈ 520 |
| Bragança | ≈ 250 | ≈ 550 |
| Lisbon | ≈ 290 | ≈ 550 |
| Évora | ≈ 320 | ≈ 540 |
| Faro | ≈ 360 | ≈ 570 |
Monthly averages calculated with PVGIS (European Commission) for a fixed panel facing south, at the optimal tilt, with 14% losses. A cloudy winter day may produce only a small fraction of this — don't rely on the sun for the essentials.
With the example in section 02 (about 150 Wh a day), a well-placed 100 W panel replaces, on an average day, one day's use — even in December. A week of rain is another story: a full battery remains your main reserve, and the panel is what tops it up.
Thinking of a small system?
Law in force since 28/08/20261
Up to 800 W, not feeding the grid: exempt from prior control in Portugal (Decree-Law 15/2022, amended by Decree-Law 130/2026). The system must prevent surplus from flowing into the grid.
2
Above 800 W: you must at least submit a prior notification to DGEG, the energy authority, before switching it on.
3
Connecting to your home's wiring is a job for a certified electrician. A mistake can cause fires or energise the grid during a blackout.
4
If the goal is power during a blackout, tell the installer from the start: you need batteries and an emergency output, and that changes the price.
05Medical equipment
If a life depends on the socket, the plan starts today.
1
Apply for priority customer status. In Portugal, anyone who depends on electrical equipment to survive or get around — or lives with someone who does — can request it from their electricity supplier. E-Redes, the network operator, will then notify you of planned interruptions.
2
Priority status doesn't guarantee power. E-Redes itself asks these customers to have backup or emergency power.
3
Talk to the doctor and the equipment supplier: how long the internal battery lasts, what alternative exists (for example, oxygen cylinders) and what to do if the power fails.
4
Refrigerated medicines: ask the pharmacist how long they keep out of the fridge, and at what temperature.
5
Write the plan down and keep it next to the equipment, with the numbers for the doctor, the supplier and 112.
06Safety
Batteries and generators: the two new risks.
Plate 08·C — Using a generator safelyOutdoors, away from doors and windows
1
Never indoors, in a garage, shed or enclosed balcony, even with doors and windows open. Carbon monoxide can't be seen or smelt, and can kill within minutes.
2
Never plug the generator into a household socket. This energises the grid and can electrocute technicians and neighbours. Powering your home's wiring needs a transfer switch installed by an electrician.
3
Switch it off and let it cool before refuelling. Store fuel outside the home, in a proper container.
4
Have a battery carbon monoxide alarm near the bedrooms. If you feel dizzy, get a headache or feel sick, get into fresh air and call 112.
1
Lithium batteries: charge them on a hard surface, away from anything flammable, and don't leave them charging while you sleep or are out.
2
Use the manufacturer's charger or one it has approved, and buy batteries from brands and shops you trust.
3
Don't charge or store them on your escape route out of the home, such as the hallway by the front door.
4
Let them cool after use, before charging again.
5
Stop using a battery that gets very hot, swells or makes noises (hissing or crackling).
07Six costly mistakes
Backup power myths — and what to do instead.
Myth
“I have rooftop panels, so I'll have power in a blackout.”
Most switch off when the grid fails. They only work with batteries and an emergency output — check with your installer.
Myth
“A 20,000 mAh power bank gives 20,000 of everything.”
mAh isn't energy. Multiply by the voltage: 20,000 mAh at 3.7 V is 74 Wh — and some is lost in conversion.
Myth
“With the garage door open, the generator can stay inside.”
It can't. Carbon monoxide builds up even with doors open. Generator outdoors, at least 6 m away.
Myth
“Plugging the generator into a socket powers the whole house.”
It's dangerous for you, your neighbours and grid technicians. Only with a transfer switch installed by an electrician.
Myth
“With a solar panel I don't need to store energy.”
In winter and on cloudy days it produces little. The panel tops up; the full battery is the reserve.
Myth
“A power bank stays good in the drawer for ever.”
It slowly discharges on its own. Check and recharge it whenever you check your grab bag.
08Recommended equipment
Store, replenish, protect.
Start with the simplest: torches, batteries and a power bank per person. A power station with a solar panel only makes sense if the numbers in section 02 call for it.
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GuardarSecção 03
Pilhas recarregáveis AA e AAA com carregador
Why: para as lanternas e o rádio. Mantém um conjunto carregado na reserva.
GuardarSecção 03
Estação de energia portátil (≈ 500 Wh)
Why: para quem precisa de alimentar vários aparelhos ou um de 230 V. Confirma a capacidade (Wh) e a potência de saída (W).
ReporSecção 04
Painel solar dobrável 100 W
Why: repõe cerca de 250 a 360 Wh num dia médio de dezembro, se estiver virado a sul e sem sombras.
GuardarSecção 03
Lanterna LED de mesa
Why: luz para uma divisão inteira, sem o risco de incêndio das velas.
SegurançaSecção 06
Detetor de monóxido de carbono
Why: indispensável se usares um gerador ou qualquer aparelho a combustão. Perto dos quartos.
Quick card
Print it. Stick it by the fuse box.
Fill it in with your family and stick it by the fuse box or on the cupboard where you keep the batteries.
Backup power · K72 sheet
1Energy per day: ______ Wh · for 3 days: ______ Wh
2Charged power banks: ____ · checked on: ______
3Spare batteries: sizes ______
4Do rooftop panels work in a blackout? yes / no
5Generator: outdoors only, 6 m away. Never into a socket.
6Medical equipment: priority status requested on ______
08·A · k72plus.comEMERGENCY: 112
- European Commission, JRC — PVGIS: photovoltaic output (SARAH-3 database, 2005–2023) — average monthly output in Porto, Bragança, Lisbon, Évora and Faro
- E-Redes — Customers with special and priority needs — who can apply, notice of interruptions, backup power
- DGEG — Electricity generation (self-consumption, in Portuguese) — exemption from prior control and prior notification
- Decree-Law 130/2026 of 29 June (amends Decree-Law 15/2022, in Portuguese) — 800 W limit without grid injection, in force since 28/08/2026
- U.S. Consumer Product Safety Commission — Portable Generator Hazards — never in enclosed spaces, never into a socket, fuel
- Ready.gov (FEMA) — Power outages — 20 ft (6 m) distance, medical equipment
- London Fire Brigade — Charging lithium batteries safely — how to charge lithium batteries, warning signs
- Visão Exame Informática — Hybrid inverters, off-grid, portable stations: what keeps power on in a blackout (2025, in Portuguese) — grid-connected panels switch off in a blackout
Reviewed 1 Oct 2026 · Next review: Mar 2027 · Spotted a mistake? Tell us.
This guide does not replace an assessment by a certified electrician or the advice of your doctor or medical equipment supplier. In an emergency, call 112.