The power goes off during dinner. The router stops, the lights disappear, the fridge-freezer is still full and your phone has 18 per cent charge left. A small portable power station might keep one or two essentials running. It may struggle with a fridge compressor or a high-wattage kettle, however.
The right size depends on two separate questions: how much power your appliances need at one moment, and how much energy they use over the outage. The first is measured in watts (W). The second is measured in watt-hours (Wh) or kilowatt-hours (kWh).
That distinction prevents a common buying mistake. A unit can have enough battery capacity for a long runtime but still lack the output needed to start a motor. Conversely, a powerful inverter may run a demanding appliance briefly while its relatively small battery empties quickly.
Start by deciding what you want to keep running
There is no single portable power station size for every household. Your answer changes depending on whether you want to support one appliance, a small group of essentials or a wider set of household loads.
- One essential: a router, phone charger, lamp or laptop usually creates a modest load, although the required runtime still matters.
- A small essentials group: you might combine broadband equipment, a few lights, phones and a laptop.
- A wider set of loads: adding a television, fridge-freezer or cooking appliance increases both the energy requirement and the need for higher continuous and surge output.
Make this decision before comparing battery sizes. Keeping the fridge-freezer cold and maintaining communications is a different job from trying to recreate normal household electricity during a prolonged outage. For a broader household planning framework, see this guide to keeping essential appliances running during a power cut.
Calculate energy use in watt-hours
For a simple appliance that runs continuously, use:
Watt-hours = watts × hours of use
If a 10 W router runs for 24 hours, the calculation is:
10 W × 24 hours = 240 Wh
Repeat the calculation for each appliance, then add the results. Do not rely on the maximum rating alone for equipment that cycles on and off. A fridge-freezer may draw substantial power while its compressor is running, then stop for a period. Its average consumption over the outage can therefore be lower than its running wattage, but the exact duty cycle varies with the appliance, room temperature, contents and usage.
For a load that runs intermittently, use:
Watt-hours = watts × total outage hours × duty cycle
A notional 80 W fridge-freezer operating with a 30 per cent duty cycle over 24 hours would be calculated as 80 × 24 × 0.3, or 576 Wh. That is a planning example, not a specification for every fridge-freezer. Check the appliance label, manual or measured consumption where possible.
Allow for conversion losses and a reserve
The battery’s advertised capacity is not normally the same as the energy available at the sockets. A portable power station has an inverter and other electronics, and these consume some energy. Efficiency also changes with load and operating conditions.
A practical estimate is:
Required nominal capacity = appliance energy ÷ estimated efficiency
If your appliances need 800 Wh and you plan around 85 per cent efficiency, the battery calculation becomes approximately 800 ÷ 0.85 = 941 Wh. You should then add a reserve rather than choosing a unit that exactly matches the arithmetic. A reserve of 10 to 20 per cent can provide planning headroom, but it is not a guarantee of runtime.
Manufacturers may publish a usable capacity, an inverter efficiency figure or a recommended minimum state of charge. Use those figures in preference to a generic allowance. If the specification only gives nominal battery capacity, treat the result as an estimate. Cold conditions, battery age, high loads and the station’s own minimum shutdown level can reduce usable energy.
Check continuous output and surge output separately
Continuous output is the power the inverter can supply during normal operation. Add the running wattage of appliances that may operate at the same time and compare that total with the station’s continuous output rating.
Surge output is the short-term power available for starting some loads. Fridge-freezers, pumps, fans and other motor-driven equipment can demand more power at start-up than they use once running. A station may have enough Wh for a fridge-freezer but still trip if its surge capability is insufficient.
Some appliances also have a high steady load. A kettle, toaster, fan heater or cooking device may use more power than a small station can provide, even if the appliance only runs for a few minutes. Check the appliance’s input rating and the station’s continuous and peak output ratings. Do not assume that a short operating time makes an appliance electrically suitable.
Worked example: one small essential
Suppose you want to run a 10 W router for 12 hours and charge two phones, with each charger drawing an average of 10 W for two hours:
- Router: 10 W × 12 hours = 120 Wh
- Phone charging: 20 W × 2 hours = 40 Wh
- Total appliance energy: 160 Wh
Allowing for an illustrative 85 per cent conversion efficiency gives about 188 Wh. Adding a 15 per cent reserve takes the planning figure to roughly 216 Wh. A station with a nominal capacity around this level might be suitable if its output rating supports the loads and its specification does not impose a larger minimum reserve.
A very small station may be convenient for this job. It is not automatically suitable for a fridge-freezer or cooking appliance simply because both are described as portable loads.
Worked example: a small group of essentials
Now add a few LED lights and a laptop:
- Router: 10 W × 24 hours = 240 Wh
- Two 10 W lights used for five hours: 20 W × 5 hours = 100 Wh
- Phone charging: 20 W × 2 hours = 40 Wh
- Laptop averaging 60 W for six hours = 360 Wh
- Total appliance energy = 740 Wh
At an illustrative 85 per cent efficiency, this becomes about 871 Wh. Add a 15 per cent reserve and the planning figure is approximately 1,001 Wh. In practice, a unit around the 1 kWh class could be a sensible starting point for this collection of loads, subject to the actual ratings and intended runtime.
The laptop figure is an example rather than a universal consumption value. A compact laptop on light workloads may draw less, while charging a powerful model or using it intensively may draw more. The data plate, charger label or manufacturer specification is the better source.
Worked example: adding a television and fridge-freezer
Using the previous small essentials group, add a television drawing an illustrative 100 W for four hours and the fridge-freezer example of 576 Wh over 24 hours:
- Existing small essentials: 740 Wh
- Television: 100 W × 4 hours = 400 Wh
- Fridge-freezer estimate: 80 W × 24 hours × 0.3 duty cycle = 576 Wh
- Total appliance energy: 1,716 Wh
After an illustrative 85 per cent efficiency allowance, the figure is about 2,019 Wh. Adding 15 per cent reserve gives approximately 2,322 Wh, or 2.3 kWh of nominal capacity as a planning target.
This example contains several variables. The television may use a different amount, the fridge-freezer’s duty cycle may be higher or lower, and the station’s actual usable capacity may not equal its headline figure. The station must also cope with the fridge-freezer’s starting surge while other devices are connected.
Cooking appliances need a different check
A small cooking appliance can consume a lot of power in a short period. For example, an illustrative 1,500 W appliance used for 12 minutes consumes:
1,500 W × 0.2 hours = 300 Wh
The energy requirement is not enormous compared with a full day’s refrigeration, but the station still needs a continuous output rating of at least the appliance’s operating demand, with suitable headroom. A unit with a 500 W inverter cannot run a 1,500 W appliance, regardless of its battery capacity.
High-power appliances can also reduce runtime sharply if used repeatedly. Record both their wattage and the number of minutes or hours you expect to use them. Avoid connecting several high-demand appliances at once unless the station’s output specification explicitly supports their combined load.
Use a simple appliance checklist
Before choosing a station, make a table with these columns:
- Appliance or charger
- Running watts
- Possible starting or surge demand
- Hours of use during the outage
- Duty cycle, if it cycles on and off
- Whether it must run continuously or can be used in turns
Find ratings on the appliance data plate, power-supply label, instruction manual or manufacturer specification. For a fridge-freezer or other cycling appliance, an energy monitor can provide a more useful real-world estimate if you already have one and know how to use it safely.
Add the watt-hours, apply the manufacturer’s efficiency or usable-capacity information, and keep a reserve. Then check three limits on the proposed station:
- Its continuous output exceeds the highest expected simultaneous load.
- Its surge output can accommodate motors and other starting loads.
- Its usable battery energy covers the planned outage period without relying on the final few per cent.
Also check the station’s socket type, output waveform information, charging limits and operating instructions. Some sensitive equipment may have specific compatibility requirements. Do not connect medical or life-support equipment based on a general wattage calculation; follow the device manufacturer’s backup guidance and consult an appropriate clinical or electrical professional.
Choose by the job, not just the battery number
As a rough planning pattern, a few hundred watt-hours may cover communications and phone charging for part of a day. Around 1 kWh may suit a small group of modest loads for a defined period. A wider combination including a television and fridge-freezer can move the calculation into the 2 kWh range or beyond. Those are examples of how the maths changes, not fixed product categories or runtime guarantees.
If you are comparing portable backup with an electric car’s appliance outlet, read the separate guidance on using V2L as portable backup power. It covers a different source of electricity and should not be treated as a substitute for checking a portable station’s own limits.
Frequently Asked Questions
Is a larger watt-hour capacity always better?
No. A larger battery can provide more runtime, but it may be heavier, costlier and slower to recharge. It also does not solve an inadequate inverter rating. Match capacity and output to the loads you have recorded.
Can a 1,000 Wh power station run a fridge-freezer for a full day?
Possibly, but capacity alone cannot confirm it. Estimate the fridge-freezer’s average consumption, allow for conversion losses and reserve, and check the station’s surge output. Actual runtime depends on the appliance’s duty cycle, temperature, battery limits and operating conditions.
How much surge output does a fridge-freezer need?
There is no universal figure. Compressor starting demand varies by model and condition. Check the appliance documentation and the power station manufacturer’s guidance rather than applying a single multiplier to every fridge-freezer.
Can I run a kettle or heater from a small portable power station?
Only if the station’s continuous output meets the appliance’s input rating and its battery has enough usable energy. Kettles, heaters and similar appliances often have high running loads, so many smaller stations are not suitable even for short use.
Record your loads before the next outage
Write down the wattage, expected runtime and any starting demand for the appliances you would actually use. Add the watt-hours, account for efficiency and reserve, then compare the result with both the station’s usable capacity and its continuous and surge output. That short exercise will usually give you a more reliable size than choosing by headline battery capacity alone.