SOLAR GENERATORDO THE MATH.

A portable power station advertises 1,000 watt-hours and a folding solar panel advertises 200 watts. Does that mean it can power a refrigerator for a day and recharge completely in five hours? Not necessarily. Those three numbers describe different quantities, and real performance depends on appliance behaviour, conversion losses, battery limits and the sunlight reaching the panel.

This guide explains how to estimate the runtime of a portable solar generator during a power cut without relying on an optimistic marketing illustration. It includes worked examples, a repeatable worksheet and the checks to make before choosing equipment. If you are considering battery backup as an option, use your own appliance measurements and manufacturer specifications; none of the examples below is a prediction of a particular device's performance.

Solar panels can generate electricity in cloud, but yield changes with the season, shading and orientation. Energy Saving Trust's August 2026 solar fact check confirms that cloudy-day generation is possible while shorter winter days reduce production. The more important figure for an outage is not a panel's peak badge, but how many watt-hours you actually put back into the battery.

First understand watts, watt-hours and kilowatt-hours

Watts (W) describe the rate of power use or generation at a moment. A light drawing 8W uses power more slowly than a laptop drawing 65W. Watt-hours (Wh) describe the amount of energy used over time. An 8W light running for five hours consumes about 40Wh. Kilowatt-hours (kWh) are just thousands of watt-hours: 1kWh equals 1,000Wh.

Battery energy and AC output power both matter. A station could have enough stored energy to run a device for hours but not enough inverter power to start it. Fridges, pumps and other motor-driven loads may draw a brief startup surge higher than their running consumption.

The basic portable power station runtime formula

For an appliance with approximately steady power use, a first estimate is:

Runtime (hours) ≈ battery capacity (Wh) × usable-energy fraction ÷ appliance load (W).

The usable-energy fraction is a planning assumption accounting for conversion losses, internal consumption and reserves. Do not choose one percentage and assert that it applies to every product. For an illustrative example below, we use 0.80 (80% of the rated watt-hours reaching the chosen load). Real efficiency may be better or worse, especially at very low output loads where the inverter's own consumption becomes significant.

Example: a hypothetical 1,024Wh station and a continuous 40W load, using the 0.80 assumption: 1,024 × 0.80 ÷ 40 = 20.48 hours. If the measured load is instead 80W, the same calculation gives 10.24 hours. Neither is a verified runtime for a particular product.

DC outputs can have different losses from 230V AC. A small USB light may run more efficiently from a correctly matched DC port than via a mains adapter and the station's AC inverter. However, never connect a device to an incompatible voltage or bypass its specified power supply to chase extra runtime.

Measure your own appliance instead of trusting a generic chart

Find the appliance's power label, manual or supplier specification. A label showing maximum input is not always the same as the average load during normal operation. A plug-in energy meter can measure compatible mains appliances over a meaningful period when electricity is available. Look at energy consumed in Wh or kWh over time as well as instantaneous watts.

For a refrigerator, measure a representative 24-hour period while doors are used normally, because the compressor cycles. Then use its average daily energy as the basis for planning. If, purely as an example, it uses 600Wh in 24 hours, its average is 25W even though its compressor may draw considerably more when running. The startup surge still needs to fit within the inverter's specification.

Electrical Safety First's appliance rating guide illustrates how widely household loads vary. If a label gives only amps (A), power may be estimated from voltage and current for some loads, but power factor and operating variation make a meter preferable where accuracy matters.

Worked example: four essentials over one evening

Imagine a household wants light, phone charging, a router and a laptop for the next evening. These are illustrative loads, not promised values for every device. Enter your measurements in place of them:

For the example 1,024Wh battery with a planning factor of 0.80, usable energy is 819.2Wh. An evening using 301Wh would leave an estimated 518.2Wh of usable energy, ignoring any additional fixed standby consumption not included in the example. More realistically, record actual battery percentage and load after a safe trial to refine your own calculation.

You can use the same method with any list: Watts × hours = watt-hours. For equipment with intermittent use, measure its average energy use across a whole day. Do not add a fridge's startup surge to its hourly energy as if the surge continued constantly: peak power and energy consumed are different checks.

How to estimate a fridge's runtime without making an unsafe assumption

Start with the fridge's measured energy use per day, then divide the battery's estimated usable Wh by that daily consumption. Suppose a fridge consumed 720Wh over 24 hours in your measurement. With the hypothetical 819.2Wh available, the arithmetic is 819.2 ÷ 720 × 24 ≈ 27.3 hours. That is an energy-only illustration, not assurance that the fridge can start or maintain safe food temperatures.

Verify the inverter's continuous and surge ratings, actual compressor behaviour, plug compatibility and the unit's operating limits. If the ambient temperature changes, the fridge is opened more often or other devices share the station, the result changes. Do not power a medical refrigerator or temperature-critical medicine using a generic calculation without specialist guidance.

When a fridge cannot be kept powered, follow the Food Standards Agency's power-cut food guidance for temperature decisions. Our prolonged power-outage guide explains how those decisions fit into a broader household response.

Now estimate solar input: peak watts are not daily energy

A panel sold as 200W is rated under defined test conditions. The US Department of Energy's photovoltaic performance guidance explains that module ratings use 1,000W/m² irradiance and a 25°C cell temperature; real output varies with sunlight, panel temperature and time of day. A 200W panel does not output 200W for every daylight hour.

For an illustration only, suppose the station actually receives an average 100W of charging input for three hours after cable and controller losses. That supplies approximately 300Wh to the charging system. If 10% of that energy is lost in the battery charging process, about 270Wh may be retained. This is a simple example: real controller efficiency, input caps, battery state and weather vary.

Do not call three clock hours 'three peak-sun-hours' unless you are actually working with an appropriate solar-energy measure. Panel production may ramp up and down constantly under clouds. Measure the station's input watts over time or consult location- and season-specific solar resource estimates to produce a better forecast.

Why a folding panel may recharge much more slowly than advertised

Four constraints often dominate real charging: panel angle and shading, season and cloud, the power station's maximum solar input, and connector/electrical compatibility. A panel may be capable of more output than the station accepts, or a collection of panels may exceed the station's permitted input voltage if connected incorrectly.

For a cloudy winter day in the UK, energy harvested may be a fraction of a clear summer day. Energy Saving Trust explains that shorter days and more frequent cloud reduce winter production. It does not provide a universal winter percentage you can multiply every panel by; location, mounting, shade and weather need to be considered.

Never improvise the wiring between panels or connect panels exceeding the battery's input limits. Confirm polarity, connector type, voltage range, current limits and whether any series/parallel arrangement is permitted in the manufacturer's manual. Put panels where they are secure against wind and cannot create trip or fall hazards; follow the equipment's moisture and weather rating.

What if the solar panel generates less than you use?

A solar-plus-battery setup is a daily energy budget, not perpetual power. If the household uses 500Wh during the day and manages to recharge only 200Wh, net stored energy falls by roughly 300Wh, before accounting for other losses. Over several poor-sunlight days, the battery may eventually reach its protective cutoff.

You have three options: reduce consumption, add a legitimately compatible energy source or accept that some devices will be unavailable when the battery is depleted. Increasing battery capacity alone does not solve insufficient recharging over a long period. It simply gives you more stored energy to use before you need another charge.

For a more complete comparison of battery-only, solar-plus-battery and fixed home systems, see the home battery backup guide. A portable station generally cannot run heavy electric heating, cooking appliances and a household's entire normal load for long without substantial capacity.

Use a simple planning table for your own station

Run the calculation twice: once for a favourable day and once for a poor-solar day. If the plan fails in the second case, you know which demand to reduce or which dependency needs a different solution. This is much more informative than asking whether a panel can power 'a house' in the abstract.

Safety: what a portable battery is not

Use a portable power station according to its manufacturer instructions for temperature, ventilation, charging, placement and permitted loads. Do not block cooling vents, charge on a bed, use damaged cables or operate near standing water. The Electrical Safety First guidance warns about overheating, overloaded adapters and unsafe charging practices.

Never connect a portable station to a home's wiring through a wall socket. Backfeeding can endanger electricians and line workers and may damage equipment. A professionally designed isolation/transfer arrangement is a different system from plugging a battery into an extension lead. Do not use a portable power station as a self-designed backup for life-support equipment; seek clinical and supplier advice.

Also confirm that any solar panel connection is within the equipment's specified input limits. A power station cannot safely accept unlimited panel wattage or voltage merely because it has an input socket.

How to test a runtime estimate without risking essentials

1. Charge the station using the approved charger and note its reported starting state.
2. Connect one non-critical appliance such as a lamp that is within the rated output.
3. Record its measured power consumption and the station's battery percentage over a set period.
4. Repeat with the ordinary collection of non-critical devices you would actually use, without exceeding any limits.
5. Compare measured consumption with the estimated energy budget, allowing for display uncertainty.
6. Record a poor-weather solar charging session separately: observed input watts, total duration and energy retained if shown.
7. Adjust your planning numbers using real results rather than promotional maximums.

Do not test by draining a battery connected to medically critical equipment, switching off essential systems or overriding the station's battery protections. Your goal is a realistic estimate, not an endurance demonstration.

When a portable solar generator is the wrong tool

It may be unsuitable when you need continuous high-power heating, an appliance with a starting surge beyond its rating, backup for an entire property or legally compliant supply to fixed home circuits. In those circumstances, a qualified installer or specialist supplier can assess whether a designed home backup system, alternative accommodation or another option better matches the need.

If your aim is only phone charging and basic light, a small dedicated power bank and battery lantern may be simpler, cheaper and easier to maintain. The equipment should follow the problem you actually want to solve—not a headline claiming indefinite off-grid electricity.

Frequently asked questions

How long will a 1,000Wh portable power station last?

It depends on actual load and usable energy. With an illustrative 80% usable-energy assumption, a steady 50W load would run for roughly 16 hours; a steady 100W load roughly eight hours. Device losses, standby draw, temperature and battery limits change real runtime.

Will a 200W solar panel charge a 1,000Wh battery in five hours?

Five hours at a continuous 200W would be 1,000Wh before losses, but that continuous output is not a realistic universal assumption. Weather, sun angle, input limits, controller efficiency and charging losses alter the result.

Can a portable battery run a refrigerator?

Sometimes, if the station supports the fridge's continuous and startup power and has enough usable energy. Measure the fridge's daily consumption and confirm compatibility rather than relying on a generic hours-per-fridge chart.

Does solar power work during a power cut?

A correctly configured standalone portable panel and compatible battery can charge independently of the mains when sunlight is available. Typical grid-connected rooftop solar does not automatically supply a house during a cut; that requires appropriately designed additional equipment.

Research and further reading

• US Department of Energy — solar panel ratings and real-world output factors.
• Energy Saving Trust — solar generation in cloud and winter, August 2026.
• Electrical Safety First — appliance power ratings.
• Electrical Safety First — safe electrical loads and extensions.