Solar Chargers UK: What They Really Deliver in British Sun

A solar charger is a small solar panel, sometimes built into a power bank, that charges phones, power banks or a 12 V battery from sunlight. In Britain a 30 W foldable charger facing the sun collects roughly 100 Wh on an average June day, five or six phone charges, but only about 20 Wh in December. Built-in power bank panels manage a few watt-hours a day.

Those figures come from our arithmetic on long-term sunlight data from PVGIS, the European Commission’s solar database, and they explain most of what follows. A solar charger works, but it works on British weather, and the energy it collects in a day is small compared with what the packaging implies. This guide covers the four kinds of solar charger on sale, what each delivers month by month in London, Manchester and Edinburgh, how to read USB and 12 V output specifications, which charge controller and battery chemistry suit a 12 V system, and when a power station or a plug-in solar kit is the better buy.

Larger folding panels of 100 to 400 W, which charge power stations for camping and vans, have their own guide: portable solar panels and what they can power. This guide stays with the small end.

The four kinds of solar charger

The name covers products that differ in size by a factor of a hundred. Sorting them by what they charge makes the market easier to read.

• Power banks with a built-in panel. A lithium battery pack, typically 10,000 to 20,000 mAh, with one small solar cell on the case or several fold-out panels. The battery is the product; the panel is a backup. Sandberg’s Solar 5-Panel Powerbank 10000, for example, stores 37 Wh and has fold-out panels with a maximum solar output of 8 W.
• Foldable USB panels. Two to six panel sections in fabric or ETFE, folding to the size of a book, with USB-A and USB-C sockets. Common ratings run from about 20 to 60 W. They charge phones, power banks and some tablets directly.
• 12 V trickle chargers and battery maintainers. A small rigid panel of 1.5 to 20 W with a cable and crocodile clips, a 12 V socket plug or an OBD connector, sold to keep car, motorcycle, caravan and boat batteries topped up while the vehicle stands.
• Panel plus charge controller. A separate panel, usually 20 to 200 W, wired through a PWM or MPPT controller to a 12 V battery. This is the setup in caravans, boats, sheds and on electric fences. Our guide to small solar panels covers sizing such systems in detail.

The first two deliver 5 V USB power or USB Power Delivery. The last two work at battery voltage, nominally 12 V, with panels that run at around 18 V at their maximum power point.

How much a solar charger delivers in British sun

A panel’s rating is measured under standard test conditions: 1,000 watts of sunlight per square metre at 25 °C cell temperature. Britain rarely delivers that for long. What matters is the solar energy reaching the panel over a day, which PVGIS publishes as monthly averages for any location. We ran its monthly radiation tool for London, Manchester, Edinburgh and Cardiff for 2005 to 2023, for a panel lying flat, tilted at 35 degrees and standing vertical, all facing south.

The daily figure can be read as peak sun hours: the number of hours at 1,000 W per square metre that would deliver the same energy. Multiply by the panel’s rated watts and a loss factor, and you have the daily energy a charger can collect.

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Two patterns stand out. Winter sunshine is a quarter or less of June’s on a tilted panel, and a fifth or less on a flat one, because the low sun strikes a flat surface at a glancing angle. And in December a vertical panel, propped against a south window or a wall, collects more than a tilted or flat one, for the same reason. In summer the order reverses.

From sunlight to watt-hours

For USB chargers we applied a loss factor of 0.7, allowing 30 percent for heat, the conversion to 5 V, cable losses and the fact that a small charger has no maximum power point tracking. That factor is our assumption, not a measured value; a hot panel or a poor cable loses more.

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Read the June figures with care. A 30 W panel with a USB output capped at 15 W cannot pass on more than 15 W at any moment, so on a bright midsummer day part of the energy is clipped. And these are averages: a clear June day can deliver half as much again, an overcast one a quarter.

What that means in phone charges

A smartphone battery holds roughly 15 to 20 Wh. Taking 18 Wh as a full charge and ignoring the extra losses inside the phone:

• A 30 W panel in June gives five or six full charges a day in theory, and in practice as many as you can plug in, because a phone stops drawing current when it is full. In December it gives about one.
• An 8 W fold-out power bank lying flat gives about one and a half charges’ worth of energy on an average June day and a sixth of one in December.
• A 2 W built-in panel gives about a third of a charge in June and a twenty-fifth in December.

For the 37 Wh Sandberg pack, filling from empty with its fold-out panels takes about a day and a half of average June sunshine near Manchester and two weeks in December, by our arithmetic. A pack of the same size with a single 2 W panel would take five or six June days and well over a month in winter. Sandberg does not publish a solar charging time; one review it quotes calls the panels “more of a backup solution”, which matches the numbers.

Data sheets: what current chargers specify

The table compares chargers from manufacturer and retailer pages viewed on 4 November 2026. Voltgeber does not test products; the values are as published, and “not stated” means we did not find the figure.

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Three points emerge. The Anker panel’s “30 W” is the panel rating; its USB outputs at 5 V and 3 A or 2.4 A allow at most 15 W and 12 W, so a single device never sees 30 W and even both ports together add up to 27 W. EcoFlow’s 45 W panel has a USB-C socket that EcoFlow says charges phones, laptops and similar devices directly, and EcoFlow sells it as a companion for its RIVER power stations; its specification list does not state the USB-C power, so whether a given laptop charges from it is something to confirm with the seller. And the 12 V maintainers are rated by panel power and current, with an 18 V maximum power voltage that is normal for a panel meant to charge a 12 V battery.

The dimensions tell the portability story. The EcoFlow panel unfolds to 98 by 30.9 cm and folds to 22.2 by 30.9 by 4 cm; the Anker PS30 unfolds to 904 by 272 mm. The Ring RSP500 is a rigid 230 by 205 mm board.

USB output: volts, amps and Power Delivery

USB power is the voltage multiplied by the current. The standard 5 V supply that every USB port can provide is limited by the current the port and cable allow: 2.4 A on many USB-A ports, 3 A on USB-C, which gives 12 W and 15 W. That is enough for a phone, slow for a tablet and too little for most laptops.

USB Power Delivery (PD) is a negotiation between charger and device. The device asks for a higher voltage, and the charger supplies it if it can. The USB Implementers Forum says that before Revision 3.1 the limit was 100 W, at 20 V with a 5 A-rated USB-C cable; Revision 3.1 added fixed voltages of 28, 36 and 48 V for up to 140, 180 and 240 W over suitable cables.

For a solar charger this has three consequences:

1. Check the USB-C specification, not the panel rating. A panel that lists only “5 V 3 A” on its USB-C port does not do PD. It will charge a phone at standard speed and will not charge a laptop that needs 20 V.
2. PD needs steady power. When a cloud passes, a panel’s output can collapse within seconds. A phone or laptop that loses power mid-negotiation may stop charging until it is unplugged and reconnected. Charging a power bank from the panel, and the device from the power bank, avoids this.
3. The power bank decides what the phone gets. Sandberg’s pack, for example, takes up to 18 W PD at its USB-C input and gives up to 20 W PD at its USB-C output. A PD power bank between panel and phone gives fast charging at night from energy collected in the day.

12 V solar trickle chargers for cars, caravans and boats

A parked car’s battery loses charge for two reasons: self-discharge inside the battery, and standby loads such as the alarm, immobiliser and control units. Ring’s specification sheets note that modern vehicles can have up to 30 on-board computers drawing a small but constant current. A solar maintainer aims to replace that loss, not to recharge a flat battery.

Worked example: a car parked near Manchester

This is an example with stated assumptions, not a measurement.

Drain. Assume a standby current of 30 milliamps, a figure in the range often quoted for modern cars; check your own car’s handbook or measure it. Over 24 hours that is 0.72 Ah, about 9 Wh at 12.5 V. Self-discharge of a healthy lead-acid battery is small by comparison: Victron gives under 2 percent a month at 20 °C for its AGM and gel batteries, about 0.04 Ah a day on a 60 Ah battery.

Supply. A small panel connected straight to a battery behaves like a current source: its current rises and falls with the sunlight, and the battery holds the voltage near 12.5 to 13 V. Daily charge in amp-hours is therefore roughly the panel’s current at maximum power multiplied by the peak sun hours. We also allowed 20 percent for the windscreen glass, which is our assumption.

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Result. From March to September a 5 W maintainer roughly matches or exceeds a 30 mA drain on average, if the car faces the sun. From November to February it supplies a third of the loss, and the battery runs down slowly; a car left for six winter weeks would lose about 20 Ah net, a third of a 60 Ah battery. A 20 W panel covers the drain year-round on average days, but in summer it supplies four to six times the drain, which raises the question of overcharging.

Connection and overcharging

How the panel connects matters as much as its size. In many cars the 12 V socket switches off with the ignition, so a maintainer plugged into it charges nothing while the car is parked. Ring sells an OBD cable that it says provides a permanently live link through the diagnostic port and is a more efficient connection than the 12 V socket. Crocodile clips straight to the battery are the most direct route.

Ring’s pages for the RSP500 and the 20 W models mention reverse-polarity protection but not charge regulation, and we found no statement either way. A small current into a large battery does little harm: 0.28 A is less than half a percent of a 60 Ah battery’s capacity per hour. A 20 W panel on a battery that is not being used is different. On a sunny week in June it can push a full lead-acid battery into continuous overcharge, which gasses and dries it. For panels above a few watts, or for any battery left unattended for months, fit a charge controller with a float stage, set for the battery type.

Caravans and boats follow the same logic on a larger scale. A leisure battery of around 100 Ah, supplying lights, a water pump and a radio, needs a panel and a controller rather than a maintainer, and the sizing method in our small solar panels guide applies.

PWM or MPPT: choosing a charge controller

A charge controller sits between panel and battery. It stops the battery overcharging, and the better ones manage charging in stages (bulk, absorption, float) with voltages set for the battery chemistry. There are two designs.

PWM controllers are, in essence, a switch. When closed, they connect the panel directly to the battery, so the panel is pulled down to battery voltage. Victron’s white paper works through an example with a typical 100 W, 36-cell panel at 25 °C. With the battery at 13 V and half a volt lost in cables and controller, the panel operates at 13.5 V and 6 A: 81 W, 19 percent less than the 100 W an MPPT controller harvests from the same panel.

MPPT controllers contain a DC to DC converter. They hold the panel at its maximum power point, around 18 V for a 36-cell panel in mild conditions, and convert the power to whatever voltage the battery needs. They also let you wire panels in series at higher voltage, which cuts cable losses on longer runs.

Victron’s paper adds a temperature effect. A panel’s voltage falls by about 4.5 percent for every 10 °C rise in cell temperature. In hot climates the panel’s voltage drops towards the battery voltage anyway, and a PWM controller loses little; the paper describes PWM as a good low-cost solution for small systems only when cell temperatures are moderate to high, between 45 and 75 °C. It states that MPPT is generally accepted to outperform PWM in a cold to temperate climate. Britain is firmly in that category: cool cells mean higher panel voltage, which a PWM controller throws away.

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The Victron data sheet adds two details worth knowing. Its controllers start charging only when the panel voltage exceeds the battery voltage by 5 V, then keep going down to 1 V above it. And the controller draws about 10 mA itself when the load output is off, so a tiny panel on a large battery is partly spent running the controller in winter.

For a 20 to 50 W panel on a car or caravan battery, a PWM controller costs little and loses a modest share in absolute watt-hours. From about 100 W upwards, or with panels in series, an MPPT controller earns back its higher price in extra energy over a few seasons, particularly in spring, autumn and winter.

Battery chemistry for 12 V solar

The battery decides how the system must be charged, how deeply it can be used and how it copes with cold.

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Lead-acid (flooded, AGM, gel). Cheap and tolerant of cold charging, but heavy, and short-lived if discharged deeply. Victron’s figures show AGM cycle life falling from 1,500 cycles at 30 percent discharge to 400 at 80 percent, and the data sheet says repeated deep discharge has a very negative effect on the service life of all lead-acid batteries. In practice that means using only about half the rated capacity. Victron also notes that self-discharge doubles for every 10 °C rise, so a hot engine bay ages a battery faster.

Lithium iron phosphate (LiFePO4). Lighter, with far more cycles and most of its capacity usable. The catch is cold. Renogy’s battery accepts charge only between 0 and 50 °C and discharges down to -20 °C. Charging a lithium cell below freezing can plate lithium metal on the anode, which damages it permanently. Many lithium batteries have a battery management system that blocks charging when cold; some, such as Renogy’s self-heating models, warm themselves first. A lithium battery in an unheated caravan or boat in a British winter needs one of those features, or a controller with a temperature sensor that stops charging below a set temperature.

Lithium batteries also need a controller with a matching charge profile. Lead-acid settings with an equalisation stage can push a lithium battery beyond its limits.

Safety

• Charge temperature. Lithium power banks and phones have narrow ranges. Apple says iPhones are designed for ambient temperatures between 0 and 35 °C and should be stored between -20 and 45 °C, and that a parked car can exceed that range; when the internal temperature is outside its normal range, charging slows or stops. Sandberg gives an operating range of 0 to 45 °C for its solar power bank. A power bank left charging on a dashboard in summer sun can exceed both, which is why its panel and its battery should not share the same sunny spot for hours.
• Fire. Lithium-ion batteries can overheat and catch fire if damaged, poorly made or charged incorrectly. The Scottish Fire and Rescue Service advises charging only in safe areas away from escape routes and flammable materials, not while asleep or away, with the approved charger, and never using a battery that is swollen, hot, hissing, leaking or smelling unusual.
• Recalls. Power banks from several brands have been recalled in the UK for fire risk. The Office for Product Safety and Standards publishes alerts and recalls on GOV.UK; check there before trusting an unknown brand from an online marketplace.
• 12 V wiring. A 12 V battery can deliver hundreds of amps into a short circuit. Fuse the positive cable close to the battery, use cable sized for the current and keep clips insulated.
• Water. An ingress rating may cover the panel but not the junction box or plug. Fold a USB panel away in heavy rain and keep the power bank or phone dry.
• Disposal. Batteries and panels belong at a recycling centre or retailer take-back point, never in household waste.

Choosing a solar charger, step by step

Common mistakes

• Expecting the rated watts. A 30 W panel delivers 30 W only in strong, direct sun with a cool panel. Consequence: flat devices. Fix: plan by watt-hours per day from the tables.
• Charging through a window. Glass, especially coated or tinted, cuts the output, and the angle is often wrong. Fix: put the panel outside where you can, or accept a lower yield.
• Leaving a power bank in the sun. Consequence: heat damage, and charging stops above the battery’s limit. Fix: put the panel in the sun and the battery in the shade with a longer cable.
• Plugging a maintainer into a socket that switches off. Consequence: no charging while the car is parked. Fix: use a permanently live connection or clips on the battery.
• No controller on a larger panel. Consequence: an overcharged and dried-out lead-acid battery. Fix: fit a controller with a float stage above a few watts.
• Charging lithium below freezing. Consequence: permanent damage. Fix: a battery with low-temperature protection or a controller that blocks cold charging.
• Buying for a power cut. Consequence: a few watt-hours on a grey winter day. Fix: keep a power bank or power station charged from the mains, and treat solar as a top-up.

When a power station or plug-in solar makes more sense

A solar charger fills a small battery slowly. That suits phones on a hike, a car standing in summer or a boat battery between trips. It does not suit running a fridge, working on a laptop all day or lowering the household bill.

A power station with a larger panel is the step up for camping, vans and power cuts. A unit of around 300 Wh to 1 kWh with a 100 to 220 W panel collects ten times what a 30 W charger does and runs mains appliances through its inverter. Our guide to portable solar panels shows how to match the two, with data-sheet values and the same PVGIS approach used here.

A fixed 12 V system is the step up for a shed, allotment, gate or camera. A rigid panel, controller and battery cost less per watt than portable gear and last longer outdoors. See our guide to off-grid solar systems in the UK and the shed example in our guide to solar panel kits.

A plug-in solar kit is the right product if the goal is a lower electricity bill. Since 27 August 2026, kits of up to 800 W output that meet the government’s Interim Product Specification, are listed on the ENA Type Test Register and are notified to the network operator have been legal in England, Scotland and Wales. They feed the home wiring through a 13 A plug and save money on everything plugged into the house; a solar charger saves next to nothing on the bill. Building a plug-in device from a small panel and a separately bought inverter is outside the rules, and plug-in batteries are excluded, as our guide to plug-in solar with a battery explains. Our guide to whether plug-in solar is legal in the UK sets out the conditions.

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The plug-in figure is our estimate by the same method, assuming about 860 W of panels tilted south and 14 percent system losses; it depends on panel size and orientation, which our plug-in guides cover; the point of the table is the scale. A solar charger is a convenience for small batteries, and it is good at that job as long as expectations match the British sky.

Next steps

• Portable solar panels: what they can power in the UK
• Small solar panels: sizes, uses and 12 V systems
• Is plug-in solar legal in the UK?