Solar panels for a shed: size the right off-grid kit

Solar panels

Generate DC electricity. Rated watts describe test-condition output, not daily energy or guaranteed UK production.

Charge controller

Regulates panel energy going into a separate battery. It must be compatible with the panel array and battery; this is not a component to guess from connector shape.

Battery

Stores energy in Wh. Battery chemistry, usable capacity, permitted temperature and protective controls matter as well as the headline number.

Inverter

Converts DC to AC for suitable appliances. Its continuous and surge limits are separate from battery capacity.

An integrated portable power station combines the controller, battery, inverter and controls in one manufacturer-supported unit. A component kit can be more configurable, but safe design of protection, cables, connectors and fixed outlets requires appropriate competence.

Watts describe demand at a moment. Watt-hours describe energy over time. Multiply each appliance's running watts by its daily hours, then add the results. Check rating plates rather than relying on the examples below.

The workshop example uses less daily energy than an all-day computer, but may need a much larger inverter because its loads are powerful while running. Motors and compressors can demand more at start-up.

Doubling rated panel wattage can increase potential generation and shorten recharge time when the battery can accept the input and the panels receive useful sun. It does not guarantee twice the energy: shading, orientation, controller limits, temperature and a full battery can all constrain output.

A 400 W array may be ample for modest, intermittent loads in brighter months. An 800 W array provides more charging potential for a daily office or workshop, but winter autonomy still depends on stored energy and weather. Do not connect panels beyond the controller or power station's stated voltage, current or power limits.

For the wider office calculation, read solar power for a garden office. The main solar calculator gives broad UK generation context but is not an off-grid kit designer.

A 2,000 W heater consumes 2,000 Wh in one hour. That is more than 28 times the basic daily example. A kettle may run briefly, but the inverter still has to support its high instantaneous load. Table saws, compressors and dust extractors can combine high running power with start-up surge.

Treat these as separate design cases. For a frequently used workshop or electrically heated building, ask a competent professional to assess whether a properly installed mains supply or another heating strategy is more practical.

• Avoid shade from the shed roof edge, trees, fences and neighbouring buildings across the whole day.
• Secure panels with a mounting system designed for the location and expected wind loads.
• Keep connectors and equipment within their stated weather-resistance limits; a garden building can be damp, dusty or very hot.
• Expect substantially less available solar energy on short, overcast winter days than in summer.
• Plan a safe alternative charging route or reduce loads rather than assuming the battery will always recover by the next day.

Plugging suitable appliances directly into a manufacturer-provided portable power station is different from installing fixed sockets in a building. Fixed wiring, protective devices, earthing and outdoor cable routes should be designed and installed by a competent electrician.

• Inspect tools, plugs and leads before use and stop if they are damaged, overheated or repeatedly trip protection.
• Keep cables away from blades, doors, standing water and trip routes.
• Check that equipment is suitable for damp or dusty conditions and that the supply supports its rating-plate load.
• Follow the battery and inverter manufacturer's ventilation, temperature, charging and storage instructions.

The UK plug-in solar guide applies to the separate mains-connected category.