This guide will help you design and size the components of a solar energy system that meets your needs.

Sizing Your Solar Panel System

To correctly size your solar system, you must ensure that over a period of time (taking into consideration the seasons and average weather and light conditions), your solar panels produce at least as much energy as your loads (laptop, TV, lighting, etc) consume.

In the UK, northern Europe and north America, a conservative estimate allows for four hours of peak sunshine in summer and one hour in winter.

The power produced by a solar panel in one hour is its power rating (Watts).

The estimated power produced in a day by the panel is obtained from multiplying the power produced in one hour by the number of hours of peak sunshine.

Thus, in the UK, a 30W panel will produce around 120 Watt-hours (30W x 4 hours) of power on an average day in summer.

1. Estimate the Total Power Usage (wattage) of Your Appliances

List all the 12V electrical appliances you intend to power from your solar system and find out how many watts each consumes (from the appliance, power adaptor or instruction booklet).

2. Calculate Your Total Daily Watt-hour Requirement

Estimate how many hours you expect to use each appliance each day. Multiply each appliance’s wattage by its estimated daily hours’ usage to calculate the daily Watt-hour requirement for that appliance. Add up the daily Watt-hour requirements of all your appliances to determine your total daily Watt-hour requirement.

3. Calculate Your Solar Panel Size

Take the total daily Watt-hour requirement you calculated in 3, and divide by the number of hours of usable light you expect in an average day (eg, four hours in summer and one hour in winter for the UK, northern Europe and north America). This will give you the minimum wattage of your solar panel.

4. Calculate Your Battery Size

Battery capacity is measured in Amp-hours (Ahrs). To calculate how large a battery you need, take the total daily Watt-hour requirement calculated in 3, and divide this by the battery voltage (12V or 24V) to convert back to Amp-hours.

You shouldn’t discharge a battery beyond half-level, so you must then multiply this figure by two to give the correct battery size.

The calculation thus far assumes you will be recharging your battery daily. If you wish to be power-autonomous for n days (ie you will only solar-charge your battery every n days), multiply the above figure by n to give you your final battery size.

5. Worked Example

You wish to run a 30W laptop for 2 hours a day and a 10W light for 4 hours a day. The daily Watt-hour requirement of the laptop is 30 x 2 = 60 Watt-hours. The daily Watt-hour requirement of the light is 10 x 4 = 40 Watt-hours. Thus, the total daily Watt-hour requirement is 60 + 40 = 100 Watt-hours. You will be using the system in summer in a location that provides 4 hours of daily sunlight. Thus, your panel needs to have a wattage of at least 100 / 4 = 25W.

Your 12V battery needs to be able to power your laptop and light for five days between charges. Thus, your battery size needs to be at least (25/12) x 2 x 5 = 20.83 Ahrs.

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