Revision history [back]

The starting point is to assess how much energy in watt-hours (Wh) is needed. Laptop batteries are typically 11 – 15V voltage with a capacity of 2,500 mAh (= 2.5 Ah) to 7,000 mAh.

Voltage (volts, V) x Current (amps, A) = Power (watts, W).

Power (watts, W) x Time (hours, h) = Energy (watt-hours, Wh).

Capacity (Ah, = 1,000 mAh) x battery voltage (V) = Energy storage (Wh).

Laptop batteries are generally between 40 and 80 Wh. It’s useful to convert from the typical laptop battery specification of milliamp-hours because the voltage that you generate may not be the same as that used by the laptop. Defining the energy required is the best way to compare different sources.

The next question is how often you need the battery recharging? A fully charged laptop will typically run for 3 to 6 hours depending on the model, activities, power settings, and age of the battery. Is this enough for one day, or several? This will tell you the minimum amount of energy you need to generate per day.

How will you charge the laptop? Laptop power supplies convert from 230 Vac (or 110 Vac) to 15 to 20 Vdc. If you have a 230 Vac source – an engine or hydro generator – you can use the standard laptop power supply. Whilst it might sound feasible to connect a DC (direct current) source, such as a solar PV panel, direct into the laptop charging socket, you will probably damage the charge regulator in the laptop. A practical solution is to use an intermediary 12V battery and either a small inverter to convert to 230 Vac or a 12 Vdc car adaptor. Inverters are easier to find (or replace) locally but may result in other loads being connected.

How to generate the energy? Unconventional sources like piezoelectric (shoes, pavements) or thermoelectric (stove or fire-charger) sound like an interesting research project. Small stove chargers are available but generally output to a USB socket which at 5.2 Vdc is not enough for a laptop. Although technically less exciting, the best approach is to use an easily available, well proven source. Typical examples would be solar PV, engine generator, wind turbine, hydro generator or, maybe, pedal generator.

For the small amount of energy required, an engine generator is overkill, and requires regular and costly fossil fuel to operate. Nepal has for many years been a world leader in the use of micro hydro for village power supplies. There may well be one available locally, in which case the laptops could be charged directly, or possibly via a portable battery (with inverter) that can be taken for charging to the hydro system once a week. Hydro systems are quite expensive to install, so would not be cost effective just to run a few laptops alone. As a village electrification supply it’s a different story.

Wind turbines are well proven and can be made locally – see Hugh Piggott’s designs at scoraigwind.co.uk which have been adapted and built in small workshops all over the world. However, windpower is very site specific and needs a well sited tall tower (see KnowledgePoint Q&A “What makes a good windpower site”).

Pedal generators have been trialled in Tanzania and proved popular at UK festivals and there are various designs available on the web. One person could generate 50 – 100 W for a sustained period so an hour’s pedalling could fully charge the laptop.

The simplest solution is a small solar PV panel with a charge controller and a small 12 V deep-cycle lead-acid battery. 10 to 40 Ah capacity (120 – 600 Wh) would work for a couple of laptops. Nepal mean daily solar irradiance is between 4 and 6 kWh/m2. This can also be expressed as peak hours which sums the variable irradiance from dawn to dusk and converts it to hours at 1 kW/m2, i.e. the same figure as for kWh/m2. A 50 W PV module (solar panel) will provide 50 times the peak hour rating – 200 to 300 Wh in this case so enough to provide some extra to cover cloudy days (but you need sufficient battery capacity to store the extra).

Costs vary locally, but a rough budget might be 50 W PV module £60 50 Ah sealed deep-cycle battery £70 5 A charge controller £20 100 W inverter (basic) £60 Simple mounting, cable, fittings £40 Total ~ £250

What do you want to achieve? If you are looking for an interesting R&D project, by all means explore unconventional energy sources. If you want a simple, cost effective solution that can be set up and run with minimum skills in most off-grid locations, a small solar PV system is the most likely choice. There are still technical challenges to consider. Can you build the components into a robust and portable format? What happens when the battery needs replacing. Do you add in USB sockets to charge phones, and if so, how do you allow for the additional load? What about a light? It’s always time well spent talking to potential users to find out what they need electricity for and what they currently use.

The starting point is to assess how much energy in watt-hours (Wh) is needed. Laptop batteries are typically 11 – 15V voltage with a capacity of 2,500 mAh (= 2.5 Ah) to 7,000 mAh.

Voltage (volts, V) x Current (amps, A) = Power (watts, W).

Power (watts, W) x Time (hours, h) = Energy (watt-hours, Wh).

Capacity (Ah, = 1,000 mAh) x battery voltage (V) = Energy storage (Wh).

Laptop batteries are generally between 40 and 80 Wh. It’s useful to convert from the typical laptop battery specification of milliamp-hours because the voltage that you generate may not be the same as that used by the laptop. Defining the energy required is the best way to compare different sources.

The next question is how often you need the battery recharging? A fully charged laptop will typically run for 3 to 6 hours depending on the model, activities, power settings, and age of the battery. Is this enough for one day, or several? This will tell you the minimum amount of energy you need to generate per day.

How will you charge the laptop? Laptop power supplies convert from 230 Vac (or 110 Vac) to 15 to 20 Vdc. If you have a 230 Vac source – an engine or hydro generator – you can use the standard laptop power supply. Whilst it might sound feasible to connect a DC (direct current) source, such as a solar PV panel, direct into the laptop charging socket, you will probably damage the charge regulator in the laptop. A practical solution is to use an intermediary 12V battery and either a small inverter to convert to 230 Vac or a 12 Vdc car adaptor. Inverters are easier to find (or replace) locally but may result in other loads being connected.

How to generate the energy? Unconventional sources like piezoelectric (shoes, pavements) or thermoelectric (stove or fire-charger) sound like an interesting research project. Small stove chargers are available but generally output to a USB socket which at 5.2 Vdc is not enough for a laptop. Although technically less exciting, the best approach is to use an easily available, well proven source. Typical examples would be solar PV, engine generator, wind turbine, hydro generator or, maybe, pedal generator.

For the small amount of energy required, an engine generator is overkill, and requires regular and costly fossil fuel to operate. Nepal has for many years been a world leader in the use of micro hydro for village power supplies. There may well be one available locally, in which case the laptops could be charged directly, or possibly via a portable battery (with inverter) that can be taken for charging to the hydro system once a week. Hydro systems are quite expensive to install, so would not be cost effective just to run a few laptops alone. As a village electrification supply it’s a different story.

Wind turbines are well proven and can be made locally – see Hugh Piggott’s designs at scoraigwind.co.uk which have been adapted and built in small workshops all over the world. However, windpower is very site specific and needs a well sited tall tower (see KnowledgePoint Q&A “What makes a good windpower site”).

Pedal generators have been trialled in Tanzania and proved popular at UK festivals and there are various designs available on the web. One person could generate 50 – 100 W for a sustained period so an hour’s pedalling could fully charge the laptop.

The simplest solution is a small solar PV panel with a charge controller and a small 12 V deep-cycle lead-acid battery. 10 to 40 Ah capacity (120 – 600 Wh) would work for a couple of laptops. Nepal mean daily solar irradiance is between 4 and 6 kWh/m2. This can also be expressed as peak hours which sums the variable irradiance from dawn to dusk and converts it to hours at 1 kW/m2, i.e. the same figure as for kWh/m2. A 50 W PV module (solar panel) will provide 50 times the peak hour rating – 200 to 300 Wh in this case so enough to provide some extra to cover cloudy days (but you need sufficient battery capacity to store the extra).

Costs vary locally, but a rough budget might be

50 W PV module £60 £60

50 Ah sealed deep-cycle battery £70 £70

5 A charge controller £20 £20

100 W inverter (basic) £60 £60

Simple mounting, cable, fittings £40 fittings£40

Total ~ £250

What do you want to achieve? If you are looking for an interesting R&D project, by all means explore unconventional energy sources. If you want a simple, cost effective solution that can be set up and run with minimum skills in most off-grid locations, a small solar PV system is the most likely choice. There are still technical challenges to consider. Can you build the components into a robust and portable format? What happens when the battery needs replacing. Do you add in USB sockets to charge phones, and if so, how do you allow for the additional load? What about a light? It’s always time well spent talking to potential users to find out what they need electricity for and what they currently use.