Laptop solar power - practical example

13 Oct 2011
Posted by Laptop Junction

Using a laptop for long periods of time away from the urban power grid requires the use of alternative power sources. When there’s enough sun, solar power is a perfect alternative.  Here’s a real life example of the electrical solar power equipment needed to power a laptop round the clock.

Laptop Solar power – essential equipment

There’s a big gap between the glossy brochure talk and really trying to power a laptop with electrical solar power. Most of the information freely available on the internet is about solar panels, solar chargers and related equipment - but not what's actually needed to power a simple laptop for days in a row.

This practical example is not about occasionally charging a laptop battery or powering a laptop a few hours on electrical solar power. The story here is about working on a laptop for most of the day, for several days in a row on electrical solar power, far away from AC mains.

Laptop solar power in Oman - Salalah
Laptop solar power in Oman - Salalah in March. The heat is already blistering early in the year.

Here’s an answer of what that takes – based on practical experience with The Machine from 2006 and onwards.

Laptop Solar power reference

1 Kwh reference - powering a laptop for a day
How much power is 1Kwh?
run a vacuum cleaner for 30 minutes

Say you got a few days of work ahead on a laptop without access to AC mains. What solar power equipment does it take to get that done? Of course assuming there’s enough sunshine.

First of all, lets assume a worst case scenario where the laptop is powered 24 hours with work being done most of the day. Work continues after sunset reverting to stored solar power during the same day.

As a benchmark reference, say it needs about 45 Watts on average. That’s about 1 Kw needed over 24 hrs – which is simply put 1 Kwh for a day. The costs for 1 Kwh from an AC mains outlet in Europe averages between 20 – 30 eurocents in 2011.

Solar power equipment

A basic solar power configuration has the following components;

Laptop solar power system components
Laptop solar power system components.

The capacity of  the battery and solar panels should be sized to that 1 Kwh reference including system inefficiency. The solar charger and DC-AC inverter are assumed to be of sufficient rating with an electrical loss of 20 - 30 % in heat and conversion.

The total system needs to collect 1.3 Kwh from the sun to compensate system losses as well. The question is; What’s the rating of the solar panel and batteries needed?

Laptop solar panel capacity

Say the sun shines on the solar panels from 10:00 am to 16:00pm producing the bulk of the required power during that time. During 8 hours of sunshine the laptop needs to be powered including enough energy to get through the remaining 16 dark hours.

So 1.3 Kw is coming from the panels in just 8 hours time. That’s about 163 Watts during sun peak hours.

Laptop solar power panels - 180 Watt each
Laptop solar power panels - 180 Watt each

In the real world Solar Panels are not angled to the sun in the most efficient way. Also panel cooling is not so efficient and the sky filled with a bit of smog / dust etc. All these factors lower the amount of power captured from the sun.

Say the solar panels operate at 60 % of their maximum capacity mounted flat on the roof without shade. Meaning the solar power panel grid needs to be sized at a minimum of 272 Watts – meaning two panels of for instance 180 Watt.

Laptop solar battery capacity

During 10:00am to 16:00pm the solar battery sees the maximum solar charge coming in. The remaining 16 hours it’s supplying power to the laptop.

8 hours of laptop power are directly supplied by the sun – meaning 0,360 Kwh (45Wh during 8 hours) is already gone. The battery is stocking up the remaining power, meaning about 1000 Watts in 8 hours. That’s a charge of 11 Amps for a 12 Volt battery – that’s 88 Ah. (8 hours * 11 Amps)

Laptop solar battery - Victron 200Ah 12 volt DC
Laptop solar battery - Victron 200Ah 12 volt DC

The battery is discharged to about 40 % of it’s rated capacity, meaning 60 % is actually available for storage. Therefore the battery needs to be rated at about 150 Ah at least.  Commercially available solar batteries are available in 200Ah rating.

Laptop solar power system - practical configuration

The Machine is equipped with two Victron 200Ah AGM batteries since there's more to power than just 1 laptop. The two 180 Watt solar panels are still there but more would be very nice of but the 320 Watt solar panel surface is already a large solar power panel surface to drive around with.

The solar battery charger is changed to a MPPT charger, adding 30 % more charge on cloudy days.

Also interesting:

Powering a laptop with human power.