Art & Freedom On The Road

How much power we needed

by Joe on 24th November 2018

Before buying solar panels and all the bits for our electric system, we had to work out how much power we needed to make and how much battery capacity we needed to store it, and so will you if you’re doing the same. People have different methods of doing this, so rather than telling you this is the best and only way I’m going to tell you what I did – and if it works for you too then that’s great!

If like me and have no experience in working with electricity then fear not, just grasp the basics like I did and you’ll be fine. You can be rough with your workings as long as you give yourself plenty of leeway and aren’t stingy on solar panels.

Firstly - Amps, Volts & Watts

Amps, Volts and Watts are all units used together to measure aspects of electricity. Solar panels typically come in watts as that’s the basic method of estimating the power they output, and your devices will usually be measured by the watts they use. The watts are usually the number of watts they use per hour (wh).

If your devices don’t have the rated watts (usually located on a sticker) then they will likely tell you how many amps and volts required. Just use this equation to work out the watts:

amp x volts = watts 

i.e. 1ah x 12v = 12wh

Calculating how much power our devices make

To give you an idea here’s what we have and roughly how much power each device uses every day

  • 45l Compressor Fridge – 40w X 12hrs = 480wh
  • Acer Laptop – 40w X 4hrs = 160wh
  • Macbook Pro – 60w X 1hrs = 60wh
  • iPhone 5S – 5w X 6hrs = 30wh
  • Samsung S7 – 5w X 6hrs = 30wh 
  • Bluetooth Speaker – 5w X 2hrs = 10wh
  • MiFi – 5w X 6hrs = 30wh
  • Lights – 2w X 4hrs = 8wh
  • Extractor Fan – 36w X 30mins =18wh

Total Watts Required Per Day – 826wh

How much solar power we needed

There are so many factors that determine the output of a panel it’s unreal. There’s the efficiency of the solar cells, the temperature (the hotter they get generally the less power they make, good to keep them cool), the efficiency of the charge controller etc, but the main thing we thought about is the amount of sun we would be getting. 

This can vary between seasons and locations massively, so as an example I worked on a worst case scenario basis and estimated we’d be getting an average of 3 hours light per day in the winter in Spain/Italy, while it may be more like 8-10 hours as the sun is low in the winter and so won’t generate it’s full capacity at any point of the day.

I then added some leeway for loss of efficiency and rounded up to 900wh as my required consumption. This meant I needed 300w worth of panels.

The battery Storage we needed

Lead Acid batteries are the most common due to their cheapness and availability. Unfortunately, they aren’t as straight forward as they should be when it comes to working out their capacity. A lead acid battery should only be drained by half and should always be half full, otherwise it’ll never charge to its full capacity again.

Batteries are measured by the amp hours they store. You may find most leisure batteries are 85ah or 110ah, when running on 12v this means they should be able to output around 1050wh and 1320wh of power respectively – but then you’ve got to half that for the power you’ll never be able to take from it, so more like 525wh and 660wh.

We decided to go for 2 x 110ah batteries, theoretically being able to store 1320wh of power – more than enough for a day.

To summarise

We decided we wanted 300w of solar panels and 220ah of lead acid battery storage for our trip to Spain/Portugal and Italy form August – February.

Good luck in calculating your requirements, if I’ve not made a lot of sense then drop me a message for any bits I can help clarify!

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