How to calculate power consumption on a sail-boat

Before deciding about your prefered way of storing electricity on board, a few things need to be considered: you have to estimate your power consumption and therefore know the necessary storage capacity. You have to know the specifications of your power sources such as how much power is provided, for how long they are available (wind vs solar vs alternator/generator) - how they charge and then of course such smaller things like how much space you have on your boat and of course cost.
As I want to renew the entire engery and storage system on our Five Senses, I did all these evaluations myself. Here I am sharing one of the steps taken: the calculation of power consumption.

All of this is clearly no rocket science: device list, power consumption (in watts or kilowatts), estimate how long the devices are operated in 24 hours (in hours) and then calculation of the consumption in ampere hours (Ah) taking into account your electrical system voltage. However, there are several different units used, so you might have to do some calculation. Here is an example table to show you how to do that:

LED lighting salon and cabins: 50 watts (rough estimate)
Operating time per 24 hours: 4 hours (estimated)
Daily consumption in kilowatt hours: (50/1000) * 4 = 0.2 kWh
We are using a 12 volts system
Conversion to Ah: 50 watts / 12 volts * 4 operating hours = 16.67 Ah

With the kWh and the Ah you are able to estimate your consumption per day. Usually, this includes on-board and navigation lights, electrical devices (hair dryer, laptop, coffee maker), navigation devices such as GPS and plotter including radar and autopilot, cooling boxes / fridge and freezer. This is the sum of your average, estimated consumption in 24 hours. It is obvious that an air conditioning system can hardly be powered by battery power. The navigation electronics created a bit of a surprise to me. Each device does not need a lot of electricity, but in total and over the sailing day and the many hours, there is a lot of overall consumption.

To estimate the battery storage capacity you need, you have to know the answers to these two questions:

How long has my stored electricity to last before I can recharge?
What kind of power storage = which type of battery do I want to use?

The first question is rather easy: This is heavily depending on your crusing plan and how long you are planning to stay off grid. Depending on where you sail, choices shoudl be made on energy sources. I collected some information and put this into another blog post to compare pro's and con's of the various alternatives.


For the second question again some background info is helpful: i.e. conventional lead acid batteries supply about 50% of their total capacity, lithium ion batteries can provide up to 80 or 90% of their storage capacity without being damaged. There are various other alternatives available and all come with their very individual specifications. Make sure you know what you need before buying any new batteries.

For our Five Senses (a 49 foot yacht in the Mediterranean and most frequently used during the day and somtimes during night), I calculated a daily consumption of around 330 Ah, or around 4 kWh. For LiIon batteries one can expect 80% of their capacity to become available, which translates into 412 Ah or almost 5 kWh of capacity per storage day. We currently have 400 Ah lead-acid batteries on board, so we are missing 130 Ah when 50% is removed, or in other words: after 14 hours without any charge, all power run devices are down.  330 Ah is a conservative calculation and an average, the power sometimes lasted longer, mainly because we also use the ship's diesel for docking and anchoring. But after a night at anchor, the batteries were mostly empty ...


Conclusion: new energy storage and energy sources are needed! Stay tuned ...

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