Living With Our Rapido 963f Le Randonneur Motor Home.

Thursday, 24 March 2011

How do I calculate my energy requirements?

If you have read my previous posts on my own boat specification, and selection process, you will have seen that there are many things to consider when either buying a narrowboat, or specifying one to be built, in that it is important to have a specification that will suit your intended use. The subject I will cover here, in a very basic form, is electricity generation, storage and usage, and the equipment needed to satisfy the way in which you want to use your boat.........
We are going to discuss electricity as an energy resource on the boat. What is used, how it is stored and how it is replaced. First we need to take a brief look at basic first principles so that it can be understood what is needed in relation to equipment. Importantly this is intended to help you understand your own requirements, not to show you how to install and connect equipment yourself. That should be left to those with professional knowledge.

Between 1973 and 1987, I qualified, and was employed as an electronics technician. Some of the first principles of electronics I learnt were basic formulas and laws that would allow the calculation of the various aspects of power, voltage resistance and current and their relationship to each other. In an attempt to make things easier to understand here, if we look at the supply of electricity as if it were water flowing down a river bed. Electrical current, measured in amperes, (amps), is the flow of water. The volume, or quantity of water carried depends on how much there is available, voltage, (volts). How much water energy, or power, (watts), we have available at a given point on the river, to turn a water wheel, depends on the relationship of the amount of water available, (volts), and the current flowing through it, (amps). The current and amount of water flowing are also related to the width of the river channel, or resistance, (ohms), to the flow.

Taking this basic theory into two formulae we can say that V, (voltage in volts) = I, (current in amps) x R, (resistance in ohms). Or V=I x R, transposed to find I=V/R or R=V/I. Are you still with me?
And W, (watts) = I, (amps) x V, (volts), again can be transposed to calculate current or voltage related to power, depending on what you want to find out. Where W, or watts  is the unit measurement of power.

Now we have the maths out of the way, we can move on to finding out what your own given requirements will be. The way electricity is used on a boat is: Generation, Storage and Use. To enable a calculation of the required equipment, you will first have to define how you want to use the boat, and then build in the equipment required to fulfil your needs.

Generation. This will include the engine alternators, solar panels, wind generators and the petrol/diesel/travel power generator if you have one.
Storage. This of course will be the battery bank.
Use. This will be the sum of all the power used by the boat's equipment. This will be the lights, TV, radio, fridge, water and shower pumps, heating pump, and even the inverter, which consumes it's own power when in use. Usually, equipment is marked with it's power consumption on a rating label, and things such as light bulbs will be individually marked on the base cap, e.g. 12v / 15w.

Group all the 12V equipment together to find the total power consumed, say 100 watts. Pop these figures into one of the above formula to find what current will be drawn as a result: I = W/V  = 100/12 = 8 amps. Multiply this over the time you intend to use the equipment, and you now have a figure in amp/hours.

The mains supplied equipment now has to be worked out in the same way, as their power rating label will be given at (x) watts at 240V. If the TV is rated at 150 watts, then the current drawn will be: I = W/V = 150/240 = 0.625 amps, multiplied by the hours intended to use to give an answer in amp/hours.

Having completed these calculations you should now have an idea on how much current your own equipment will, (roughly), draw. Storage, or your batteries, are rated in amp/hours. In theory, a 100 amp/hour battery can have 1 amp drained from it for 100 hours or 8 amps drained from it for roughly a day. However, in practice this doesn't work due to various factors such as age and condition of the battery. The battery properties also mean that they will last longer if a lower current is drawn, less if more is drawn. An average 100a/h battery will be flattened and be of no practical use long before 50% of it's marked capacity has been drawn. So, some leeway has to be built into the size of battery bank required, but bear in mind, that the amount of charging capacity available also has to be taken into consideration, as the amount of time required for generation equipment to run in order to fully charge the batteries back to full capacity.

I earlier gave the formula V=IxR. In a 12V system, the thickness, and thus the resistance of the wiring is an important consideration. A substantial drop in voltage will be noted at the appliance, if a long run of cable of insufficient thickness is used to connect it to the battery. This could be enough to stop the appliance from working in some cases. When cable is purchased, it also is rated, and figures can be substituted in the above formula to calculate the correct cross section of wire to use. Voltage is measured across two terminals, current flows through a cable. I would strongly recommend all wiring be left to the professionals, who will know the specification of materials to use, as in extreme cases, because of heat generated in incorrect installations, the result is fire!

Now I know all this might sound a little complicated, but what you are aiming for is to calculate the total capacity of the battery bank that would be required to run all the boat's equipment whilst moored, with no engines running, for the time you want to do so. To achieve this you need to:
Add up the consumption of all the equipment you want to use, and over how long.
Add up all the charging capacity produced by the various means of electricity generation you have available, and then calculate how long those generators have to run to replenish the battery bank once depleted after running the boat's equipment whilst moored! Full circle!

The answer should allow you to buy a boat with an electrical specification which suits you, or will allow you to  arrange to have such a specification built in when you order your new build, but the builder should be able to offer advice on this in any case. If you have previously experienced flat batteries, you can now work out why that may be, if it isn't down to faulty equipment.

As you can see in my boat specification, Kelly Louise is fitted with two 110a/h domestic batteries, charged by a 70amp engine alternator, with access to a C-Tek M100 <Click> marine multi stage charger when external mains supply, either shore power or generator is available. There is a smaller, single battery used only to start the engine, which is supplied by the other 50amp engine alternator, with it's own access to another C-Tek multi stage charger, a multi XS3600 <Click>. Each element of the system is matched to each other, in that the size of the battery bank couldn't be increased without increasing the size of the charger etc. With this set up we can survive overnight on the energy stored in the domestic batteries, but need to travel every day to replenish them properly. If we stay in one place more than one night, then we have to bring in further charging, either by the main engine, or if possible without annoying others, by using our small generator to power the C-Teks. In other words, only suitable for occasional holiday use or on its mooring with shore power. This set up would be unacceptable for extended cruising or live aboard. However, it suits our needs at the moment.

If you get it right, you should now be able to use the boat at rest without the batteries going flat. Good luck!

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