Sunday, 14 October 2012

Power Budget (1)

There is not a great deal of difference between boats, caravans and other recreational vehicles other than one floats along and the other trundles on wheels. Many of the domestic electrical items used in one, can be used in the other. A refrigerator and a television set are typical of what you may find in a boat and a caravan. 

Boats however often have a much larger leisure battery bank fitted  (500 - 1000 Ampere hour are not uncommon) In some instances a mains voltage alternator called a "travel pack" can also be fitted to the boat engine and provide mains power for full sized domestic white goods like a washing machine.

However, I am of the belief that your leisure battery bank should be able to provide the needed electrical power source. That it should also be able to provide power for periods of up to 48 hours without requiring recharging. As far as practicable, you should aim to achieve this power budget, without the need to use the boat engine.

Before you can calculate the size of your leisure battery bank you will need some idea of your typical power consumption over the target period. Start to create a list of essential items, you will need, then workout their typical power consumption.

Using our boat lighting as an example you can also calculate your lighting power budget. First walk through your boat and count the number of lights fitted. Break down the numbers into areas. Break down the areas to the number of lights controlled by each switch. Because in different areas you requirements will change.

When we purchased Rosie the lighting configuration was as highlighted in the list below. The figures in brackets are the power consumption for each bulb in the unit of power consumption which is expressed as watts. 
  • Saloon:      6 (20w) and 4 (20w)
  • Kitchen:      4 (20w)
  • Bathroom:  4 (20w)
  • Bedroom:    6 (20w)
  • Aft cabin:    2 (15w)
  • Navigation: 3 (15w)
  • Tunnel:        1 (40w)
Saloon        10 x 20 = 200 watts.
Kitchen        4 x 20 =   80 watts.
Bathroom    4 x 20 =   80 watts.
Bedroom      6 x 20 =  120 watts.
Aft Cabin      2 x 15 =    30 watts.
Navigation   3  x 15 =    45 watts.
Tunnel          1  x 40 =   40 watts.

Giving a grand total of 595 Watts. 

The conversion of Watts to Amps at fixed voltage is governed by the equation Amps = Watts/Volts

595/12 = 49.58 Amperes.  However, when I measured the actual amps used with all the items turned on. Power consumption measured 56.2 Ampares on Rosies digital ammeter. The actual power used will vary a little for each bulb. The figures quoted on the packaging are nominal. All the little additional bits in our case totalled about 6 amps or 72 watts just over 2 watts per bulb.

If all the lights were to be left switched on, 0ur 500ah leisure battery bank would be depleted very quickly. In 5 hours our battery bank would have given up 281 amperes which is more than half of its theoretical capacity. (500ah) I use the word theoretically as the quicker you discharge any battery the less total capacity the battery has to deliver. In 5 hours at this rate our leisure batteries would have been well on their way to being discharged.

Geek Mode On: There is a formula (Peukert's exponent) for calculating the above effect. It is known as Peukert's equation. The formula changes depending upon battery type and age. 

However, it is highly unlikely that we would ever have the situation where all the lights would be on simultaneously. There will be times when one or more sets of lights will be in use and the amount of time that the lights are on will vary. When you realise how much is being used you become quite frugal and disciplined. You soon learn to turn off lights that are not needed. We soon realised that a single lighting switch would often control up 4 to 6 lights. So as a quick fix I removed every other halogen bulb. It was surprising that the reduced amount of light was hardly noticeable.

We decided that we would change where possible the type of lighting used on board Rosie to reduce power consumption. However, we decided not to change any of the wiring or fittings. The internal Halogen light bulbs were to be replaced by LED equivalent fitting types.

LED lights are the ideal solution. Light Emitting Diode can be used in a variety of places and for a range of activities. Not only do they look great, they also consume farless energy than regular incandescent and halogen bulbs.

LED lights are very efficient and also improving all the time. We now needed to select the correct type of lights. The lights would be where possible in the appropriate part of the colour spectrum for best effect.

Also the brightness of the LED lights would be tailored to our needs.  When we watch TV, we like to have a warm subdued colour from the wall lights. However, where we sit and read we wanted an overhead light that would approximate to a more natural white. In the bathroom and kitchen we wanted a more white light. In the bedroom a more warm subdued colour  However, the reading spot lamps over the bed to be a bit whiter.  LED's are available in Very Warm White, Warm White, White and Blue-White.  The colour of light is measured on the Kelvin Scale.

So far we have not felt the need to change the the Aft Cabin, Navigation  and Tunnel lights in any way as we rarely use them. The only time we would use them would be when we were on the move. The 480 watts (40 ampere) of halogen internal lighting has been changed to 44 watts (3.7 ampere) of LED. Typically we use about 1 ampere when the lights are in use. That's a total of about 12 watts.

So our lighting budget we estimated at 8 hours at 2 ampere hours for a 24 hour period.  Giving a 50% more overhead than we estimate we would use.  (16ah)

When it came to the television, we chose a flat screen as it is space saving. It has a digital receiver and DVD/CD player built in. So it has a reduced number of external boxes and cables for a typical home entertainment centre. It also runs off 12 volt and consumes 50 watts. (4.5 amperes) We tend to watch TV in an evening, 6 o'clock news through to 11pm at the latest. we gave an extra 20% overhead at 6 hours in a 24 hour period. (27ah)

We have a shoreline fridge. It draws 6.5 amps when running under normal conditions and seems to run for about 20 minutes  of each hour.  (52ah)

Most other electrical items on Rosie are very intermittent. Pumps, macerator and other sundry items I put in the in the 1 ah range (24ah)

So Rosies power budget is around 120ah per day.

To offset this deficit we have solar panels. The amount of power generated varies but we have been averaging 24.5ah each day for the last 140 days. So that brings us into the 100ah per day range. That equates to an average 4.1 ah discharge rate.

I will be re-installing the solar panels ready for next cruising season. I want to install them in such a way that they can be pointed into the sun to collect more of the sunlight.


1 comment:

  1. I like this power budget calculation for home. But by using of LEDs in our home, we can save more electricity and there is not necessary to calculate the power budget


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