3 March 2011

Electrical consumption

I have bought two LED lighting units from Sea Rolf LED (http://www.searolf.com/) at a total of £10.60 incl p&p The claimed current draw is only 40mA per unit. I will try to wire them in over the next coupe of days. I wired in the connection for them last night, to my third switch and it eventually worked (I pulled out the GPS connection at first, which has very fine wires.)
I have done some rough calculations of power consumption to see how long my battery (7Ahr capacity) might last with the solar panel recharging it. It looks very positive, but I have had to guess at the GPS power consumption. I can't find figures anywhere. Does anyone know the power consumption of a Garmin 72 GPS?

Power consumption calculations

Per day power usage on weekend sail (nominally sailing for a maximum of 5 hrs per day)

Lighting: 2hrs/night at   80mA = 0.080A  so 0.16Ahr per night
Depth:    5hrs/day   at  22mA = 0.022A  so 0.11Ahr per day
GPS:      5hrs/day   at 100mA = 0.100A so 0.50Ahr per day  (this is a guesstimate)
Totals         for a weekend cruise       0.77Ahr per day i.e. approx 0.75Ahr/day

Battery capacity (nominal)                         7.0Ahr
Usable capacity say 50%                          3.5Ahr

Therefor fully charged battery should run electrics for 4+ days with no further input.


Solar Panel output                              1.0W at 12V gives 0.08A (nominal)
Say 50% efficiency                            0.5W at 12V gives 0.04A

Power generated for say 8 hours per day                         0.32Ahr
Power per week (7 days)                                                2.20Ahr


Therefor one week's solar charging gives power needs for approx 3 days sailing.
Solar panel halves discharge rate i.e. battery power available for about 9 days.
after two days sailing, only 0.75Ahr discharged and recharge time approx 2 days.


If I sail at most three days per week (if only, I have a day job), the solar panel and battery should cover all needs for the season.
If I go for a week's cruise to the English RAID, I should have plenty of capacity, but it will take over a week to recharge the system.  That shouldn't be a problem, I think I will want at least a week off after that.

Interesting to see how it will work out in practice.


  1. I have an old Garmin GPS45 and the spec states
    Input 4AAbatteries or 5-40vDC
    Usage .75 watts
    Battery life 10 hrs normal mode and up to 20hrs in battery saver mode
    The old models were power hungry and I think they have improved but its the display that takes the power. I dont think you are far out with your estimate of .100A

  2. Thanks Roger. If that is right I think I might have more reserve than I calculated. If it is 0.75W at 12volts, I make that about 0.06A.
    I fitted one of the LED units this evening and I am astounded by how well it works. I will fit the other tomorrow evening. The colour of the light is meant to be "warm white" which means it is a bit yellowish. But nicer than the fierce blue-white you get from some LEDs.

  3. Julian I assume that the .75 watts would be the draw on the batteries which would be about 5 volts so your original calculation would be about right. However I have given the data straight from the manual so you can place your own interpretation on it.

  4. I run my GPS72 on batteries rather than "mains" - so why not do the same and just take it out of the power equation?? They only use a couple of AA's and I easily get 8-10 hours on that.. batteries cost peanuts, I get them from B&Q in big multipacks and keep a half dozen on the boat for spares/emergencies... do like the idea of your LED's though....

  5. I find the AA batteries always run flat just when I need them. And peanuts add up. But mainly I needed to wire in the depth sounder and I had a battery tank so it seemed to make sense to wire in everything. I can still use AAs if the main battery conks out.