Could this post be anywhere near as popular as...

THE BUDGET BLOG - (here it is!)

I was genuinely surprised at the number of page hits the budget blog got and that readers either found it interesting or useful. This blog delves further into the psyche of a narrowboat cupboard lurker (my electrical information panels are in a cupboard). But now here is...

THE POWER AUDIT!!! DUN..DUN...DUUURRRRNN....

What is it, do you need it, how do you make one and what do you do with the information once you have the result.

Firstly, a power audit is simply everything electrical you use on the boat and how long you use it. The idea is that once you know how much energy you use, you can make two assessments that are of benefit if they are balanced, namely 'what size battery bank do I need and how do I replace the energy that I've consumed'.

Power audits aren't just for the continuous cruiser though. In fact it would probably be more important to me if I was a marina based leisure dweller out for a few days at a time. I wouldn't have invested in solar panels if I was plugged into landline electric forty odd weeks a year and I certainly wouldn't have a petrol generator for charging, I would want enough battery capacity to run everything I needed for at least a couple of days without even thinking about battery SOC or DOD (state of charge and depth of discharge respectively). A charging regime is probably easier for continuous cruisers as we move mooring every day or so (OK maybe every week for us) and charge the domestic battery bank to a certain degree every time.

Secondly, do you need one. Strictly speaking no, as long as you are happy with your power management at the moment. But if you are looking at buying new batteries or anything to charge them, or getting some new electrical equipment that may have an impact (say changing from a 240v fridge to a 12v one, or going from a portable radio to a Goldmine Epilogue signature audio system) then the balance of the battery bank capacity, charging ability/regime and power usage will be awry.

Thirdly, how do I do one. Pen, paper and calculator is fine but as I have an old laptop with Excel I used a spreadsheet. First off for each item you need to know how many amps it draws. Some state this on the info plate but most are in watts that have to be converted to amps - watts divided by volts = amps. Your 240v appliance isn't really running from the mains but from your 12v batteries via an inverter so you don't divide by 240. Unfortunately the inverter uses power itself so to make an allowance for that, any 240v item is divided by 10 rather than 12.

As an example let's take a 20" TV. Most seem to average about 40w (watts) divided by 10 = 4a (amps) x however long you use it (say 3 hours a day) = 12ah that needs to come out of your battery bank (amp hours - possibly amper hours ??). If you have a12v TV then you save a bit from not running the inverter. A new 12v Cello 22" TV/DVD uses 25w so 25w / 12v = 2.1a x 3 hours is 6.3ah or about half as much as a 240v one and the inverter. The downside of 12v appliances is that it probably costs more. The TV is about 1.5 times the price, a 12v fridge is between 4 or 5 times the price!

* technical disclaimer - those who know the author will appreciate his limited technical knowledge, those who know him well will understand his ability to sound convincing as he makes what he thinks, sound rather like knowledge - blog comments with corrections, amendments and further knowledge will be most appreciated.

So here is my power audit. I've split it up into seasons firstly because it is easier to work out what you use and secondly there may (will) become a time that we are marina based part of the year and if we only cruise in the summer I'll know what power is needed. Don't ask me how but I think I have rewritten the blogger code in HTML so you can input your own figures via my blog!!

Or Power Audit - This could quite possibly be a link to download my excel spreadsheet that somehow I have made available in SkyDrive

Lastly, what do you do with the resultant information. You really need to know a little bit about available power in a battery first. If you take the open wet cell leisure battery of 110ah capacity you should not use more than 50% of that capacity (or drop the voltage below 12.25v) as nasty things start to happen and your batteries will have started an early and rapid demise - at 12.15v your batteries cells have started to die. Google sulphation for far more accurate description than I'm going to try here.

These voltages are resting voltages ie no load being applied

The next bit of bad news is that to recharge your battery to 80% doesn't take too much doing but the last stretch to 100% can take many hours. Unless you are regularly hooked up to 240v landline and a multi-stage charger a genuine 100% charge is unlikely. So each 110ah battery gives me a usable 33ah. I use a maximum of 107ah per day so I could possibly scrape through with 3 x 110ah batteries (actually if you look at my spreadsheet you will see that we use more power in summer because the fridge cycles on for longer as the boat gets hotter - quite often 30°c on a sunny summers day - however on said sunny days the 400w of solar panels can put in more juice than we use so you could theoretically knock a good few amps off the summer usage.

If I was a marina based leisure boater I would want a battery bank of no less than 660ah so we could last at least two full days without the worry of charging and more importantly damaging the battery bank. As we cruise, cook and therefore charge the batteries daily, I'll try reducing my bank of batteries from 5 x 110ah to 3 x 110ah (or maybe 135ah ones if I can get a good deal.

In real terms that means a saving of about £140 for the battery purchase, 50kg less weight on the side of the boat that could do with 100kg less and 18 battery cells to check (about every 10 weeks) instead of 30.

What you take out has to go back in, and a bit more besides. The rough rule of thumb in a balanced system is that you have to put back in about 130% of what you take out. The primary reason for this is that your charging system is not made of magic (I look forward to adding the most suitable blog comment here). So if you use 120a then you would have to use a 120a charging system for 1 hour 18 minutes, a 60a system for a little under 3 hours etc.

Well I do hope you found that useful or at least interesting. Next Blog, we're off to a boat launch...