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How to do a Power Audit

How to do a Power Audit

Location: your boat (or other dwelling place), doing a power audit

The big question for everyone who wants to have a boat is, “how much power do I need?”. This was not a problem back when canal boats were first used, you had a horse to pull the boat, oil to light the lamps and no fridge and no electronic gadgets. Now it is all different.

Now we have an engine powering the boat, a battery bank, electric lights, fridges, and all sorts of electronic gadgets from phones to GPS systems to TVs. All this stuff requires power, and the source of power on a boat is your battery bank.

Batteries are expensive. They also have varying different capacities and discharge rates, depth of discharges etc. You don’t want to have too many as that will be just wasted money, and you don’t want too few as you will constantly be having to run the generator or engine to charge up the batteries. To work out the right amount for you you will have to do a power audit.

To work out the right amount of batteries you will have to do a power audit.Click To Tweet

In explaining how to do a power audit I will assume that you have the most common depth of discharge on your batteries, which is 50% (almost all of them except lithium ion have this depth of discharge and lithium ion has 80%).

Also it is best to work out the power needs for the worst case scenario so I will do a worked example showing winter use.

The first thing you need to do when doing a power audit is to list everything that will be on the boat that requires electric power. The most common items are; fridge, lights, mobile phone, TV, computer. On my boat the items are:

12V fridge

lights

water pump

mobile phone

mobile internet device

tablet computer

computer

microwave

Next you need to work out how many hours, and / or how much power each item will use.

12V fridge – 12 hours, 72 W per hour (although the fridge is running all the time it only uses power when the temperature rises too much)

lights – 8 hours, 10 W per hour (two lights with 5 W bulbs)

water pump – 0.25 hours, 4.4 Amps per hour

mobile phone – one charge per day, 1.2 Ah

mobile internet device – one charge per day, 1.5 Ah

tablet computer – one charge every 3 days, 7.3 Ah

computer – 2 hours, 45 W per hour

microwave – 6 mins 0.8 kW per hour + 0.2 kW inverter

The next step it to covert all the kW per hour in to Ah. To covert kW per hour in to amps per hour you need to divide by the voltage. It is 12V if you can just plug them in to the batteries or 240 V if they run on mains power (if you do have them then remember to add in extra power for the inverter).

12V fridge – 12 hours, 6 Amps per hour

lights – 8 hours, 0.85 Amps per hour

water pump – 0.25 hours, 4.4 Amps per hour

mobile phone, one charge per day (from 50% discharge), 1.2 Ah

mobile internet device – one charge per day, 1.5 Ah

tablet computer – one charge every 3 days, 7.3 Ah

computer – 2 hours, 3.75 Amps per hour

microwave – 6 minutes, 83.5 Amps per hour

Next you need to multiply the Amps per hour by the number of hours you have the item on.

12V fridge – 12 hours, 6 Amps per hour, 72 Ah per day

lights – 8 hours, 0.85 Amps per hour, 6.8 Ah per day

water pump – 0.25 hours, 4.4 Amps per hour, 1.1 Ah per day

mobile phone, one charge per day, 1.2 Ah per day

mobile internet device – one charge per day, 1.5 Ah per day

tablet computer – one charge every 3 days, 7.3 Ah, 2.5 Ah per day

computer – 2 hours, 3.75 Amps per hour, 7.5 Ah per day

microwave – 6 minutes 83.5 Amps per hour, 8.4 Ah per day

Now you need to add up all the Ah for all the devices. Make sure that you use the amount of Ah per day.

12V Fridge – 72 Ah

lights – 6.8 Ah

water pump – 1.1 Ah

mobile phone – 1.2 Ah

mobile internet device – 1.5 Ah

tablet computer – 2.5 Ah

computer – 7.5 Ah

microwave – 8.35 Ah

total – 100.95 Ah per day, which I will round to 101 Ah per day

The total is the amount of Ah that you will use everyday, personally I will add 25% to be on the safe side as I don’t want to be under powered. This will also leave power for things which happen intermittently like bilge pumps.

Total – 101 Ah per day

25% of total – 25.25 Ah

New total – 126.25 Ah per day

As I said near the beginning of this article, batteries commonly have a 50% depth of discharge. This means that you can only drain them halfway before they start to get damaged. So take your new total of Ah and then double it.

Total usage with safety margin – 126.25 Ah per day

Total battery capacity needed to provide this – 252.5 Ah

If you want to remain off grid for more than one day without running the engine or a generator then you will have to multiply the total battery capacity needed by the number of days you need it for.

For example if I wanted to be able to go off grid for three days before running my engine or a generator then I would have to multiply the total battery capacity needed by three.

Total battery capacity needed to provide one day of electricity – 252.5 Ah

Total battery capacity needed to provide three days of electricity – 757.5 Ah

There are ways of reducing the amount of battery capacity needed, one is to get better batteries that can be discharged more, if you have 80% battery capacity you can have more power per Ah in the battery,another way is to have another power source. I have 600 W of solar panels. In the summer they provide a lot of power, but in the winter they only produce about 57 Ah per day (on average). If I was to incorporate this in to my calculations they would look like this.

Total battery capacity needed to provide one day of electricity – 252.5 Ah

Total electricity generated by solar panels – 57 Ah per day

New total battery capacity needed (taking solar panels in to consideration) – 195.5 Ah per day

Total battery capacity needed to provide three days of electricity – 586.5 Ah, which you can see is a lot less than without the solar panels.

If you want a spreadsheet that will help you to do a power audit that you can use then you can find a link here to the waterways world website which has a spreadsheet and an interactive calculator.

If you want to find out more about solar panels then read my post here.

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