Sailing and solar panels – how much energy do you need?

Solar Panels and how to work out how many you need is a crucial part of your planning when cruising.

We are getting SV Marty Alessa ready to cross oceans (still not sure which way, Indian or Atlantic) and of course electricity onboard is at the top of the list to sort out. Need to run our fridge, washing machine (yes, we have a fully automatic one on board), lights, pumps, navigational equipment, appliances, charge our computers and whatever other gadgets we can plugin. Loads of stuff. Yes, yes, we know we could probably do without half of it but if I am travelling the world and the boat is my home then I want as many of the comforts of a land-based home as I can have dammit!!

Where to start?

Start with some basic arithmetic. Of course, you can download some fancy app that will do all the work for you but I wanted to really understand it myself. Luckily Captain Mike is a veeeeery patient electronics engineer so he could explain in a way I understood.

Work out the wattage of all these gadgets and how many hours you run each one every day, what does my cell phone use to charge, my computer, whatever. Add it up. How do you work out the wattage? Read on the back of appliances! An easy way is to buy a DC current clamp meter, a great way to measure in practice how much power different things use. Switch everything off. Clamp your clamp meter on to the input of the 12-volt side of your inverter while switching on various 220-volt devices, one at a time. In this way you will see how much current drain each device causes on your battery. Make a list of all your 220-volt appliances and do the above measurement with all of them. This makes it easy to work out your daily load on your 12-volt power caused by 220-volt appliances. If you already have 12-volt appliances all over your boat, then instead of clamping on your inverter, you will clamp to the main supply of your domestic switchboard. You can either switch on everything you use in the day and measure as a whole “day mode” or do one by one. Similarly, you can do “night mode” measuring, switch on anchor lights, radar etc and measure the total current drain.

To get from the amps you have just measured to watts, multiply by 12. Or 24. If you have a 24-watt system. To get from power consumption in watts to actual energy you need to multiply by the number of hours the appliance was used.  The energy used is measured in watt-hours or (Wh) 

On a 12V system, 1 amp hour is therefore 12 watt-hours. For example, if you run the fridge (typically 4 amps) for one hour, then this would equate to 4 amp-hours or 48 watt-hours. Add up your watt-hours over a 24 hour period for everything that you have calculated and you will arrive at a total watt-hour figure for daily use.

Let’s say you arrive at 1600 watt-hours to run all your goodies over a 24 hour period. This means your solar panels will need to produce that.

In order not to have to run your engine to charge the batteries, your solar panels will need to replenish this amount of energy every day.

How to work out how many solar panels you need? We have found, in the winter sun here at the moment, our 150 watt rated panels are only producing around 80 watts for about 6 hours per day. Don’t be fooled by solar panels showing 150 watts of energy, you will rarely get that out of your solar panel.

So, we need 1600 watt-hours at the moment, we have 8 hours of sunlight so we divide 1600 by 8 hours and that gives you 200 watts. So all the solar panels have to produce at least 200 watts together. Now comes the final part of this sum. When we buy them we should double that figure because the rated power is just a theoretical power. We ended up buying 3 x 150watt solar panels to cater for our daily needs.

We had to build an aluminium structure to hold the 3 new panels; originally we only had one solar panel.

We decided to wire our panels in parallel because they then work independently of each other if you put them in series and there is shade covering just one little cell then the whole system produces nothing during that shaded period. However, you do apparently get fancy panels that have bypass diodes that prevent this problem. Not in Africa though! We had to beef up our wiring between the panels and the solar regulator that we had to buy, to cater for the increased electrical current.

Of course, the sun isn’t going to shine in the same way every day so you need to be able to store your excess energy produced on good days in your batteries.

That is a whole separate issue……deciding how many and which batteries are best for your needs.

Read about the battery decisions here.

Captain Mike and Nikki.


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