We probably spent more time puzzling over the question of electric power than any other. For civilised living and safe sailing we didn’t want to run out of 12V power, or damage our service (domestic) batteries by draining them too far.

Should we buy more batteries? And assuming we didn’t want a petrol generator on board, which form of additional charging should we use: Solar, wind turbine, water turbine or hydrogen fuel cell?

In the end for various reasons we had more nights hooked up to shore power than we originally expected, so we rarely ran low. And all along we had a back-up option of running the engine (under load) in order to charge the batteries using the alternator.

The question has three parts:

  • Demand: How much power do we need?
  • Storage: What battery capacity do we need?
  • Supply: How best can we re-charge the batteries?

If you’re interested in the detail (including links to longer articles I found useful), read on. Otherwise, feel free to jump straight to the conclusion.

This large yacht (anchored next to us, off Skye) has committed to wind power.

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Demand: There are many articles and calculators on the web, aimed at helping you estimate how much power you may consume. There’s no real substitute for real-world testing, but in practice there are only a few pieces of equipment with heavy consumption: The fridge (e.g. 30Ah for 24hrs) and the instruments plus autohelm (later we measured 40Ah for a 10h passage).

Yachting World (from Atlantic crossings): https://www.yachtingworld.com/yachts-and-gear/generating-power-on-board-64034

Yachting World (2015 article for cruisers): https://www.yachtingworld.com/gear-reviews/wind-water-and-solar-power-65797

Storage: We have two service batteries, each 120Ah. They’re AGM (sealed gel). That’s a total capacity of 240Ah. My understanding is that you can only use half this (batteries must not be allowed to go below 50%) so that gives us 120Ah to use before we must recharge.

Incidentally I also worried (from experience) about the possibility of our engine start battery failing, and I considered fitting a switch that would quickly allow us to use the domestic battery for this purpose. However there’s a much simpler (and less potentially damaging) solution available in the form of a dedicated emergency battery. I bought the Noco Genius Boost GB40 lithium jump-start battery.

Here’s Nigel Calder on battery management (detailed)

Supply: Aside from a petrol generator, which seemed like a bulky way to duplicate what the engine could already do, there are four main options we considered.

  1. Water: It is possible to generate power while sailing along, using a water-generator such as the Watt & Sea hydrogenerator. Brilliant but expensive. And of course useless if you need to generate power when moored, which is what we needed. 
  • Wind: This has obvious appeal in the UK where there’s plentiful wind, and even better in combination with solar for windless days. We quickly ruled it out because we didn’t want a huge gantry installed on Nova, with its associated windage when we are trying to sail efficiently. We also didn’t like the idea of constant noise, although recent models are much quieter than in the past.
  • Hydrogen Fuel Cell: This was hugely attractive because they are silent, instant and as by-products they produce only water and a tiny amount of CO2. The most popular maker is Efoy (e.g. Comfort 140) and a newer manufacturer is Hydromax (e.g. Hydromax 150). They’re expensive (over £3,000), but in the end it was the fuel that was the show-stopper – it is costly and proprietary, needing to be ordered from the manufacturer rather than obtained locally.
  • Solar: In the end it was solar power we chose. I went into great detail calculating insolation at particular latitudes (see this detailed PBO article), but my conclusions were a blend of science and guesswork. Offset by a little motoring in and out of harbour, I estimated we’d need at least 100Ah (= 1,200Wh) to recharge each day. On an average day, we might get 33Ah from each 100W of solar panel.

I concluded we’d need 30W panel for trickle charging and 300W to supply real daily needs when at anchor/mooring.

I also looked at many different suppliers, with three in the final short-list to make custom panels for us (a) Sorbian Energie (in Italy). Mattio Rizzo (technical) and Karin Mussino (sales) were very helpful and I’d certainly consider them again. (b) Link Solar, an alternative, cheaper, Chinese manufacturer. Shoven Dean (manager) was responsive, but I had intrinsic caution about buying from China. (c) Solarcloth System (CEO: Alain Janet), who make thin-film solar ‘cloth’ – these novel panels are so thin they have even been fitted to sails. They are no more expensive than other panels per unit of power, and although only half as efficient (i.e. larger for the same power) they are claimed to perform better in indirect sunlight.

So what did we do? 

We bought a Victron 100/20 MPPT regulator (to control the charge to the batteries) and two thin-film solar panels from Solarcloth: A 60W panel for the sprayhood (permanently mounted) and a 224W custom-made ‘bimini’ panel to put out (facing the sun) when moored. This was half the wattage I thought we’d need, but we decided to try it and if needed buy another panel later.

The successful 60W panel mounted with velcro on Nova’s sprayhood

Result: The smaller panel has performed well. It is often in shade, yet seems to generate in proportion to the sunlight it receives (which is not true of all panels). It serves very well to trickle charge the batteries when we are absent. The larger panel was less successful. When it eventually arrived (Solar Cloth later told us their 10-week delivery time is ‘approximate’, so it reached us in Edinburgh) we found it takes up much more space than we anticipated to store it when not in use, and developed a fault with the terminals. That’s a problem we understand Solar Cloth have now rectified. In the end, we have returned the larger panel.

The large solarcloth ‘bimini’ in action on the island of Rona

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Conclusion

With hindsight, for a journey like ours, I’d simply fit a larger battery bank (e.g. 360Ah) and a small solar panel, mainly for top-up purposes when leaving the boat on a mooring. Given how much we were able to plug in and recharge, we could have managed with that. The number of occasions when it would be inadequate would have been few, and could be compensated for by running the engine (under load). That would have saved a lot of time, expense and experimentation. Our consumption turned out to be just 40-60Ah for 24hrs, which is on a new boat with all-LED lights and an efficient fridge, which we sometimes turned off at night. To be generous in future I will plan for 100Ah per 24h period, allowing for a long day sailing passage with autohelm.

What has been your experience with power? We’d welcome your comments.