Going Big

(Warning: Math!!)

Most cruising sailboats run on the same type of electrical circuits as cars – 12 volts. They have batteries to store the electricity (yes, they look just like car batteries) and they generate electricity using the engine, solar cells and wind turbines.

Sailors can guesstimate how much power they need if they know how many amps each electrical device uses. Sometimes the number of amps is stamped on the device and other times it is given as “watts”. Since watts = amps x volts, you can get the amperage draw on a 12V circuit by dividing watts by 12. The sticker on our Group 27 deep cycle battery says it can output 23 Amps for 200 Minutes. So it can store:
23 Amps x 200 minutes / 60 minutes/hour = 76 Amp hours.

Our daily power usage looks like this:
LED anchor light 2W x 12 hours = 2W/12V x 12 hours = 2 Amp hours
Engel Fridge (on low) = 1 Amp x 24 hours = 24 Amp hours
Shane’s Laptop = 90W/12V x 4 hours = 30 Amp hours
LED lights = 4 x 0.5W/12V x 5 hours = 0.8 Amp hours
Fans = 3 x 0.5A x 6 hours = 9 Amp hours
Android and iPad = 3 x 0.5A x 4 hours – 6 Amp hours
LPG Gas controller = 1 x 1A x 3 hours = 3 Amp hours
WiFi antenna and router = 0.5A x 12 hours = 6 Amp hours
Total = 81 Amp hours

We run the Garmin GPSMAP 740S GPS Chartplotter (about 0.75A) while under way but this is compensated by other systems not used much underway – WiFi, fans, laptop.

Assuming our 190W solar array averages half it’s capacity for 12 hours on a sunny day we produce:
85W x 12 hours/12 volts = 85 Amp hours


So in short, our power usage drains one battery per day and on a sunny day our solar array refills one battery.

Up until today, we only had two deep cycle batteries. Our SunSaver Duo solar controller (the device with regulates the power coming out of the solar array) was set to distribute the power equally between the two batteries. Every morning I would switch the batteries. The idea was to hope the solar would cope with the daytime usage and the battery would still be full charged in the evening.

With this setup I was finding we would end up with the resting battery at full charge by mid afternoon and the battery in use only at about 75% charge. From mid-afternoon until night time the 50% of the solar generation was being wasted because the battery at rest could not take more charge. By morning, the battery in use would be down to 25-50%. Since deep cycle batteries prefer to only be discharged to 40-50% we were killing our batteries slowly. This also meant if we woke up to a rainy day we would need to run the noisy engine to generate power.

My conclusion – we have enough power production but our storage capacity was inadequate efficiently store it. Similar problem with battery charging. Each battery cannot absorb more than 120W (10A at 12V). Our alternator can generate up to 30 Amps, so 20 Amps is wasted if one battery is already full.

The solution: go big.


Today I replaced one of the Group 27 marine batteries with three Group 24 batteries in parallel. This will now be our “house” battery bank. The Group 24 batteries can output 23 Amps for 150 Minutes. So the battery bank can generate:
3 x 23 Amps x 150 minutes / 60 minutes/hour = 172 Amp hours.
Just as importantly, there will be enough capacity in the batter bank to consume all our our energy production. Our maximum energy production is 30 Amp hours/hour (Engine) + 10 Amp hours/hour (Solar) = 40 Amp hours/hour. With four batteries (three new plus one old) we can now absorb 4 x 10 Amp hours/hour = 40 Amp hours/hour.

I changed the solar controller to 90/10, which means 90% of solar power generated will be directed to “house” battery bank. We will use primarily, resting it about one day every two weeks.

Next… more power generation. Just kidding. The trick is to keep our power consumption at a minimum. It is easy to see how sailors get stuck in a vicious cycle – more power usage -> more power generation -> more power storage -> more power usage -> more power generation -> more power storage… and soon their boats start looking like space stations!