Michele and I are very fortunate to have a plethora of electronic gadgets on our boat. We have a hydraulic autopilot, 4KW HD radar, two vhf radios, 7 inch touchscreen multifunction device (chartplotter), microwave, TV, water pumps, bilge pumps, etc… you get the point. All of these gadgets, both essential and superfluous, take up a surprising amount of electricity. We estimate that our electrical usage while at anchor will be approximately 110-125 amp hours a day. While at sea it will be higher due to the autopilot and radar being active for the majority of the time while sailing. While this is a lot of electrical demand, it is by no means insurmountable with today’s technology.

In this part of the series I will focus on the sizing and selection of our system. The first step in sizing the system was to ascertain what our actual daily usage will be both at anchor and at sea. There are several useful calculators online that give reasonably accurate estimates, but we wanted to go a bit further. We decided to install a digital battery monitor that tells us the percentage charged, amp hours used, amp hours available, current amps, voltage, and historical data so we can know precisely how much electricity remains in our batteries at any given time. The Victron Energy BMV-702 meets all of these needs in a very easy to install and professional looking package.

Most battery monitors utilize a high amperage shunt that is installed between the batteries and load/generation in an electrical system. The shunt is then wired to the battery monitor. With the BMV-702 it is as simple as running a computer network cable (included!) from the shunt to the battery monitor.

Battery Monitor Shunt

The 50mV shunt I installed for the battery monitor

The next step in the installation of the monitor is to decide where to mount the unit itself. We had a 1970’s era voltage meter that no longer worked taking up a large space in our electrical panel area. After removing the old Danforth meter we had a serious hole left in the bulkhead, however…


Modern battery monitor size vs 70’s era voltage meter

Thankfully my dad was able to fabricate a black lexan panel that matched our existing panels quite nicely.

BMV-702 installed with new panel

BMV-702 installed with new panel

With the new monitor installed and running, we were able to accurately determine our actual electrical usage averages 90 Ah a day. Granted we are at dock right now so our usage is different than it will be while at anchor, but we now have a very good estimate to size our system with. We decided to add a buffer of 33% to account for increased inverter usage and other possibilities.

To meet all of this electrical demand, many people choose to install and utilize a diesel generator to meet their electrical demands while cruising. This is an acceptable choice for day sailing, hopping between marinas, etc that can quickly become expensive if used as the main source of power for a vessel. The maintenance and fuel for a generator can quickly meet or exceed the maintenance costs of the main engine on a cruising sailboat.

We decided to go with a hybrid setup that utilizes both solar and wind power sources simultaneously. This has the benefit of high power generation while also diversifying the source of power… on rainy days we will generate less solar power, but most likely more wind power. We also will have as a last resort backup our engine alternator that can charge the batteries to nearly full in a few hours. The next part of this series will showcase our solar panel, charge controller, and custom solar panel mount installation.