Continuing preparation of "Every Day" Part 13

We spent a few of weeks in St. Croix over New Years. It gave us some time to think about power usage for the boat. Ultimately, we are trying to guess consumption and power production, so we can choose the correct size generator.

We are installing 5- 75 watt solar panels above the dodger and a 400 watt wind turbine on top of the mizzen mast. This should generate almost 200 amp-hours per day. That is 5 times our power consumption when we are not on the boat (bilge pumps, refrigeration, anchor light, etc) and about the amount we will use when we are on the boat (including making water but not including air conditioning).

What we're told is that when at anchor, you rarely use the air conditioning. When your at a dock, however, your rarely pointing into the wind and therefore don't get sufficient air flow through the boat and may need A/C. Unlike Chicago, many docks around the world don't have power and a generator may be needed.

Here is the debate. A diesel engine should be run at a minimum of 75% of capacity. This is especially true of a diesel generator. If we assume that we will be running all the A/C and making water and charging the batteries, we need a 7.5 or 8 KW generator. If we assume that solar and wind generation will be sufficient for day to day usage, and only need 2 of the three A/C, then we only need 4.5 kW generator.

A friend of ours who has taken his Alden 52 down to the Caribbean 4 separate trips over the past 12 years. He has a 6 KW generator and no alternative power and has never been able to load his generator even close to capacity. He also confirmed that if you expect to be at anchor most of the time you never use the A/C.

It is obvious since we have added a lot of alternative energy that the 12.5 kW generator we had was way too big for the boat. In fact, we are probably going to sell it to a friend who has a house down in St. Croix and it will provide back up generation for them and will run their entire house.

One thing interesting is how much power costs on a boat. A generator, for purchase, maintenance, and fuel over the life of the generator is about $2.00 per kW-hr. Solar, which for land based systems is very expensive, is very economical for a boat at $0.50 per kW-hr. Wind is even more economical, it even approaches land based power generation, at about $0.25 per kW-hr. Just for reference electricity for our house here in Chicago with taxes and transmission fees is about $0.18 per kW-hr.

One last factor is that we are told you don't want to make water in a port because of the potential of oil contamination that destroys the reverse osmosis filters. Also the boat does have a limited head tank capacity therefore we expect to leave the hook a minimum of once a week to empty tanks, make water and charge the batteries. Since our alternator can generate about 125 amps. it is like having a 3.0 kW generator and is enough to run one A/C unit but most likely will be used to make water and top off the batteries.

So we are still trying to figure our what size generator and are working out how to hook the power into the existing power on the boat and how to protect the boat from stray electrical problems when connected to shore power.