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Old 06-29-1999
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Creating a 12-Volt Spreadsheet

Generating, storing, and using 12-volt energy on board must be viewed as a total system. Attempts to set up the best complement of batteries, inverters, alternators, and chargers without a comprehensive 12-volt spreadsheet are usually disappointing. And since every sailor uses electricity differently, a 12-volt spreadsheet needs to be developed.

Begin your spreadsheet with the manufacturer's spec sheet for each of the electrical items on the boat. A good retailer's catalog may sometimes be substituted. The spreadsheet can be developed on a computer or done the old-fashioned way with paper, pencil, and calculator.

We've found that two spreadsheets are necessary, and we do them side-by-side. One is for underway when the running lights, nav instruments, and radios get a lot of use. The other is headed "At Anchor" when the entertainment appliances, anchor, and reading lights demand more power.

When setting up your spreadsheet, don't forget to incorporate items that you plan to add in the future. Heavy electrical users like12-volt refrigeration, SSB radio, or radar will upset the whole plan if not included.

If you have little experience living with your systems, err toward the generous side when estimating the time each electrical appliance will be used. It's better to plan using the water maker three hours per day and then later discovering it's actually used only two hours.

Set up the column with these headings:

Appliance, Watts, Amps, Hours at Anchor, Hours Underway, Total Hours at Anchor, and Total Hours Underway

Sit at the boat's 12-volt panel and fill out the Appliance column. Don't forget the hidden items such as pumps, propane solenoid, bilge blower, or loads not directly labeled. Items such as interior lights can be grouped together under one heading.

Have no fear of the Watts and Amps columns. Electrical parts manufacturers quote the usage of appliances either way and they are both measures of power consumption. If the electrical draw is quoted in amps, you can leave the watts column blank.

Specification sheets showing 12-volt draw in watts will require a simple calculation to convert it to amps. For your spreadsheet, the formula is: watts divided by volts = amps. In round numbers, a 12-volt battery is fully charged at 13 volts and completely discharged at 12 volts. A battery being charged by a generating source will usually read more than 14 volts. To convert watts to amps, we normally use12.75. Thus, our formula becomes watts /12.75 = amps.

Example: a halogen reading light is quoted by the manufacturer as 20 watts. That's: 20 / 12.75 = 1.57 amps.

If an inverter is running 120-volt loads, use the same formula. Thus using the microwave for popcorn would look like this: 1,200 watts / 12.75 volts = 94.12 amps of 12-volt power.

Next, estimate the number of hours each appliance is used during a 24-hour day. Do this twice; once in the At Anchor column, and again in the Underway column. Use tenths of hours (six minutes = one-tenth hour) to keep the math simple.

Finally, multiply the amps by the number of hours used, round to two decimal places and fill in the results in the Totals columns. Again, you must do this twice, once for Total At Anchor and once for the Total Underway columns. These results are the amount of 12-volt power used by each appliance in 24 hours.

Now your spreadsheet should look like this:

12-Volt Spreadsheet
Hours UsedTotalHours UsedTotal
Appliance/ElectronicsWattsAmpsat Anchorat AnchorUnderwayUnderway
Running Lights 3.000.000.0010.0030.00
Reading Lights20.001.573.455.410.000.00
Anchor Light10.000.789.507.450.000.00
Navigation Instruments 0.500.000.0024.0012.00
Refrigerator 6.008.5051.008.0048.00
LPG Solenoid16.001.251.401.760.500.63
Microwave Popcorn1200.0094.120.109.410.000.00
Totals75.03 90.63
Denominator for Watts Conversion = 12.75

At the bottom of the sheet, add the two Totals columns. Most sailors find they use much more juice in the Underway column than in the At Anchor. Boats with power-hungry electronics, such as radar, SSB radio and electronic charting often use two or three times as much 12-volt power when sailing.

Keep your spreadsheet in your maintenance log. Add new appliances to the spreadsheet when you install them and review it occasionally to see if your estimates on usage hours change.

Your spreadsheet is the basis to properly sizing all other components of your 12-volt system. Many experts advise having two to four times the maximum amount of amperage used daily in each battery bank. For the sample spreadsheet, where 90 amps were used in 24 hours of sailing, this would result in banks of 180-to-360-amp hours. Perhaps 270-amp-hour banks would be a good compromise. These batteries would be one-third drawn down underway, but only one-fourth depleted in 24 hours at anchor.

Our next two articles will deal with proper sizing of charging systems based on your spreadsheet.

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