The Admin's Blog

By Pianotuna

I’ve been harping on about doing an energy audit. Here are some suggestions as to how to do so.

1. by direct measurement

2. by doing calculations.

For direct measurement, find a watt meter such as the inexpensive Kill-a-Watt meter, plug in the rv shore power cord. Total load will need to be kept under 1800 watts or the meter may become a rather poor door stop.

My suggestion is to measure for between three and six days.

Once you know the wattage used per day, it is an easy jump to figuring out battery capacity. If you wish to be totally “off the grid” then use the six day number to size a lead acid battery bank.

If you are using a more exotic battery chemistry (LI or Carbon Foam) then just use the three day number plus 20%.

This will allow three days of zero charge from the solar panels without going into energy conservation mode.

To size the panel wattage for a large system use 60 watts of panels per 100 amp-hours of storage. That will be the minimum number of watts to allow for equalization.

If there are weight or space limitations, as battery bank size gets smaller move upwards on the wattage. However, there is not much point in going above 150 watts per 100 amp-hours of storage, because the acceptance rate of a lead acid jar at 85% state of charge is 12.5 amps (or 150 watts) . Extra wattage would only get you 85% faster, but may not get you to 100%. As we talked about earlier, lead acid jars need to get to 100%, if they are to have a long life span.

Another possible approach is to have a truly massive solar array with a fairly small battery bank. The idea is to essentially run the RV from the panels while only having enough bank to get through a night. I do not favor this method as it would require a generator back up to the solar.

For those of you who wish to do a calculation instead here is a link to the rather special spreadsheet which includes an energy audit, that N8GS (Gale Scholten) has created to help size solar battery charging systems!

This sheet is useful to those who do a direct measurement, too.


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