My house has a 1,000-gallon underground tank to store fuel oil for the heating season. Instead of filling the tank on a scheduled basis, which means the amount you pay varies as the price of fuel oil changes, it's better to purchase oil during the summer when the demand is lowest and fuel oil is less expensive. During the more expensive winter months, it's best to purchase only the amount of addition fuel you need, if any. This system has saved me about $700 a year in fuel oil costs, which makes it well worth the effort.

To buy fuel just in time, I need to know how much fuel I have left and the rate at which I consume it. Unfortunately, due to the way the filler lines for my tank are installed, measuring the fuel level is unreliable. Instead, to figure out how much is remaining in the tank, I must keep track of how much fuel I've used since it was last filled.

The furnace's oil gun draws fuel oil from the tank and burns it to produce heat. The oil gun has a 1.25 tip on it, which means that it burns 1.25 gallons of fuel oil per hour. Said another way, it takes 22.5 seconds to burn one ounce of oil. To track the consumption of fuel oil, I need to track how long the gun is burning fuel on a daily basis. I hooked up a relay switch in parallel with the oil gun motor. The relay is connected to a Powerflash module . When the oil gun turns on or off, the relay causes the module to send X10 On and Off commands to the power line. I use XTension to measure the time between each on and off signal, calculate the accumulated time, and calculate the fuel consumed each day, week, month, and season.

lists the units that are necessary for this hack.

Table 1. Required units

To ensure I don't miss any signals from the universal module, as can happen occasionally with X10, I also use a hacked motion detector to send a wireless signal directly to my computer when the fuel gun starts and stops. The motion detector's dusk sensor is wired to the same relay that's connected to the universal module. When the relay is activated, XTension receives a wireless signal from the motion detector and a power-line signal from the universal module.

TIP

The scripts that identify and adjust for missed commands are used in both the Line Pump and Wireless Pump units. Either could miss a command, and having the scripts in both places helps ensure accuracy.

The pseudo unit called The Oil Pump does all the calculation of elapsed time and fuel consumed. This is its On script:

write log "Pump on"
write log "the time delta of the pump is " & (time delta of (thisUnit))
set value of "PtimesOn" to (value of "PtimesOn") + 1
turnon "Attic fan" in 8 * minutes—this fan is turned on when the main furnace air 
handling fan comes on.
write log "Attic fan schedules" color green
save database for unit thisUnit
write log "END OF ON SCRIPT"

This script increments the PtimesOn pseudo unit that's used for statistical tracking. It also schedules a fan in the attic to come on, thanks to an appliance module , to help circulate the furnace's hot air.

The Off script for The Oil Pump does all the heavy lifting of calculations:

set secondsOn to time delta of (thisUnit)
write log "the time delta of the pump is " & (secondsOn)
set value of "PtimesOff" to (value of "PtimesOff") + 1
set tenthsOn to secondsOn / 6
set value of "PavgOn" to (((value of "PdayOn") * 6) / (value of "PtimesOn"))

The elapsed time since the unit was turned on, calculated using the time delta provided by XTension, is converted from seconds to minutes. The daily average time turned on (PavgOn) is updated using this information.

If the current elapsed time is the highest today so far, PmaxOn is updated to set a new high score:

if (value of "PmaxOn") < secondsOn then
    set value of "PmaxOn" to secondsOn
end if

Likewise, if this is the shorted elapsed time, PminOn is updated to reflect the new value:

if (value of "PminOn") > secondsOn then
    set value of "PminOn" to secondsOn
end if

Total usage for the season is stored as a unit property:

set t to (Get Unit Property "TimeOn") 
Set Unit Property "TimeOn" to (t + secondsOn) -- in seconcds 
set S to (Get Unit Property "SeasonUsage") 
Set Unit Property "SeasonUsage" to round ((S + pOil) * 100) / 100 -- in 
ounces of oil

This calculation determines the per-hour usage and updates a unit property:

set h to (Get Unit Property "HourUsage")
set h to round (h * 100) / 100
Set Unit Property "HourUsage" to round ((h + pOil) * 100) / 100

Daily, weekly, and monthly totals are also calculated and stored:

set D to (Get Unit Property "DayUsage")
Set Unit Property "DayUsage" to round ((D + pOil) * 100) / 100
set w to (Get Unit Property "WeekUsage")
Set Unit Property "WeekUsage" to round ((w + pOil) * 100) / 100
set M to (Get Unit Property "MonthUsage")
Set Unit Property "MonthUsage" to round ((M + pOil) * 100) / 100

Finally, save database for unit causes XTension to write all the unit properties to its database file:

save database for unit thisUnit

Usually, XTension saves to disk hourly; this statement ensures the latest info is saved in case a problem occurs.

XTension's unit properties provide a handy way to store the calculated values. provides a comprehensive list, but you don't need to create these in advance. XTension will create them when you store a value.

Table 0. XTension's unit properties

You might wonder why these values are kept in unit properties, but some (PavgOn, for example) are stored using pseudo units. All values could be stored as unit properties, but properties aren't visible in the Master List and aren't as easily accessible when you access XTension using a web browser . I like to keep some of the values more accessible so that I can check on them easily and ensure that the system is not missing too many signals or that the average consumptions aren't inordinately high.

Now that the data is being gathered, let's do some analysis. A scheduled script that executes hourly appends the information to a comma-delimited text file:

set MyDate to ((month of (current date)) & (day of (current date)) & ":" &
(round (time of (current date)) / hours) as string) as text
set t to (Get Unit Property "TimeOn" from unit "The Oil Pump")
set h to (Get Unit Property "HourUsage" from unit "The Oil Pump")
set D to (Get Unit Property "DayUsage" from unit "The Oil Pump")
set w to (Get Unit Property "WeekUsage" from unit "The Oil Pump")
set M to (Get Unit Property "MonthUsage" from unit "The Oil Pump")
set S to (Get Unit Property "SeasonUsage" from unit "The Oil Pump")
try
--in case the file was left open
    close access alias "G4 X HD:Applications:XTensionFolder:OilUsageGraph.
txt" 
on error the error_message number the error_number
end try
set MyFileRef to open for access alias "G4 X HD:Applications:XTensionFolder:
OilUsageGraph.txt" with write permission
set MyData to read MyFileRef

set eof of MyFileRef to 0–append to end of file
set MyData to ((MyData) & (MyDate & "," & h & "," & D & "," & w & "," & M & "," & (t / 6) & 
"," & S) as text) & return 
write MyData to MyFileRef 
close access MyFileRef