# Predicting daily electricity load - fitting time series

I want to predict inter-day electricity load. My data are electricity loads for 11 months, sampled in 30 minute intervals. I also got the weather-specific data from a meteorological station (temperature, relative humidity, wind direction, wind speed, sunlight). From this, I want to predict the electricity load until the end of the day.

I can run my algorithm until 10:00 of the present day and after that it should give the prediction of loads in 30 minute intervals. So, it should tell the load at 10:30, 11:00, 11:30 and so on until 24:00.

My first attempt was to create a linear model in R.

BP.TS <- ts(Buying.power, frequency = 48)
a <- data.frame(
Time, BP.TS, Weekday, Pressure, Temperature, RelHumidity, AvgWindSpeed, AvgWindDirection, MaxWindSpeed, MaxWindDirection, SunLightTime,
)
a <- a[(6*48+1):nrow(a),]

start = 9716
par(mfrow=c(5,2))
for (i in 1:10) {
train <- a[1:(start+(i-1)*48),]
summary(reg <- lm(log(BP.TS)~., data=train, na.action=NULL))
pred <- exp(predict(reg, test))

plot(test$BP.TS, type="o") lines(pred, col=2) cat("MAE", mean(abs(test$BP.TS - pred)), "\n")
}


This is not very succesful. Now I try to model the data with ARIMA. I used auto.arima() from the forecast package. These are the results I got:

> auto.arima(BP.TS)
Series: BP.TS
ARIMA(2,0,1)(1,1,2)[48]

Call: auto.arima(x = BP.TS)

Coefficients:
ar1      ar2     ma1    sar1     sma1    sma2
1.1816  -0.2627  -0.554  0.4381  -1.2415  0.3051
s.e.  0.0356   0.0286   0.033  0.0952   0.0982  0.0863

sigma^2 estimated as 256118:  log likelihood = -118939.7
AIC = 237893.5   AICc = 237893.5   BIC = 237947


Now if I try something like:

reg = arima(train$BP.TS, order=c(2,0,1), xreg=cbind( train$Time,
train$Weekday, train$Pressure,
train$Temperature, train$RelHumidity,
train$AvgWindSpeed, train$AvgWindDirection,
train$MaxWindSpeed, train$MaxWindDirection,
train$SunLightTime, train$Buying.2dayago,
train$MovingAvgLastNDaysLoad, train$X1, train$X2, train$X3, train$X4, train$X5, train$X6, train$X7, train$X8, train$X9,
train$X11, train$X12, train$X13, train$X14, train$X15, train$X16, train$X17, train$X18,
train$MovingAvgLast7DaysMidnightTemp )) p <- predict(reg, n.ahead=21, newxreg=cbind( test$Time,
test$Weekday, test$Pressure,
test$Temperature, test$RelHumidity,
test$AvgWindSpeed, test$AvgWindDirection,
test$MaxWindSpeed, test$MaxWindDirection,
test$SunLightTime, test$Buying.2dayago,
test$MovingAvgLastNDaysLoad, test$X1, test$X2, test$X3, test$X4, test$X5, test$X6, test$X7, test$X8, test$X9,
test$X11, test$X12, test$X13, test$X14, test$X15, test$X16, test$X17, test$X18,
test$MovingAvgLast7DaysMidnightTemp )) plot(test$BP.TS, type="o", ylim=c(6300,8300))
par(new=T)
plot(p$pred, col=2, ylim=c(6300,8300)) cat("MAE", mean(p$se), "\n")


• you try very general models for specific problem. Did you any research before trying these models? Google search on "forecasting electricity loads" gives plenty of links. – mpiktas Jan 25 '11 at 8:45
• There is loads of literature on this problem. Check out forecasters.org/ijf/journal-issue/320 for a few papers published in the IJF. – Rob Hyndman Jan 25 '11 at 11:57
• Do you know why your linear model is not successful? And you've put a massive amount of predictors into your model - without much thought? - I would find the best 4 or 5 predictors first, and see how they go (in the linear model). – probabilityislogic Jan 25 '11 at 13:29
• @mpiktas: I did some articles, however they are very implementation-scarce. One I found very instructive is sciencedirect.com/…. In this article, they model each hour by a different time series. However, I may not have enough data to do that. – Grega Kešpret Jan 25 '11 at 15:17
• Also, they got very good results with Regression with autocorrelated errors. I tried to do that with arima() function in R. A lot of the predictors are from that article. @Rob Hyndman: Thank you for the link, I will look at those articles. Is there an article that you would suggest to start with? I would like it to be as implementation-close as possible. @probabilityislogic: Yes, my linear model is not succesful probably because the dynamics of my system is not linear. That's why I tried to model the system with arima() - autoregression with autocorrelated errors. – Grega Kešpret Jan 25 '11 at 15:18

I've played around with electrical demand models, and I can tell you that it's a good idea to start "zoomed out". Each region has its own characteristics, but the general idea is the same.

Electric demand is a function of many variables. Starting with the slowest moving terms.

1. General Economic Activity is the slowest moving term (typically the 3 to 8 year time frame). This term is typically related to Gross Domestic Product for the area. Electrical Demand may generally grow faster than GDP, but the electrical demand "ups" during good economic times, and demand "downs" during recessions provide an obvious link to GDP. See the blue line in the first graph below.

2. Next, is the Seasonal Term (annual time frame). For instance in the U.S., the Summer Peak shows up in August, the Winter Peak shows up in January, the Spring Trough shows up in April and the Fall Trough shows up in November. See the red line in top two graphs below. In the second graph, I have shown the Seasonal Term to be constant for each month, but you can easily improve that by a linear or non-linear relationship for each month (monthly time frame).

3. You are now down to the daily time frame. The bottom graph shows the Electrical Demand for Texas for one 24 hour period (12/22/2010). The Day-time Peak was at 7:00PM (19:00) and the Night-time Trough was at 4:00AM (04:00). This time frame is where you want to consider holidays, weekends, weather, etc. However, keep in mind that those other variables (in 1 and 2 above) are also affecting your results.

So, from your description, you have data for 11 months. Look at the first graph below and assume that you have data for 11 months. Is that enough to get an idea of the Seasonal Term for the year? I would use a minimum of 10 years of monthly data to get a feel for the Seasonal Term. The idea here is to tweek the structure of your daily model differently during months of "rapid seasonal change" versus months of "slow seasonal change".

Next, I would play around with the size and structure of the "data window" that you will use to estimate your daily model. For example, will you get a better daily model if you include daily fall and winter data when estimating a summer daily model? Or, is it better to use 10 rescaled "summer data windows", one for each year in 10 years of data, when estimating a summer daily model?

Once you get all of the deterministic terms working well, then, and only then would I go after the ARIMA terms.

• Dear bill_080, thank you for your elaborate answer. I know that more data would help, but unfortunately I have only 11 months of data. Is it possible to construct a model on some other data and use this model for my data? Then, how should one go about tweaking the model when applying it to the new data? Is that feasible at all? Am I looking in the right direction, or am I just doomed because of too little data that I have in my specific case? Thank you. – Grega Kešpret Jan 25 '11 at 23:23
• @Grega: You're somewhat doomed in that you need more data. However, the electrical demand data that you need may be easier to find than you think. For instance, in Texas here's a link: ercot.com/gridinfo/load/index What area of the world are you trying to forecast? – bill_080 Jan 26 '11 at 0:12
• @bill_080: I'm trying to forecast an electricity load of a factory in central Japan. My question still stays. Is it possible to devise a model on some other data and apply it to my specific case? But wouldn't dynamics of the system be different then? – Grega Kešpret Jan 26 '11 at 0:24
• @bill_080, +1 for detailed and insightfull answer – mpiktas Jan 26 '11 at 6:51
• @bill_080: Your U.S. Seasonal Component is very close to what I'm seeing in the microcosm of our condominium association's usage. Which I find a bit strange. (Though our summer peak is much larger.) And your underlying trend looks familiar, too, which is even weirder to me. – Wayne Apr 1 '11 at 21:48

@Grega:

I ran out of room in the comment area, so I started another answer.

From your comment, that's the problem. Each system/grid has its own "signature". It's a combination of system dynamics, age, weather, local economics, cultures, traditions, etc. In Japan, its worse. Some sections of the country run at 50Hz and other sections run at 60Hz. These grids connect at high voltage DC stations, which means some areas behave like electrical "islands" (totally different behavior than their neighbors just a few miles away). If you're zeroed-in on one factory, the predictability goes down even more. Fewer users means higher uncertainty.

No matter how you do this, it's going to be messy.

I would filter out the daily/weekday/weekend/holiday component to get a Seasonal Term. How? A 31 day CENTERED moving average? 51 day CMA? XX day CMA? You'll have to experiment with that, but I would make it a variable so you can tweek it later. Whatever filter you end up with, keep in mind that it stops short of either end of your data (a 31 day centered moving average will start on the 16th day of your raw data and end on the 16th day from the end of your raw data).

Next, the best you can do with 11 months of data is to "make up" month 12 (draw a line from your filtered series at its end to its beginning). Next, subtract your Seasonal Term (the filtered data) from your raw data to get a residual. Fit the residual to your weather data, allowing for factors for day-of-the-week and holidays.

Some factors that you may need to add are:

1) A "production run" factor. Whatever they make at this factory, if they switch from one product/category to another, the power demand required to make one product may be different than what is required for another product.

2) A "change over" factor. This is when they shift from one product/category to another. Sometimes it takes days of preparation for the switch.

3) A "work shift" factor. If they have three shifts per day, power demand for the late shift will probably be significantly different than the day shifts.

Good luck. As you probably know, this kind of a problem can get real frustrating.

====== Edit to answer Grega's first comment (01/25/2011) ====================

@Grega: Answering your first comment, I'm afraid it doesn't. The idea behind a model like this would be to have multiple "similar instances" of your 32 future points, so you can fit those points, and then predict new points. You don't have typical "similar instances" because yesterday was not the same day-of-the-week as today. You have to use last week's same day-of-the-week and the previous week's same day-of-the-week, etc. By the time you get several "similar instances" (say 20), you're typically more than 20 weeks in the past (a holiday may screw up one or more of your weeks). At that point, you're in a whole different season of the year. So, in order to use those days in another season, you need to remove the Seasonal Term from the raw data.

It's a sloppy situation, but it's the best you can do with 11 months of data.

• Just one more thought, I don't know if it's obvious from my initial post. I am not predicting many days (or months/years) in advance. I am trying to predict maximum 16 hours in advance (with 30 minutes sampling, this means 32 points). Does the approach change with that fact? – Grega Kešpret Jan 26 '11 at 2:49
• @Grega, I added an answer to your comment as an edit in the above answer. – bill_080 Jan 26 '11 at 4:21
• sorry nothing related to the original question, but do you know anything about the australian electricity market? Seams quite stable, especially with all their coal easily available? – RockScience Jan 26 '11 at 9:57
• @fRed: I don't know much about Australia. However, I do know that the power market is changing all over the world. Coal is cheap and stable, but the CO2 guys are after them. Combined Cycle plants (gas fired turbines with waste heat recovery) can now achieve fairly high efficiencies. Things are changing fast. – bill_080 Jan 26 '11 at 16:06
• yes very interesting topic – RockScience Jan 27 '11 at 6:27