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Forecasting electricity demand with Python
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Introduction¶
A time series is a succession of chronologically ordered data spaced at equal or unequal intervals. The forecasting process consists of predicting the future value of a time series, either by modeling the series solely based on its past behavior (autoregressive) or by using other external variables.
When working with time series, it is seldom needed to predict only the next element in the series ($t_{+1}$). Instead, the most common goal is to predict a whole future interval (($t_{+1}$), ..., ($t_{+n}$)) or a far point in time ($t_{+n}$). Several strategies allow generating this type of prediction, skforecast has implemented the following:
Recursive multi-step forecasting: since the value $t_{n-1}$ is required to predict $t_{n}$, and $t_{n-1}$ is unknown, it is necessary to make recursive predictions in which each new prediction is based on the previous one.
Direct multi-step forecasting: this method involves training a different model for each step.
This document shows an example of how to use forecasting methods to predict hourly electricity demand. Specifically, it introduces skforecast, a simple library that contains the classes and functions necessary to adapt any scikit-learn regression model to forecasting problems.
More examples in skforecast-examples.
Use Case¶
A time series with electricity demand (MW) for the state of Victoria (Australia) from 2011-12-31 to 2014-12-31 is available. It is intended to generate a forecasting model capable of predicting the next day's energy demand at the hourly level.
# Data manipulation
# ==============================================================================
import numpy as np
import pandas as pd
# Plots
# ==============================================================================
import matplotlib.pyplot as plt
import seaborn as sns
%matplotlib inline
from statsmodels.graphics.tsaplots import plot_acf
from statsmodels.graphics.tsaplots import plot_pacf
plt.style.use('fivethirtyeight')
# Modelling and Forecasting
# ==============================================================================
from sklearn.linear_model import Ridge
from lightgbm import LGBMRegressor
from sklearn.preprocessing import StandardScaler
from sklearn.metrics import mean_absolute_error
from skforecast.ForecasterAutoreg import ForecasterAutoreg
from skforecast.ForecasterAutoregDirect import ForecasterAutoregDirect
from skforecast.model_selection import grid_search_forecaster
from skforecast.model_selection import backtesting_forecaster
# Warnings configuration
# ==============================================================================
import warnings
warnings.filterwarnings('ignore')
Data¶
The data used in this document were obtained from the R tsibbledata package. The dataset contains 5 columns and 52,608 complete records. The information in each column is:
Time: date and time of the record.
Date: date of the record.
Demand: electricity demand (MW).
Temperature: temperature in Melbourne, capital of the state of Victoria.
Holiday: indicator if the day is a public holiday.
# Data download
# ==============================================================================
url = ('https://raw.githubusercontent.com/JoaquinAmatRodrigo/skforecast/master/' +
'data/vic_elec.csv')
data = pd.read_csv(url, sep=',')
data.info()
The Time column is stored as a string. To convert it to datetime, the function pd.to_datetime() is used. Once in datetime format, and to make use of pandas functionalities, it is set as an index. In addition, since the data has been recorded every 30 minutes, the frequency ('30min') is indicated.
# Data preparation
# ==============================================================================
data = data.copy()
data['Time'] = pd.to_datetime(data['Time'], format='%Y-%m-%dT%H:%M:%SZ')
data = data.set_index('Time')
data = data.asfreq('30min')
data = data.sort_index()
data.head(2)
One of the first analyses to perform when working with time series is to verify whether the series is complete.
# Verify that a temporary index is complete
# ==============================================================================
(data.index == pd.date_range(start=data.index.min(),
end=data.index.max(),
freq=data.index.freq)).all()
print(f"Number of rows with missing values: {data.isnull().any(axis=1).mean()}")
# Fill gaps in a temporary index
# ==============================================================================
# data.asfreq(freq='30min', fill_value=np.nan)
Although the data are at 30-minute intervals, the goal is to create a model capable of predicting hourly electricity demand, so the data must be aggregated. This type of transformation can be very fast, combining the Pandas time type index and its resample() method.
It is very important to use the closed='left' and label='right' arguments correctly in order not to introduce future information into the training, leakage). Suppose that values are available for 10:10, 10:30, 10:45, 11:00, 11:12 and 11:30. If the hourly average is to be obtained, the value assigned to 11:00 must be calculated using the values of 10:10, 10:30, and 10:45; and that of 12:00, with the value of 11:00, 11:12, and 11:30.
For the 11:00 average value, the 11:00 point value is not included because, in reality, the value is not yet available at that exact time.
# Aggregating in 1H intervals
# ==============================================================================
# The Date column is eliminated so that it does not generate an error when aggregating.
# The Holiday column does not generate an error since it is Boolean and is treated as 0-1.
data = data.drop(columns='Date')
data = data.resample(rule='H', closed='left', label ='right').mean()
data
The dataset starts on 2011-12-31 14:00:00 and ends on 2014-12-31 13:00:00. The first 10 and the last 13 records are discarded so that it starts on 2012-01-01 00:00:00 and ends on 2014-12-30 23:00:00. In addition, to optimize the hyperparameters of the model and evaluate its predictive capability, the data are divided into 3 sets, training, validation, and test.
# Split data into train-val-test
# ==============================================================================
data = data.loc['2012-01-01 00:00:00': '2014-12-30 23:00:00']
end_train = '2013-12-31 23:59:00'
end_validation = '2014-11-30 23:59:00'
data_train = data.loc[: end_train, :]
data_val = data.loc[end_train:end_validation, :]
data_test = data.loc[end_validation:, :]
print(f"Train dates : {data_train.index.min()} --- {data_train.index.max()}")
print(f"Validation dates : {data_val.index.min()} --- {data_val.index.max()}")
print(f"Test dates : {data_test.index.min()} --- {data_test.index.max()}")
Graphic exploration¶
When it is necessary to generate a forecasting model, plotting the time series values could be useful. This allows identifying patterns such as trends and seasonality.
Full time series
# Time series plot
# ==============================================================================
fig, ax = plt.subplots(figsize=(12, 4))
data_train.Demand.plot(ax=ax, label='train', linewidth=1)
data_val.Demand.plot(ax=ax, label='validation', linewidth=1)
data_test.Demand.plot(ax=ax, label='test', linewidth=1)
ax.set_title('Electricity demand')
ax.legend();
The above graph shows that electricity demand has annual seasonality. There is an increase centered on July and very accentuated demand peaks between January and March.
Section of the time series
Due to the variance of the time series, it is not possible to appreciate with a single chart the possible intraday pattern.
# Zooming time series chart
# ==============================================================================
zoom = ('2013-05-01 14:00:00','2013-06-01 14:00:00')
fig = plt.figure(figsize=(12, 6))
grid = plt.GridSpec(nrows=8, ncols=1, hspace=0.6, wspace=0)
main_ax = fig.add_subplot(grid[1:3, :])
zoom_ax = fig.add_subplot(grid[5:, :])
data.Demand.plot(ax=main_ax, c='black', alpha=0.5, linewidth=0.5)
min_y = min(data.Demand)
max_y = max(data.Demand)
main_ax.fill_between(zoom, min_y, max_y, facecolor='blue', alpha=0.5, zorder=0)
main_ax.set_xlabel('')
data.loc[zoom[0]: zoom[1]].Demand.plot(ax=zoom_ax, color='blue', linewidth=2)
main_ax.set_title(f'Electricity demand: {data.index.min()}, {data.index.max()}', fontsize=14)
zoom_ax.set_title(f'Electricity demand: {zoom}', fontsize=14)
plt.subplots_adjust(hspace=1)
When zooming in on the time series, a clear weekly seasonality is evident, with higher consumption during the work week (Monday to Friday) and lower consumption on weekends. It is also observed that there is a clear correlation between the consumption of one day and that of the previous day.
Annual, weekly and daily seasonality
# Boxplot for annual seasonality
# ==============================================================================
fig, ax = plt.subplots(figsize=(7, 3.5))
data['month'] = data.index.month
data.boxplot(column='Demand', by='month', ax=ax,)
data.groupby('month')['Demand'].median().plot(style='o-', linewidth=0.8, ax=ax)
ax.set_ylabel('Demand')
ax.set_title('Demand distribution by month')
fig.suptitle('');
It is observed that there is an annual seasonality, with higher (median) demand values in June, July, and August, and with high demand peaks in November, December, January, February, and March.
# Boxplot for weekly seasonality
# ==============================================================================
fig, ax = plt.subplots(figsize=(7, 3.5))
data['week_day'] = data.index.day_of_week + 1
data.boxplot(column='Demand', by='week_day', ax=ax)
data.groupby('week_day')['Demand'].median().plot(style='o-', linewidth=0.8, ax=ax)
ax.set_ylabel('Demand')
ax.set_title('Demand distribution by week day')
fig.suptitle('');
Weekly seasonality shows lower demand values during the weekend.
# Boxplot for daily seasonality
# ==============================================================================
fig, ax = plt.subplots(figsize=(9, 3.5))
data['hour_day'] = data.index.hour + 1
data.boxplot(column='Demand', by='hour_day', ax=ax)
data.groupby('hour_day')['Demand'].median().plot(style='o-', linewidth=0.8, ax=ax)
ax.set_ylabel('Demand')
ax.set_title('Demand distribution by the time of the day')
fig.suptitle('');
There is also a daily seasonality, with demand decreasing between 16:00 and 21:00 hours.
Holidays and non-holiday days
# Violinplot
# ==============================================================================
fig, ax = plt.subplots(nrows=1, ncols=1, figsize=(7, 3.5))
sns.violinplot(
x = 'Demand',
y = 'Holiday',
data = data.assign(Holiday = data.Holiday.astype(str)),
palette = 'tab10',
ax = ax
)
ax.set_title('Distribution of demand between holidays and non-holidays')
ax.set_xlabel('Demand')
ax.set_ylabel('Holiday');
Holidays tend to have lower consumption.
Autocorrelation plots
# Autocorrelation plot
# ==============================================================================
fig, ax = plt.subplots(figsize=(7, 3))
plot_acf(data.Demand, ax=ax, lags=60)
plt.show()
# Partial autocorrelation plot
# ==============================================================================
fig, ax = plt.subplots(figsize=(7, 3))
plot_pacf(data.Demand, ax=ax, lags=60)
plt.show()
The autocorrelation and partial autocorrelation plots show a clear association between one hour's demand and previous hours, as well as between one hour's demand and the same hour's demand on previous days. This type of correlation is an indication that autoregressive models can work well.
Recursive autoregressive forecasting¶
A recursive autoregressive model (ForecasterAutoreg) is created and trained from a linear regression model with a Ridge penalty and a time window of 24 lags. The latter means that, for each prediction, the demand values of the previous 24 hours are used as predictors.
Forecaster training¶
# Create and train forecaster
# ==============================================================================
forecaster = ForecasterAutoreg(
regressor = Ridge(random_state=123),
lags = 24,
transformer_y = StandardScaler()
)
forecaster.fit(y=data.loc[:end_validation, 'Demand'])
forecaster
Backtest¶
How the model would have behaved if it had been trained with the data from 2012-01-01 00:00 to 2014-11-30 23:59 and then, at 23:59 each day, the following 24 hours were predicted is evaluated. This type of evaluation, known as Backtesting, can be easily implemented with the function backtesting_forecaster(). This function returns, in addition to the predictions, an error metric.
# Backtest
# ==============================================================================
metric, predictions = backtesting_forecaster(
forecaster = forecaster,
y = data.Demand,
initial_train_size = len(data.loc[:end_validation]),
fixed_train_size = False,
steps = 24,
metric = 'mean_absolute_error',
refit = False,
verbose = True
)
# Plot
# ==============================================================================
fig, ax = plt.subplots(figsize=(12, 3.5))
data.loc[predictions.index, 'Demand'].plot(ax=ax, linewidth=2, label='real')
predictions.plot(linewidth=2, label='prediction', ax=ax)
ax.set_title('Prediction vs real demand')
ax.legend();
# Backtest error
# ==============================================================================
print(f'Backtest error: {metric}')
Hyperparameter tuning¶
In the trained ForecasterAutoreg object, the first 24 lags and a Ridge model with the default hyperparameters have been used. However, there is no reason why these values are the most appropriate.
To identify the best combination of lags and hyperparameters, a Grid Search with validation by Backtesting is used. This process consists of training a model with diferent combinations of hyperparameters and lags and evaluating its predictive capacity. In the search process, it is important to evaluate the models using only the validation data and not to include the test data, which are used only to evaluate the final model.
# Hyperparameter Grid search
# ==============================================================================
forecaster = ForecasterAutoreg(
regressor = Ridge(random_state=123),
lags = 24, # This value will be replaced in the grid search
transformer_y = StandardScaler()
)
# Lags used as predictors
lags_grid = [5, 24, [1, 2, 3, 23, 24, 25, 47, 48, 49]]
# Regressor's hyperparameters
param_grid = {'alpha': np.logspace(-3, 5, 10)}
results_grid = grid_search_forecaster(
forecaster = forecaster,
y = data.loc[:end_validation, 'Demand'],
param_grid = param_grid,
lags_grid = lags_grid,
steps = 24,
metric = 'mean_absolute_error',
refit = False,
initial_train_size = len(data[:end_train]),
fixed_train_size = False,
return_best = True,
verbose = False
)
# Grid Search results
# ==============================================================================
results_grid
The best results are obtained by using the lags [1, 2, 3, 23, 24, 25, 47, 48, 49] and a Ridge configuration {'alpha': 215.44}. By specifying return_best = True in the grid_search_forecaster() function, at the end of the process, the forecaster object is automatically retrained with the best configuration found and the complete dataset (train + validation).
forecaster
Backtest with test data¶
Once the best model has been identified and trained, its error in predicting the test data is calculated.
# Backtest final model
# ==============================================================================
metric, predictions = backtesting_forecaster(
forecaster = forecaster,
y = data.Demand,
initial_train_size = len(data[:end_validation]),
fixed_train_size = False,
steps = 24,
metric = mean_absolute_error,
refit = False,
verbose = False
)
fig, ax = plt.subplots(figsize=(12, 3.5))
data.loc[predictions.index, 'Demand'].plot(linewidth=2, label='real', ax=ax)
predictions.plot(linewidth=2, label='prediction', ax=ax)
ax.set_title('Prediction vs real demand')
ax.legend();
# Error backtest
# ==============================================================================
print(f'Backtest error: {metric}')
After optimization of lags and hyperparameters, the prediction error has been reduced from 289.5 to 251.9.
Prediction intervals¶
A prediction interval defines the interval within which the true value of "y" can be expected to be found with a given probability.
Rob J Hyndman and George Athanasopoulos, in their book Forecasting: Principles and Practice, list multiple ways to estimate prediction intervals, most of which require that the residuals (errors) of the model are normally distributed. When this property cannot be assumed, bootstrapping can be resorted to, which only assumes that the residuals are uncorrelated.
# Backtest with test data and prediction intervals
# ==============================================================================
metric, predictions = backtesting_forecaster(
forecaster = forecaster,
y = data.Demand,
initial_train_size = len(data.Demand[:end_validation]),
fixed_train_size = False,
steps = 24,
metric = 'mean_absolute_error',
interval = [10, 90],
n_boot = 500,
in_sample_residuals = True,
verbose = False
)
print('Backtesting metric:', metric)
predictions.head(5)
# Plot
# ==============================================================================
fig, ax = plt.subplots(figsize=(12, 3.5))
data.loc[predictions.index, 'Demand'].plot(linewidth=2, label='real', ax=ax)
predictions.iloc[:, 0].plot(linewidth=2, label='prediction', ax=ax)
ax.set_title('Prediction vs real demand')
ax.fill_between(
predictions.index,
predictions.iloc[:, 1],
predictions.iloc[:, 2],
alpha = 0.3,
color = 'red',
label = 'prediction interval'
)
ax.legend();