Background

After installing solar panels, a smart electricity meter, and a solar battery - the next obvious step was a smart energy tariff.

Octopus (join and we both get £50) have an "Agile" tariff. Unlike a normal tariff - with a set price for electricity - this tariff fluctuates every 30 minutes. Prices depend on wholesale costs which means they can go negative. That's right, you can get paid to soak up excess power.

Of course, they can also spike considerably. Unlike the failed Texas experiment, here the maximum price is capped at £1/kWh.

Every day at about 1600, the next day's prices are published on Octopus's website. And they're also made available via a simple REST API.

So, it's relatively simple to generate a line graph and display it on the eInk screen.

Code

(You can treat this code as MIT Licenced if that makes you happy.)

Calling the API for the half-houly prices is:

$url = "https://{$API_KEY}:@api.octopus.energy/v1/products/" .
       "AGILE-FLEX-22-11-25/electricity-tariffs/E-1R-AGILE-FLEX-22-11-25-C/standard-unit-rates/";

Your API_KEY is unique - and you'll need to check which tariff you're on.

The data is retrieved as JSON and converted:

$content = file_get_contents($url);
$data = json_decode($content);

The JSON is full of entries like this:

"results": [
{
  "value_exc_vat": 13.6,
  "value_inc_vat": 14.28,
  "valid_from": "2023-11-01T22:30:00Z",
  "valid_to": "2023-11-01T23:00:00Z",
  "payment_method": null
},
{
  "value_exc_vat": 18.4,
  "value_inc_vat": 19.32,
  "valid_from": "2023-11-01T22:00:00Z",
  "valid_to": "2023-11-01T22:30:00Z",
  "payment_method": null
},

They're newest first, so need to be reversed:

$tariffs = array_reverse( $data->results );

Then it's a case of looping through them and grabbing today's data:

$userTimeZone = new DateTimeZone('Europe/London');
$now = new DateTime('now', $userTimeZone);

$nowPosition = 0;
$datay = array();
$datax = array();

foreach ( $tariffs as $tariff ) {
    $dateStringFrom = $tariff->valid_from;
    $dateStringTo   = $tariff->valid_to;
    $dateTimeFrom = new DateTime($dateStringFrom, new DateTimeZone('UTC'));
    $dateTimeTo   = new DateTime($dateStringTo,   new DateTimeZone('UTC'));

    if ($now >= $dateTimeFrom && $now <= $dateTimeTo) {
        $costNow = $roundedInteger = (int)round( $tariff->value_inc_vat );
        $hour   = intval( $dateTimeFrom->format('G') ); //  No leading 0
        $minute = intval( $dateTimeFrom->format('i') );
        $offset = ($minute == 0) ? 0 : (($minute == 30) ? 1 : null);
        $nowPosition = (2 * $hour) + $offset + 0.5;
        $until = $dateTimeTo->format('H:i');
    }

    if ($dateTimeFrom->format('Y-m-d') == $now->format('Y-m-d')) {

        $datax[] = $dateTimeFrom->format("H:i");
        $cost = $roundedInteger = (int)round( $tariff->value_inc_vat );
        $datay[] = $cost;
    }   
}

Drawing the graph uses the venerable JPGraph:

$path = 'jpgraph/';
set_include_path(get_include_path() . PATH_SEPARATOR . $path);
require_once ('jpgraph/jpgraph.php');
require_once ('jpgraph/jpgraph_line.php');
require_once ('jpgraph/jpgraph_plotline.php');

// Size of graph
$width  = 600;
$height = 600;

// Setup the graph
$graph = new Graph($width,$height);
$graph->SetScale("intlin");
$graph->SetMargin(35,0,45,20); // L R T B

$graph->SetUserFont('dejavu/DejaVuSansMono.ttf');
$graph->title->SetFont(FF_USERFONT,FS_NORMAL,25);

$graph->SetBox(false);

$graph->title->Set( $now->format('l') . "'s Electricity Prices\n" . $costNow . "p / kWh until {$until}" );
$graph->title->SetColor('#000');

$graph->ygrid->Show(true);
$graph->xgrid->Show(true);

$graph->xaxis->SetTickLabels( $datax );

$graph->xaxis->SetColor('#000');
$graph->yaxis->SetColor('#000');

$graph->xaxis->SetFont(FF_USERFONT, FS_NORMAL, 10);
$graph->yaxis->SetFont(FF_USERFONT, FS_NORMAL, 14); 

// Just let the maximum be autoscaled
$graph->yaxis->scale->SetAutoMin(0); 

// Only show up until 23:00
$graph->xaxis->scale->SetAutoMax(46);

$graph->xaxis->SetTextLabelInterval(2); 
$graph->SetTickDensity(TICKD_DENSE, TICKD_DENSE);  

// Create the line plot
$p1 = new LinePlot($datay);
$graph->Add($p1);
$p1->SetStepStyle();
$p1->SetColor('#000');

//                 Direction, position,     colour@alpha, width
$l1 = new PlotLine(VERTICAL,  $nowPosition, 'black@.8',   13);
// Add vertical highlight line to the plot
$graph->AddLine($l1);

// Output line
$graph->Stroke();

Next steps

I dunno? Add some details about carbon emissions? Battery stats? Let me know what you think in the comments.