community/electricity-price-widget.lua

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+-- name = "Electricity spot price"
+-- description = "Day-ahead spot price of electricity"
+-- data_source = "https://api.energy-charts.info/"
+-- type = "widget"
+-- foldable = "true"
+-- author = "Hannu Hartikainen <hannu.hartikainen@gmail.com>"
+-- version = "1.0"
+
+json = require "json"
+prefs = require "prefs"
+
+url_base = "https://api.energy-charts.info/price?bzn=%s&end=%d"
+
+price_data = nil
+price_interval = nil
+price_unit = nil
+unit_multiplier = 1
+
+next_fetch_ts = 0
+next_redraw_ts = nil
+
+-- parameters and default values
+function on_load()
+    -- bidding zone: see "Available bidding zones" in https://api.energy-charts.info/
+    if not prefs.bidding_zone then
+        prefs.bidding_zone = "FI"
+    end
+
+    -- VAT percentage
+    if not prefs.vat_percentage then
+        prefs.vat_percentage = 24
+    end
+
+    -- change threshold percentages for showing changes in folded format
+    if not prefs.oneline_threshold_high then
+        prefs.oneline_threshold_high = 1.3
+    end
+    if not prefs.oneline_threshold_low then
+        prefs.oneline_threshold_low = 0.7
+    end
+
+    -- url to open when clicked
+    if not prefs.click_url then
+        prefs.click_url = "https://www.sahkonhintatanaan.fi/"
+    end
+end
+
+function get_url()
+    -- at most about 35 hours are known in advance; fetch all known prices
+    local end_offset = 2*24*60*60
+    local end_ts = os.time() + end_offset
+    return string.format(url_base, prefs.bidding_zone, end_ts)
+end
+
+function on_alarm()
+    if os.time() > next_fetch_ts then
+        http:get(get_url())
+    end
+end
+
+function on_network_result(result, code)
+    if code >= 200 and code < 299 then
+        parse_result(result)
+        draw_widget(true)
+    end
+end
+
+function on_tick(t)
+    local ts = os.time()
+    if next_redraw_ts and ts > next_redraw_ts then
+        draw_widget(true)
+    end
+end
+
+function on_click()
+    if not ui:folding_flag() then
+        system:open_browser(prefs.click_url)
+    else
+        draw_widget(false)
+    end
+end
+
+function parse_result(result)
+    price_data = json.decode(result)
+    local price_count = #price_data.unix_seconds
+    if price_count < 2 then
+        return
+    end
+
+    price_interval = price_data.unix_seconds[2] - price_data.unix_seconds[1]
+    if price_data.unit == "EUR/MWh" then
+        price_unit = "c/kWh"
+        unit_multiplier = 0.1
+    else
+        price_unit = price_data.unit
+        unit_multiplier = 1
+    end
+
+    -- assume next day is known 8 hours before it starts
+    -- (eg. Nord Pool Spot typically publishes dayahead prices at 14 local time)
+    local next_fetch_offset = 8*60*60
+    local end_of_data_ts = price_data.unix_seconds[price_count] + price_interval
+    next_fetch_ts = end_of_data_ts - next_fetch_offset
+end
+
+function get_current_idx()
+    local t = os.time()
+    for i = 1, #price_data.unix_seconds do
+        local ts = price_data.unix_seconds[i]
+        if ts < t and t < (ts+price_interval) then
+            return i
+        end
+    end
+end
+
+function price(i)
+    return price_data.price[i]
+end
+
+function get_display_price(idx)
+    local mul = (1.0 + (prefs.vat_percentage / 100.0)) * unit_multiplier
+    -- NOTE: float rounding in string.format doesn't work so do it here
+    return math.floor(100.0 * price(idx) * mul + 0.5) / 100.0
+end
+
+function get_display_time(idx)
+    return os.date("%H", price_data.unix_seconds[idx])
+end
+
+function format_price(idx)
+    return string.format("%0.2f", get_display_price(idx))
+end
+
+function format_price_and_unit(idx)
+    return string.format("%s %s", format_price(idx), price_unit)
+end
+
+function format_oneline(idx)
+    local more_prices = ""
+    local more_count = 0
+    local cur_price = price(idx)
+    for i = idx+1, #price_data.price do
+        if price(i) > prefs.oneline_threshold_high * cur_price
+            or price(i) < prefs.oneline_threshold_low * cur_price then
+            more_count = more_count + 1
+            more_prices = more_prices .. string.format("⋄ <i>%s′</i> <b>%s</b> ", get_display_time(i), get_display_price(i))
+            cur_price = price(i)
+            if more_count > 3 then
+                break
+            end
+        end
+    end
+    return string.format("<b>%s</b> %s", format_price_and_unit(idx), more_prices)
+end
+
+-- NOTE: using timestamps would be better than indices, but the chart element
+-- doesn't support times spanning multiple days properly
+function make_chart_data(idx)
+    local chart = {}
+    for i = idx, #price_data.price do
+        table.insert(chart, {
+            i-1,
+            get_display_price(i)
+        })
+    end
+    return chart
+end
+
+function draw_widget(fold)
+    ui:set_folding_flag(fold)
+    local idx = get_current_idx()
+    if not idx then
+        ui:show_text("Error: no current price data")
+        -- request fetch on next on_alarm and don't redraw before that
+        next_fetch_ts = 0
+        next_redraw_ts = nil
+        return
+    end
+
+    next_redraw_ts = price_data.unix_seconds[idx+1]
+    ui:set_title(string.format("Electricity spot price: %s", format_price_and_unit(idx)))
+    ui:show_chart(make_chart_data(idx), "x: int, y: float", "", true, format_oneline(idx))
+end