diff --git a/nix/pkgs/default.nix b/nix/pkgs/default.nix
index b9e25bc9ff..ea3c666345 100644
--- a/nix/pkgs/default.nix
+++ b/nix/pkgs/default.nix
@@ -69,4 +69,5 @@
     extraPkgs = pkgs: with pkgs; [ openssl gnome.zenity ];
   };
   vault-acme = pkgs.callPackage ./vault-acme { };
+  nixos-size = pkgs.callPackage ./nixos-size { };
 } // (import ./heptapod-runner args)
diff --git a/nix/pkgs/nixos-size/combined-builder.sh b/nix/pkgs/nixos-size/combined-builder.sh
new file mode 100755
index 0000000000..13a7d02b76
--- /dev/null
+++ b/nix/pkgs/nixos-size/combined-builder.sh
@@ -0,0 +1,3 @@
+#!/bin/sh
+
+echo -ne "$sp1\n$sp2\n" > $out
diff --git a/nix/pkgs/nixos-size/combined.nix b/nix/pkgs/nixos-size/combined.nix
new file mode 100644
index 0000000000..b6b63fecbc
--- /dev/null
+++ b/nix/pkgs/nixos-size/combined.nix
@@ -0,0 +1,13 @@
+{ builder ? ./combined-builder.sh }:
+
+derivation {
+  system = builtins.currentSystem;
+  name = "nixos-size-combined";
+  inherit builder;
+
+  sp1 = builtins.storePath /run/current-system;
+  sp2 = builtins.storePath /run/booted-system;
+
+  preferLocalBuild = true;
+  allowSubstitutes = false;
+}
diff --git a/nix/pkgs/nixos-size/default.nix b/nix/pkgs/nixos-size/default.nix
new file mode 100644
index 0000000000..faef6b4c6a
--- /dev/null
+++ b/nix/pkgs/nixos-size/default.nix
@@ -0,0 +1,15 @@
+{ writeShellApplication, nix_2_3, gawk }:
+
+writeShellApplication {
+  name = "nixos-size";
+  runtimeInputs = [ nix_2_3 gawk ];
+  text = ''
+    test -e /run/booted-system || exit 0
+
+    combined_path="$(nix-build ${./combined.nix} --no-out-link --arg builder ${./combined-builder.sh})"
+
+    echo "nixos_closure_size{type=\"booted-system\"} $(nix path-info -S /run/booted-system | awk '{ print $2 }')"
+    echo "nixos_closure_size{type=\"current-system\"} $(nix path-info -S /run/current-system | awk '{ print $2 }')"
+    echo "nixos_closure_size{type=\"combined\"} $(nix path-info -S "$combined_path" | awk '{ print $2 }')"
+  '';
+}
diff --git a/ops/nixos/lib/common.nix b/ops/nixos/lib/common.nix
index b32ce466b8..4f385987d4 100644
--- a/ops/nixos/lib/common.nix
+++ b/ops/nixos/lib/common.nix
@@ -243,23 +243,33 @@ in
     system.activationScripts.node-exporter = {
       text = ''
         test -d /run/prometheus-textfile-exports || mkdir /run/prometheus-textfile-exports
+
         my_version_string="$(cat "$systemConfig/nixos-version")"
         my_hash_string="$(readlink -f "$systemConfig" | ${pkgs.gnugrep}/bin/grep -Eo '\b[0-9a-df-np-sv-z]{32}\b')"
         my_specialisation="$(cat "$systemConfig/specialisation-name" 2>/dev/null || true)"
         echo "nixos_running_system{version=\"$my_version_string\", hash=\"$my_hash_string\", specialisation=\"$my_specialisation\"} 1" > /run/prometheus-textfile-exports/running_system.prom
 
-        my_version_string="$(cat "/run/booted-system/nixos-version")"
-        my_hash_string="$(readlink -f "/run/booted-system" | ${pkgs.gnugrep}/bin/grep -Eo '\b[0-9a-df-np-sv-z]{32}\b')"
-        my_specialisation="$(cat "/run/booted-system/specialisation-name" 2>/dev/null || true)"
-        echo "nixos_booted_system{version=\"$my_version_string\", hash=\"$my_hash_string\", specialisation=\"$my_specialisation\"} 1" > /run/prometheus-textfile-exports/booted_system.prom
+        if test -e /run/booted-system; then
+          my_version_string="$(cat "/run/booted-system/nixos-version")"
+          my_hash_string="$(readlink -f "/run/booted-system" | ${pkgs.gnugrep}/bin/grep -Eo '\b[0-9a-df-np-sv-z]{32}\b')"
+          my_specialisation="$(cat "/run/booted-system/specialisation-name" 2>/dev/null || true)"
+          echo "nixos_booted_system{version=\"$my_version_string\", hash=\"$my_hash_string\", specialisation=\"$my_specialisation\"} 1" > /run/prometheus-textfile-exports/booted_system.prom
+        fi
+
+        ${depot.nix.pkgs.nixos-size}/bin/nixos-size > /run/prometheus-textfile-exports/nixos_size.prom
       '';
     };
     boot.postBootCommands = lib.mkAfter ''
       test -d /run/prometheus-textfile-exports || mkdir /run/prometheus-textfile-exports
-      my_version_string="$(cat "/run/booted-system/nixos-version")"
-      my_hash_string="$(readlink -f "/run/booted-system" | ${pkgs.gnugrep}/bin/grep -Eo '\b[0-9a-df-np-sv-z]{32}\b')"
-      my_specialisation="$(cat "/run/booted-system/specialisation-name" 2>/dev/null || true)"
-      echo "nixos_booted_system{version=\"$my_version_string\", hash=\"$my_hash_string\", specialisation=\"$my_specialisation\"} 1" > /run/prometheus-textfile-exports/booted_system.prom
+
+      if test -e /run/booted-system; then
+        my_version_string="$(cat "/run/booted-system/nixos-version")"
+        my_hash_string="$(readlink -f "/run/booted-system" | ${pkgs.gnugrep}/bin/grep -Eo '\b[0-9a-df-np-sv-z]{32}\b')"
+        my_specialisation="$(cat "/run/booted-system/specialisation-name" 2>/dev/null || true)"
+        echo "nixos_booted_system{version=\"$my_version_string\", hash=\"$my_hash_string\", specialisation=\"$my_specialisation\"} 1" > /run/prometheus-textfile-exports/booted_system.prom
+      fi
+
+      ${depot.nix.pkgs.nixos-size}/bin/nixos-size > /run/prometheus-textfile-exports/nixos_size.prom
     '';
     system.extraSystemBuilderCmds = lib.mkAfter ''
       echo "${if config.my.specialisationName == null then "" else config.my.specialisationName}" > $out/specialisation-name