# From configuration.nix
  systemd.user.services.batteryMonitor = {
    path = [ pkgs.bash pkgs.acpi pkgs.libnotify ];
    wantedBy = [ "multi-user.target" ];
    description = "Notifies when battery is low and suspends";
    serviceConfig = {
      Type = "simple";
      ExecStart = "${pkgs.bash}/bin/bash %h/.wm-scripts/battery-monitor.sh";
    };
  };

  systemd.user.timers.batteryMonitor = {
    timerConfig = {
      OnUnitInactiveSec = "2s";
      AccuracySec = "1s";
    };
    wantedBy = [ "timers.target" ];
  };


λ systemctl --user cat batteryMonitor
# /nix/store/bbakz8q4wcyhbh43n86f5z07i8jlsadv-unit-batteryMonitor.service/batteryMonitor.service
[Unit]
Description=Notifies when battery is low and suspends

[Service]
Environment="LOCALE_ARCHIVE=/nix/store/1xpr86xg998h5acn4zqrx58xjdjqnnds-glibc-locales-2.31/lib/locale/local>
Environment="PATH=/nix/store/2jysm3dfsgby5sw5jgj43qjrb5v79ms9-bash-4.4-p23/bin:/nix/store/891p25c0g3acijh6g>
Environment="TZDIR=/nix/store/xg8nmhp28a5xf6ifgg8n93nrwbp8ghvm-tzdata-2019c/share/zoneinfo"

ExecStart=/nix/store/2jysm3dfsgby5sw5jgj43qjrb5v79ms9-bash-4.4-p23/bin/bash %h/.wm-scripts/battery-monitor.>
Type=simple

λ systemctl --user enable batteryMonitor
The unit files have no installation config (WantedBy=, RequiredBy=, Also=,
Alias= settings in the [Install] section, and DefaultInstance= for template
units). This means they are not meant to be enabled using systemctl.