diff --git a/Cargo.lock b/Cargo.lock
index 9ecc0aa..cddd331 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2859,6 +2859,7 @@ dependencies = [
  "serde",
  "serde_json",
  "sysinfo",
+ "tempfile",
  "thiserror 2.0.18",
 ]
 
diff --git a/crates/core/Cargo.toml b/crates/core/Cargo.toml
index 1ac7d0d..4fae7aa 100644
--- a/crates/core/Cargo.toml
+++ b/crates/core/Cargo.toml
@@ -11,3 +11,6 @@ thiserror = "2"
 sysinfo = "0.35"
 async-trait = "0.1"
 num_cpus = "1"
+
+[dev-dependencies]
+tempfile = "3"
diff --git a/crates/core/src/lib.rs b/crates/core/src/lib.rs
index 3845d15..1e48006 100644
--- a/crates/core/src/lib.rs
+++ b/crates/core/src/lib.rs
@@ -5,6 +5,7 @@ pub mod model_registry;
 pub mod paths;
 pub mod process_watch;
 pub mod recommendation;
+pub mod power;
 pub mod types;
 
 pub use error::{MagnotiaError, Result};
diff --git a/crates/core/src/power.rs b/crates/core/src/power.rs
new file mode 100644
index 0000000..37000f0
--- /dev/null
+++ b/crates/core/src/power.rs
@@ -0,0 +1,27 @@
+//! Power-state probe for inference thread tuning.
+//!
+//! Reports whether the machine is running on AC or battery so callers
+//! can drop thread counts when energy matters more than throughput.
+//! Linux uses the documented sysfs ABI under
+//! `/sys/class/power_supply/`. macOS and Windows return `Unknown` for
+//! now; native probes (IOPSGetProvidingPowerSourceType,
+//! GetSystemPowerStatus) are deferred.
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum PowerState {
+    OnAc,
+    OnBattery,
+    Unknown,
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn power_state_variants_are_distinct() {
+        assert_ne!(PowerState::OnAc, PowerState::OnBattery);
+        assert_ne!(PowerState::OnAc, PowerState::Unknown);
+        assert_ne!(PowerState::OnBattery, PowerState::Unknown);
+    }
+}