diff --git a/esphome/components/noise/noise.cpp b/esphome/components/noise/noise.cpp
index 7434cb6489..f1cce31eea 100644
--- a/esphome/components/noise/noise.cpp
+++ b/esphome/components/noise/noise.cpp
@@ -27,9 +27,8 @@ void NoiseContext::load_psk(psk_t &out) const {
 }
 
 #ifdef USE_NOISE_SPARE_EPHEMERAL
-static constexpr size_t PRIVATE_KEY_SIZE = 32;
-static constexpr size_t PUBLIC_KEY_SIZE = 32;
-static_assert(PRIVATE_KEY_SIZE + PUBLIC_KEY_SIZE == SPARE_EPHEMERAL_SIZE);
+static constexpr size_t PRIVATE_KEY_SIZE = SPARE_EPHEMERAL_KEY_SIZE;
+static constexpr size_t PUBLIC_KEY_SIZE = SPARE_EPHEMERAL_KEY_SIZE;
 uint8_t spare_ephemeral[SPARE_EPHEMERAL_SIZE];  // NOLINT(cppcoreguidelines-avoid-non-const-global-variables)
 
 void prepare_spare_ephemeral() {
diff --git a/esphome/components/noise/noise.h b/esphome/components/noise/noise.h
index 5797402a7b..6090440e92 100644
--- a/esphome/components/noise/noise.h
+++ b/esphome/components/noise/noise.h
@@ -45,13 +45,14 @@ const LogString *noise_err_to_logstr(int err);
 // One responder ephemeral key pair generated ahead of time (about 60 ms on
 // ESP8266), refilled by the api server while idle and consumed by the next
 // handshake of any noise transport; an empty slot means the handshake
-// generates its own key.
+// generates its own key. The private key stays in RAM until consumed.
 // Private key then public key; zero when empty
-static constexpr size_t SPARE_EPHEMERAL_SIZE = 64;
+static constexpr size_t SPARE_EPHEMERAL_KEY_SIZE = 32;
+static constexpr size_t SPARE_EPHEMERAL_SIZE = 2 * SPARE_EPHEMERAL_KEY_SIZE;
 extern uint8_t spare_ephemeral[SPARE_EPHEMERAL_SIZE];  // NOLINT(cppcoreguidelines-avoid-non-const-global-variables)
 // Polled every api loop tick, so it must inline. A clamped X25519 private key
 // always has bit 254 set, so that byte doubles as the ready flag.
-inline bool has_spare_ephemeral() { return (spare_ephemeral[31] & 0x40) != 0; }
+inline bool has_spare_ephemeral() { return (spare_ephemeral[SPARE_EPHEMERAL_KEY_SIZE - 1] & 0x40) != 0; }
 /// Fill the slot; blocks for the base point multiply
 void prepare_spare_ephemeral();
 /// Hand the slot's key pair to a handshake that has not started and wipe the
diff --git a/tests/components/noise/test_noise_handshake.cpp b/tests/components/noise/test_noise_handshake.cpp
index 7e07b18cc3..6621aae23b 100644
--- a/tests/components/noise/test_noise_handshake.cpp
+++ b/tests/components/noise/test_noise_handshake.cpp
@@ -161,7 +161,7 @@ TEST(NoiseResponderHandshakeTest, FullHandshakeAndTransportRoundTrip) {
 // responder_e receives the ephemeral public key the responder put on the
 // wire (the clear text start of its message, taken before the initiator
 // consumes the buffer in place)
-static void run_handshake(NoiseResponderHandshake &responder, uint8_t responder_e[32]) {
+static void run_handshake(NoiseResponderHandshake &responder, uint8_t responder_e[SPARE_EPHEMERAL_KEY_SIZE]) {
   const psk_t psk = make_psk(7);
   ASSERT_EQ(responder.init(ctx_for(psk), PROLOGUE, sizeof(PROLOGUE)), 0);
   Initiator initiator(psk, PROLOGUE, sizeof(PROLOGUE));
@@ -170,8 +170,8 @@ static void run_handshake(NoiseResponderHandshake &responder, uint8_t responder_
   ASSERT_EQ(responder.read_message(msg, msg_len), 0);
   size_t reply_len = 0;
   ASSERT_EQ(responder.write_message(msg, sizeof(msg), reply_len), 0);
-  ASSERT_GE(reply_len, 32u);
-  std::memcpy(responder_e, msg, 32);
+  ASSERT_GE(reply_len, SPARE_EPHEMERAL_KEY_SIZE);
+  std::memcpy(responder_e, msg, SPARE_EPHEMERAL_KEY_SIZE);
   ASSERT_EQ(initiator.read_message(msg, reply_len), 0);
   ASSERT_EQ(responder.action(), Action::ACTION_SPLIT);
 }
@@ -179,7 +179,7 @@ static void run_handshake(NoiseResponderHandshake &responder, uint8_t responder_
 TEST(SpareEphemeralTest, EmptySlotLeavesHandshakeToGenerate) {
   ASSERT_FALSE(has_spare_ephemeral());
   NoiseResponderHandshake responder;
-  uint8_t responder_e[32];
+  uint8_t responder_e[SPARE_EPHEMERAL_KEY_SIZE];
   run_handshake(responder, responder_e);
   EXPECT_FALSE(has_spare_ephemeral());
 }
@@ -209,11 +209,11 @@ TEST(SpareEphemeralTest, ConsumeHandsTheKeyToANewState) {
 TEST(SpareEphemeralTest, SlotKeyIsOnTheWireAndConsumedOnce) {
   prepare_spare_ephemeral();
   ASSERT_TRUE(has_spare_ephemeral());
-  uint8_t expected_pub[32];
-  std::memcpy(expected_pub, spare_ephemeral + 32, sizeof(expected_pub));
+  uint8_t expected_pub[SPARE_EPHEMERAL_KEY_SIZE];
+  std::memcpy(expected_pub, spare_ephemeral + SPARE_EPHEMERAL_KEY_SIZE, sizeof(expected_pub));
 
   NoiseResponderHandshake first;
-  uint8_t responder_e[32];
+  uint8_t responder_e[SPARE_EPHEMERAL_KEY_SIZE];
   run_handshake(first, responder_e);
   // The spare, not a generated key, went out; and it went out once
   EXPECT_EQ(std::memcmp(responder_e, expected_pub, sizeof(expected_pub)), 0);