diff --git a/sys/dev/acpica/acpi.c b/sys/dev/acpica/acpi.c
--- a/sys/dev/acpica/acpi.c
+++ b/sys/dev/acpica/acpi.c
@@ -732,7 +732,8 @@
 	goto out;
 
     /* Register ACPI again to pass the correct argument of pm_func. */
-    power_pm_register(POWER_PM_TYPE_ACPI, acpi_pm_func, sc);
+    power_pm_register(POWER_PM_TYPE_ACPI, acpi_pm_func, sc,
+	acpi_supported_stypes);
 
     acpi_platform_osc(dev);
 
@@ -4740,7 +4741,8 @@
     if (!cold || resource_disabled("acpi", 0))
 	return;
 
-    power_pm_register(POWER_PM_TYPE_ACPI, acpi_pm_func, NULL);
+    power_pm_register(POWER_PM_TYPE_ACPI, acpi_pm_func, NULL,
+	acpi_supported_stypes);
 }
 
 SYSINIT(power, SI_SUB_KLD, SI_ORDER_ANY, acpi_pm_register, NULL);
diff --git a/sys/kern/subr_power.c b/sys/kern/subr_power.c
--- a/sys/kern/subr_power.c
+++ b/sys/kern/subr_power.c
@@ -39,13 +39,14 @@
 #include <sys/systm.h>
 #include <sys/taskqueue.h>
 
-enum power_stype	 power_standby_stype	= POWER_STYPE_STANDBY;
-enum power_stype	 power_suspend_stype	= POWER_STYPE_SUSPEND_TO_IDLE;
-enum power_stype	 power_hibernate_stype	= POWER_STYPE_HIBERNATE;
+enum power_stype	 power_standby_stype	= POWER_STYPE_UNKNOWN;
+enum power_stype	 power_suspend_stype	= POWER_STYPE_UNKNOWN;
+enum power_stype	 power_hibernate_stype	= POWER_STYPE_UNKNOWN;
 
 static u_int		 power_pm_type	= POWER_PM_TYPE_NONE;
 static power_pm_fn_t	 power_pm_fn	= NULL;
 static void		*power_pm_arg	= NULL;
+static bool		 power_pm_supported[POWER_STYPE_COUNT] = {0};
 static struct task	 power_pm_task;
 
 enum power_stype
@@ -70,6 +71,26 @@
 	return (power_stype_names[stype]);
 }
 
+static int
+sysctl_supported_stypes(SYSCTL_HANDLER_ARGS)
+{
+	int error;
+	struct sbuf sb;
+	enum power_stype stype;
+
+	sbuf_new(&sb, NULL, 32, SBUF_AUTOEXTEND);
+	for (stype = 0; stype < POWER_STYPE_COUNT; stype++) {
+		if (power_pm_supported[stype])
+			sbuf_printf(&sb, "%s ", power_stype_to_name(stype));
+	}
+	sbuf_trim(&sb);
+	sbuf_finish(&sb);
+	error = sysctl_handle_string(oidp, sbuf_data(&sb), sbuf_len(&sb), req);
+	sbuf_delete(&sb);
+
+	return (error);
+}
+
 int
 power_sysctl_stype(SYSCTL_HANDLER_ARGS)
 {
@@ -86,7 +107,8 @@
 
 		if (new_stype == POWER_STYPE_UNKNOWN)
 			return (EINVAL);
-		/* TODO Check to see if the new stype is supported. */
+		if (!power_pm_supported[new_stype])
+			return (EOPNOTSUPP);
 		if (new_stype != old_stype)
 			*(enum power_stype *)oidp->oid_arg1 = new_stype;
 	}
@@ -97,6 +119,9 @@
 static SYSCTL_NODE(_kern, OID_AUTO, power, CTLFLAG_RW, 0,
     "Generic power management related sysctls");
 
+SYSCTL_PROC(_kern_power, OID_AUTO, supported_stype,
+    CTLTYPE_STRING | CTLFLAG_RD, 0, 0, sysctl_supported_stypes, "A",
+    "List supported sleep types.");
 SYSCTL_PROC(_kern_power, OID_AUTO, standby, CTLTYPE_STRING | CTLFLAG_RW,
     &power_standby_stype, 0, power_sysctl_stype, "A",
     "Sleep type to enter on standby");
@@ -116,7 +141,8 @@
 }
 
 int
-power_pm_register(u_int pm_type, power_pm_fn_t pm_fn, void *pm_arg)
+power_pm_register(u_int pm_type, power_pm_fn_t pm_fn, void *pm_arg,
+    bool pm_supported[POWER_STYPE_COUNT])
 {
 	int	error;
 
@@ -125,6 +151,16 @@
 		power_pm_type	= pm_type;
 		power_pm_fn	= pm_fn;
 		power_pm_arg	= pm_arg;
+		memcpy(power_pm_supported, pm_supported,
+		    sizeof(power_pm_supported));
+		if (power_pm_supported[POWER_STYPE_STANDBY])
+			power_standby_stype = POWER_STYPE_STANDBY;
+		if (power_pm_supported[POWER_STYPE_SUSPEND_TO_IDLE])
+			power_suspend_stype = POWER_STYPE_SUSPEND_TO_IDLE;
+		else if (power_pm_supported[POWER_STYPE_SUSPEND_TO_MEM])
+			power_suspend_stype = POWER_STYPE_SUSPEND_TO_MEM;
+		if (power_pm_supported[POWER_STYPE_HIBERNATE])
+			power_hibernate_stype = POWER_STYPE_HIBERNATE;
 		error = 0;
 		TASK_INIT(&power_pm_task, 0, power_pm_deferred_fn, NULL);
 	} else {
diff --git a/sys/sys/power.h b/sys/sys/power.h
--- a/sys/sys/power.h
+++ b/sys/sys/power.h
@@ -93,7 +93,8 @@
 
 typedef int (*power_pm_fn_t)(u_long _cmd, void* _arg, enum power_stype _stype);
 extern int	 power_pm_register(u_int _pm_type, power_pm_fn_t _pm_fn,
-			void *_pm_arg);
+			void *_pm_arg,
+			bool _pm_supported[static POWER_STYPE_COUNT]);
 extern u_int	 power_pm_get_type(void);
 extern void	 power_pm_suspend(int);