diff --git a/src/cost-calculator.js b/src/cost-calculator.js
index ad8f3c9..f4e3cec 100644
--- a/src/cost-calculator.js
+++ b/src/cost-calculator.js
@@ -1,26 +1,33 @@
 'use strict';
 
-const fs = require('fs');
+const fs = require('fs').promises;
 const path = require('path');
 
 /**
  * Load rate configuration from rates.json.
  *
- * @returns {Object} rate configuration
+ * @returns {Promise<Object>} rate configuration
  */
-function loadRatesConfig() {
+async function loadRatesConfig() {
   const cfgPath = path.join(__dirname, 'rates.json');
   try {
-    const data = fs.readFileSync(cfgPath, 'utf8');
+    const data = await fs.readFile(cfgPath, 'utf8');
     return JSON.parse(data);
   } catch (e) {
     if (e.code === 'ENOENT') {
       return {};
     }
     if (e instanceof SyntaxError) {
-      console.error(`Invalid JSON in rate configuration: ${e.message}`);
+      console.error('Invalid JSON in rate configuration', {
+        path: cfgPath,
+        error: e.message,
+      });
     } else {
-      console.error(`Error loading rate configuration: ${e.message}`);
+      console.error('Error loading rate configuration', {
+        path: cfgPath,
+        error: e.message,
+        code: e.code,
+      });
     }
     throw e;
   }
diff --git a/test/unit/calculator.test.js b/test/unit/calculator.test.js
index 2f308da..e277637 100644
--- a/test/unit/calculator.test.js
+++ b/test/unit/calculator.test.js
@@ -3,14 +3,14 @@ const fs = require('fs');
 const path = require('path');
 const { calculateCharges, loadRatesConfig } = require('../../src/cost-calculator');
 
-function testFileConfig() {
+async function testFileConfig() {
   const usage = [
     { account: 'education', date: '2024-01-15', core_hours: 100, gpu_hours: 10 },
     { account: 'research', date: '2024-02-01', core_hours: 50, gpu_hours: 5 },
     { account: 'special', date: '2024-02-01', core_hours: 100, gpu_hours: 20 },
     { account: 'other', date: '2024-02-01', core_hours: 10 }
   ];
-  const config = loadRatesConfig();
+  const config = await loadRatesConfig();
   const charges = calculateCharges(usage, config);
   assert.strictEqual(charges['2024-01'].education.cost, (100 * 0.015 + 10 * 0.15) * 0.5);
   assert.strictEqual(charges['2024-02'].research.cost, 50 * 0.01 + 5 * 0.1);
@@ -42,24 +42,24 @@ function testInvalidUsageIgnored() {
   assert.deepStrictEqual(charges, {});
 }
 
-function testMissingConfig() {
+async function testMissingConfig() {
   const cfgPath = path.join(__dirname, '../../src/rates.json');
   const backup = cfgPath + '.bak';
   fs.renameSync(cfgPath, backup);
   try {
-    const cfg = loadRatesConfig();
+    const cfg = await loadRatesConfig();
     assert.deepStrictEqual(cfg, {});
   } finally {
     fs.renameSync(backup, cfgPath);
   }
 }
 
-function testInvalidConfig() {
+async function testInvalidConfig() {
   const cfgPath = path.join(__dirname, '../../src/rates.json');
   const original = fs.readFileSync(cfgPath, 'utf8');
   fs.writeFileSync(cfgPath, '{ invalid json', 'utf8');
   try {
-    assert.throws(() => loadRatesConfig(), SyntaxError);
+    await assert.rejects(loadRatesConfig, SyntaxError);
   } finally {
     fs.writeFileSync(cfgPath, original, 'utf8');
   }
@@ -102,19 +102,22 @@ function testRoundingTotals() {
   assert.strictEqual(june.round.cost, 0.22);
 }
 
-function run() {
-  testFileConfig();
+async function run() {
+  await testFileConfig();
   testPassedConfig();
   testInvalidUsageIgnored();
-  testMissingConfig();
-  testInvalidConfig();
+  await testMissingConfig();
+  await testInvalidConfig();
   testNegativeInputs();
   testRoundingTotals();
   console.log('All calculator tests passed.');
 }
 
 if (require.main === module) {
-  run();
+  run().catch(err => {
+    console.error(err);
+    process.exit(1);
+  });
 }
 
 module.exports = run;