diff --git a/backend/src/lib/cost-calc.js b/backend/src/lib/cost-calc.js
index 2731509..6e110f7 100644
--- a/backend/src/lib/cost-calc.js
+++ b/backend/src/lib/cost-calc.js
@@ -162,7 +162,7 @@ export function computeCost({ tariff, windows, consumption, daysInPeriod }) {
   if (tariff.is_time_of_use && windows.length > 1 && tariff.fallback_split_pct && Object.keys(tariff.fallback_split_pct).length) {
     split = {}
     for (const [key, pct] of Object.entries(tariff.fallback_split_pct)) {
-      const win = windows.find(w => w.label.toLowerCase().includes(key.toLowerCase()))
+      const win = windows.find(w => w.label.toLowerCase() === key.toLowerCase())
       if (!win) continue
       const share = consumption * (Number(pct) / 100)
       const cost = share * Number(win.unit_rate_pence_per_unit)
diff --git a/backend/src/routes/tariffs.js b/backend/src/routes/tariffs.js
index 3c9285e..c3d919b 100644
--- a/backend/src/routes/tariffs.js
+++ b/backend/src/routes/tariffs.js
@@ -1,6 +1,16 @@
 import { requireAuth, requireCap } from '../auth.js'
 import { pool } from '../db.js'
 
+// Mirrors computeCost()'s condition in cost-calc.js — the split is only actually
+// applied (and only actually needs to sum to 100) once there's more than one
+// window to divide consumption across.
+function splitPctError(isTimeOfUse, fallbackSplitPct, windowCount) {
+  if (!isTimeOfUse || windowCount <= 1 || !fallbackSplitPct || !Object.keys(fallbackSplitPct).length) return null
+  const total = Object.values(fallbackSplitPct).reduce((s, v) => s + Number(v), 0)
+  if (Math.round(total) !== 100) return `Fallback split must total 100% (currently ${total}%)`
+  return null
+}
+
 export async function tariffRoutes(app) {
   app.addHook('preHandler', requireAuth)
 
@@ -45,6 +55,8 @@ export async function tariffRoutes(app) {
     if (!Array.isArray(b.windows) || b.windows.length === 0) {
       return reply.status(400).send({ error: 'At least one rate window is required' })
     }
+    const splitErr = splitPctError(b.is_time_of_use === true, b.fallback_split_pct, b.windows.length)
+    if (splitErr) return reply.status(400).send({ error: splitErr })
 
     const client = await pool.connect()
     try {
@@ -84,6 +96,19 @@ export async function tariffRoutes(app) {
     if (!existing.length) return reply.status(404).send({ error: 'Tariff not found' })
     const t = existing[0]
     const b = req.body || {}
+
+    const isTimeOfUse = b.is_time_of_use ?? t.is_time_of_use
+    const fallbackSplitPct = b.fallback_split_pct ?? t.fallback_split_pct
+    let windowCount
+    if (Array.isArray(b.windows)) {
+      windowCount = b.windows.length
+    } else {
+      const { rows: wc } = await pool.query('SELECT COUNT(*)::int AS n FROM tariff_rate_windows WHERE tariff_id = $1', [req.params.id])
+      windowCount = wc[0].n
+    }
+    const splitErr = splitPctError(isTimeOfUse, fallbackSplitPct, windowCount)
+    if (splitErr) return reply.status(400).send({ error: splitErr })
+
     const { rows } = await pool.query(
       `UPDATE tariffs SET name = $1, supplier = $2, effective_from = $3, effective_to = $4,
                           standing_charge_pence_per_day = $5, ccl_rate_pence_per_unit = $6, ccl_exempt = $7,
diff --git a/frontend/src/pages/Tariffs.tsx b/frontend/src/pages/Tariffs.tsx
index a4c9a62..da803d9 100644
--- a/frontend/src/pages/Tariffs.tsx
+++ b/frontend/src/pages/Tariffs.tsx
@@ -93,6 +93,10 @@ export default function Tariffs() {
   async function save() {
     if (!form) return
     if (!form.name.trim() || !form.category_id) { setError('Name and category are required'); return }
+    if (form.is_time_of_use && form.windows.length > 1) {
+      const total = form.windows.reduce((s, w) => s + (Number(w.split_pct) || 0), 0)
+      if (Math.round(total) !== 100) { setError(`Fallback split must total 100% (currently ${total}%)`); return }
+    }
     setSaving(true)
     setError(null)
     try {
@@ -271,7 +275,13 @@ export default function Tariffs() {
 
             <div className="modal-actions">
               <button className="btn" onClick={() => setForm(null)}>Cancel</button>
-              <button className="btn btn-primary" onClick={save} disabled={saving}>{saving ? 'Saving…' : 'Save'}</button>
+              <button
+                className="btn btn-primary"
+                onClick={save}
+                disabled={saving || (form.is_time_of_use && form.windows.length > 1 && Math.round(splitTotal) !== 100)}
+              >
+                {saving ? 'Saving…' : 'Save'}
+              </button>
             </div>
           </div>
         </div>
diff --git a/seed-app.js b/seed-app.js
index b7938c4..c467dbf 100644
--- a/seed-app.js
+++ b/seed-app.js
@@ -10,7 +10,7 @@ const pool = new Pool({ connectionString: process.env.DATABASE_URL })
 
 await pool.query(`
   INSERT INTO apps (slug, name, description, base_path, icon, theme_color, category, internal_host, internal_port)
-  VALUES ('utilities', 'Utilities', 'Meter readings, tariffs and energy cost tracking', '/utilities', 'Zap', '#1e6091', 'Operations', '10.10.10.127', 3080)
+  VALUES ('utilities', 'Utilities', 'Meter readings, tariffs and energy cost tracking', '/utilities', 'Zap', '#1e6091', 'Hotel', '10.10.10.127', 3080)
   ON CONFLICT (slug) DO UPDATE SET
     name          = EXCLUDED.name,
     description   = EXCLUDED.description,