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Octopus Cosy vs Flux Tariff Comparator UK
Side-by-side comparison of Octopus Cosy (heat-pump-tuned), Flux (battery-tuned) and the standard variable. Annual £, peak-window share, export income.
Interactive Cosy vs Flux tariff comparator
Worked example
A heat-pump household using 4,500 kWh/yr, with ~30% of consumption shifted into the Cosy cheap windows by timer, no battery, no solar export:
- Standard variable
- £1439/yr
- Octopus Cosy
- £1283/yr
- Octopus Flux
- £1399/yr
- Cheapest
- Cosy
You'll probably want this next
Cosy works for heat pumps; Flux works for batteries
Pair the right kit with the right tariff. Start with the heat pump or battery decision and the tariff follows.
Heat pump payback →Cosy and Flux do different jobs
Cosy is built for heat pumps. It gives you three cheap windows per day (4–7am, 1–4pm, 10pm–midnight) where the unit rate drops to roughly half the standard rate, plus a 4–7pm peak rate that's higher to discourage use. If your heat pump is on a timer and a hot-water cylinder is sized to charge during cheap windows, you can shift 35–45% of consumption into the cheap rate — a real saving of £200–500/yr versus a flat tariff.
Flux is built for batteries (and ideally solar). It pays the highest UK domestic export rate during 4–7pm peak (28p/kWh), so a battery that filled overnight and exports during the peak window earns a wide arbitrage spread. Pair it with a south-facing solar array and the export income compounds. Without a battery, Flux is roughly equivalent to a flat tariff for most homes — the peak import rate cancels the off-peak savings.
Quick decision rule
- Heat pump, no battery, no solar → Cosy. Stack the heating timer with the cheap windows.
- Battery + solar (or solar+EV) → Flux. The peak export rate is unmatched in the UK domestic market.
- Battery, no solar → Flux still wins thanks to arbitrage; double-check with the battery payback calculator .
- None of the above → Stay on flat or look at Agile , which has a wider potential spread but more variance.
Frequently asked questions
- What are the Cosy cheap windows?
- Octopus Cosy gives you three cheap-rate windows per day — typically 4–7am, 1–4pm, and 10pm–midnight. Total: 9 hours/day at half the standard rate. The 4–7pm peak rate is higher to discourage use during grid stress. With a heat-pump timer charging the hot water cylinder during cheap windows, you can comfortably shift 35–45% of consumption.
- Is Flux better than Agile for batteries?
- Different shapes. Agile is fully dynamic — half-hourly variable rates that can occasionally go negative. Flux is banded — three fixed rates a day for both import and export. Flux export is the highest fixed rate in the UK domestic market (28p peak). Agile can beat Flux on optimal weeks but underperform on others. See our [Octopus Agile savings calculator for the dynamic version.
- Do I need a smart meter for Cosy or Flux?
- Yes, half-hourly readings via a SMETS2 smart meter (or upgraded SMETS1). If yours is in 'dumb' mode, get Octopus to fix that first. Both tariffs require an active smart meter import-export connection.
- What's the standing-charge difference?
- Cosy and Flux standing charges sit close to each other (around 60p/day in 2026) — and close to the Ofgem cap. We treat them as equivalent for simplicity. The difference between standing charges contributes about £20–40/yr at most, which is small relative to unit-rate savings.
- Can I switch between Cosy and Flux?
- Yes, but each switch is processed in 14–28 days and you'll see one-month gaps in the data. If your circumstances change (you install a battery, swap to a heat pump), the switch is usually worth doing. Don't switch monthly to chase savings — Octopus may flag it. Consider a [home battery only after you've decided on the tariff.
What v1 doesn't model
- Day-to-day variation. Both tariffs have the same banded rates every day; this calculator uses a steady-state annual model.
- Standing charge variation. Cosy and Flux standing charges sit close to each other; we treat them as identical for simplicity.
- Octopus Saving Sessions / DFS. Both tariffs let you opt in to demand-flexibility events — typically £30–80/yr extra. Not modelled.
- Smart-meter requirement. Both tariffs require a smart meter sending half-hourly data. If yours is in dumb mode, you're not eligible until that's resolved.
Related calculators
Ranked by topic overlap with this tool.
- Solar Panel Savings Calculator UK — Full scenario comparison with battery modelling, SEG export income and 25-year stress tests.
- Octopus Agile Savings Calculator UK — Agile vs flat-tariff savings by DNO region, with load-shift modelling and cheap-window detection.
- Solar + Battery + EV Combined Savings Calculator UK — Combined savings from solar + battery + EV charging — compounding interactions, 15-year payback on the whole stack.
- Solar Export Tariff Comparator UK — Compare UK SEG tariffs side-by-side — Octopus, E.ON, British Gas, EDF and more. See exactly what each pays for your export kWh.
- Home Battery Payback Calculator UK (No Solar) — Standalone home battery payback for UK households on Octopus Flux, Cosy or Go — the arbitrage maths without solar. Tariff presets, capacity sizing, 10-year net.