Why a battery only pays back when it's sized correctly
The maths on a home battery is simple once you write it down. You buy electricity cheaply (off-peak grid, or your own solar) and you avoid buying electricity expensively (evening peak, especially Nov–Feb 4–7pm).
The pay-back depends on three things: tariff differential (how big the gap is between cheap and expensive electricity), battery size relative to your evening demand (too small and it empties before peak ends; too big and you've paid for capacity you don't use), and round-trip efficiency (95% on modern lithium-iron-phosphate batteries).
Most quotes get this wrong by selling you a size that fits a round number, not your actual demand.
How Thermova sizes a battery
We start with twelve months of your half-hourly electricity data (you can pull this from your smart meter app or we'll request it from your supplier). We model your evening peak demand under your existing tariff, then again under each viable time-of-use tariff (Octopus Cosy, Intelligent Octopus Go, etc.).
From that we work out the battery size that covers your typical winter evening peak from the bottom of the discharge curve — and where adding more capacity stops paying back.
What's included as standard
| Demand analysis | Twelve months of smart-meter data modelled against current and time-of-use tariffs |
|---|---|
| Battery unit | LFP (lithium-iron-phosphate) battery sized to your evening peak (typical: 5kWh–13kWh) |
| Inverter | Hybrid inverter if pairing with solar; AC-coupled inverter if retrofit to existing solar; standalone if grid-only |
| Smart-tariff integration | Battery scheduled to charge off-peak and discharge during peak automatically |
| DNO application | G98/G99 application handled on your behalf |
| Monitoring app | See live state of charge, daily savings, and payback tracking |
| Commissioning | Performance check on the day, walkthrough on the app, handover paperwork |
The two pay-back routes
Route 1: Solar + battery
You generate excess solar during the day, store it, and use it in the evening. Best for households with daytime emptiness and evening loads (heat pump, cooking, EV charging).
Route 2: Time-of-use tariff arbitrage
You charge the battery from the grid at cheap off-peak rates (some tariffs go below 8p/kWh) and discharge during peak (often 30p+ /kWh). Works without solar — useful for homes where roof orientation doesn't suit panels.
Most installs use a hybrid of both routes: solar fills the battery in summer, off-peak grid in winter.
How we differ from a typical battery quote
- Sized on your data, not a brochure. We use twelve months of half-hourly data, not a guess from your floor area.
- Tariff-aware design. We model the battery against current Octopus, EDF and British Gas time-of-use tariffs to show you the real pay-back.
- Works with what's already there. AC-coupled retrofit available if you already have a solar inverter you don't want to rip out.
- Founder-led. Graham signs off the design.
Frequently asked questions
Do I need solar to get a battery?
How big should my battery be?
What batteries do you install?
Will my battery work in a power cut?
What's the typical pay-back period?
Can a battery be added to my existing solar?
Explore further
Next step
See if a battery actually pays back for your home
Book a free assessment. We model your half-hourly data against the current best time-of-use tariffs and give you a real pay-back number — not a brochure figure.