Case studies
Evidence based solutions to domestic energy questions involving battery payback, panel counts, north-facing roofs, heat pump bills, and when extra PV is worth the cost.
Case study archive
A practical walkthrough based on a specific home and business questions.
Five reports from 1 kW to 16 kW design heat loss show why annual space heating is not proportional to fabric loss once solar gain, occupant heat, and standby heat are included.
Heat loss and running costsSix reports compare Flux and a Cosy-style heat pump tariff at 5, 10 and 15 kWh — annual bills, winter versus summer costs, and which tariff suits smaller batteries.
Flux vs Cosy by battery sizeSix reports compare domestic morning/evening demand with business-hours demand, showing SOC/load profiles, export-rate sensitivity, battery price ranges, and payback.
Business battery paybackFour reports compare no battery, a 5 kWh pack, off-peak night charging, and battery-only (no solar) for 3.6 kW solar and 3,500 kWh/year use — bills, payback, and export risk.
Solar battery worth it?A look into whether adding additional batteries to an existing 23 kWh battery stack makes sense
Extra battery paybackA month-by-month comparison of Solar Butter calculator output against actual home app readings for PV generation, grid import, export, load, and heat pump electricity.
Calculator vs actual homeThese two case studies compare the same 10 kW array on two roof layouts: south-facing and east/west. They show how roof direction changes clipping and how much money a smaller inverter can cost over a year.
A 2,800 kWh/year home case study comparing three off-grid-style options, showing that a much larger array is what finally removes winter imports in the model.
Off-grid sizingHistoric simulations for the same 8 kW south-facing array and battery: a 3.68 kW export-limited inverter versus an 8 kW inverter, trimming grid export by about 880 kWh/year and total benefit by about £106/year.
3.68 kW export limit comparisonRegional heat pump running costs compared for London, Glasgow, Gloucester, Paris, Marseille, and California, including 10 south-facing solar panels, a battery, household load, and report downloads.
Heat pump running costsA pre-1920s house compared with newer and low-carbon properties, showing how insulation, radiator flow temperature, solar, batteries, and Economy 7 change heat pump running costs.
Old house heat pumpsA large-house case study comparing Cosy against an EV tariff for a 9.9 kW heat loss home with 8 kW of solar and a 32 kWh battery.
Heat pump vs EV tariffA beginner-friendly case study using an 8-panel south-facing roof model to show what happens annually, what still happens in winter, and how to think about payback.
8 south panels, winterA comparison of the current system and a north-roof upgrade, based on two reports, showing why the payback still looks unattractive in this particular instance.
North-facing roofA comparison of a heat pump only home against a PV, battery, and Cosy tariff setup, showing how far the annual bill can be pushed down.
Heat pump + PV billsA comparison between a quoted 16-panel system against a 20-panel version, showing the extra 4 panels add about £210/year and are mostly an export-value decision.
More panels retrofit system