Decision-grade spreadsheet models for EV, solar, battery and heat pump investments — comparing real cashflows over time and showing which assumptions actually drive the outcome.
Built for analytical homeowners — the kind of people who want to see the assumptions, not receive a pitch.
Decisions we're modelling first
Most calculators give you one answer. We show you the three variables that actually determine whether that answer changes. That's a more useful frame for a €30–80k capital decision.
Nominal annual cashflows for every year of your ownership horizon — fuel, energy, maintenance, financing, residual value. No black-box outputs. Every number is traceable.
NPV comparison, cost-per-kilometre, cumulative cost crossover — presented together so you understand the full financial picture, not just when the lines cross.
Change one variable — mileage, electricity price, fuel price, residual value — and see how much the conclusion shifts. Surfaces the 2–3 drivers that actually matter for your scenario.
What this is — a capital allocation model for EV vs ICE decisions with charging strategy and solar interaction, built for people who prefer to understand their inputs and challenge their assumptions rather than trust a number from a manufacturer's website.
How it's built — explicit cashflows in nominal €, a small scenario set, and "what must be true" sensitivity so you can see which conditions keep the decision rational — and which ones flip it.
Market presets included — Ireland, UK, Germany, Netherlands and Generic EU quick-start defaults, all user-editable with guidance on where to find your own current rates.
No affiliate links. No installer referrals. No manufacturer partnerships. The model's job is to give you an honest answer — whatever that happens to be.
Blue cells are inputs. Every formula is open. The methodology doc explains every modelling choice — including the deliberate limitations. No black boxes.
Solar changes the EV economics meaningfully. Most tools model them in isolation. We treat EV charging strategy, solar yield, and grid tariff as one interconnected decision.
The methodology document clearly states what the model doesn't do — no hourly dispatch, no live API data, no automated incentive fetching. Knowing the limits is part of using it well.
WattDecide is built by professionals with over 17 years of analytical experience in the renewable energy sector — energy modelling for wind, solar and storage projects at a commercial scale.
We also own EVs, solar panels, battery storage and the full smart-home setup. We built WattDecide because the tools available to homeowners making these decisions were — with few exceptions — either marketing tools dressed up as calculators, or technically sound but impossible to use without a finance background. We wanted something in between: rigorous, transparent, and usable by anyone prepared to engage with the numbers.
Everything you need to model your own household energy decision — download once, use forever, update the inputs whenever your situation changes.
One structured support interaction — a 20-minute model review or a detailed email walkthrough — to help you apply the model to your specific situation. Limited availability.
Refunds: if the model isn't a fit, email within 7 days of purchase with your reason. Refunds are at our discretion to prevent abuse of digital downloads.
WattDecide is for decision support and education — not financial advice.
Launching soon
No drip campaigns. No weekly newsletters. One notification when v1 ships — then it's your call.
Unsubscribe any time. No affiliate links. No installer leads. Ever.