Solar & battery system calculator
Model your panels, an optional battery and EV, and your real tariff together for every 30 minutes of a whole year. You'll see how self-sufficient you'd be, what you'd save, and how long the system would take to pay for itself.
How this is calculated
Each array's yearly generation comes from long-term satellite solar data for your location, and a typical weather year from the same data sets the timing through the year, split into half-hour periods for tariffs, inverter limits, household use, EV charging and battery state of charge. The battery retains its state between periods and seasons rather than restarting each day. Energy the battery takes from the grid counts as grid when your home uses it, so self-sufficiency only counts your own solar.
Your "current rate today" and the post-install import rate are asked for separately on purpose - lots of people switch tariff or supplier alongside installing, so using the same figure for both would understate or overstate the saving. Leave them the same if you're not planning to switch. The first-year figures use current output; the long-term projection applies your chosen degradation assumptions. Usage and weather are held constant while prices change by the percentage you enter in Financials. Treat this as a planning estimate rather than a quote.
Common questions
What panel technology does the calculator use?
This calculator is for new installations, so it uses the newer crystalline-silicon panel model. There's no panel type option, because older panel types would only add confusion when comparing a current quote.
How is this different from the other two calculators?
The Annual Generation calculator covers panels only, and the Battery-Only calculator covers a battery on its own - neither combines the two with a real tariff. This one models an array, an optional battery, and your import/export rates together in one simulation, so the self-consumption and cost figures come from how they actually interact, not three separate numbers added up by hand.
Do I have to include a battery?
No - leave the battery box unticked to model panels alone against your real tariff, or tick it to see how much difference adding one makes to the same system.
How is the self-consumption split calculated?
We work through every 30 minutes of a whole year, which is 17,520 steps. Your solar output follows a typical weather year for your postcode, scaled to match the expected yearly output of each array. Each step is matched against what your home is using at that time of day, with more used on winter days than summer ones, as measured in real homes. The battery keeps its charge from one step to the next, so what it stores on a sunny afternoon is there for your evening, and a run of dull winter days shows up as well. Anything the battery takes from the grid is counted as grid when your home uses it. Cheap overnight rates, inverter and export limits all feed into the results too.
Why does this ask for two different tariff rates?
Your "current rate today" sets the no-solar baseline you're compared against, while the import rate is what you'd pay on any grid shortfall once the system is installed - often a different tariff or supplier. Set them the same if you're not planning to switch.
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Solar, battery, car charging and energy tariff costs, all in Wattlas.
