Answer three quick questions and see a starting point for your system size and battery count. This is an educational tool to frame the conversation, not a formal design recommendation. A real design should factor your utility provider, rate plan, roof layout, and actual energy usage.
The tool turns three numbers into a starting point. It is intentionally simple and conservative; a full design models your actual hour-by-hour usage.
| Step | Logic |
|---|---|
| System size | (Annual usage × offset target) ÷ roof yield, then rounded to the nearest whole 400W panel so the kW and panel count always match. Savings targets 80% offset; Independence targets 120% to cover battery round-trip losses and overnight charging; Custom uses your chosen offset (30 to 150%). |
| Batteries (Savings) | Sized to cover your evening peak window, roughly 22% of a day's usage, at 90% usable depth. Most homes land on one unit. |
| Batteries (Independence) | Sized to carry your overnight load, roughly 42% of a day's usage, at 90% usable depth. |
| Batteries (Custom) | Whatever count you set with the stepper. Coverage updates live as you add or remove units. |
| Annual offset | (System size × yield) ÷ annual usage. |
| Battery run time at night | Usable battery storage ÷ your average overnight draw. Overnight energy is a seasonal share of that day's usage: about 40% in winter and spring, up to 50% in summer when the AC keeps cycling after dark. Night length is seasonal too: roughly 10 hours in summer up to 13.5 hours in winter. The season toggle re-runs this with that season's usage, overnight share, and night length, so summer runtime reads shorter and winter longer. Formula: usable storage ÷ (overnight energy ÷ night hours), shown in 30-minute steps. |
| Battery unit size | Fixed at 13.5 kWh usable, the size of one Tesla Powerwall 3. Homeowners stack whole units, so the tool varies the number of batteries rather than the size of each. |
| Monthly chart | The yellow bars spread your annual production across the 12 months with a fixed Arizona solar curve (strongest March to June). The blue bars spread your annual usage across the year with a typical Arizona cooling-driven curve (heaviest in July and August). Both are built from your annual totals using typical shapes, not your actual monthly data; the chart shows where production and consumption line up and where they do not, which is the gap batteries are built to close. |
Assumptions: 400W panels for the panel count; 90% usable battery depth; an Arizona monthly production curve. Real numbers shift with shading, panel choice, roof orientation, and your specific rate plan. This is not a quote and does not include pricing, incentives, or payback.
This tool is for education only and produces rough estimates to help you understand how usage, sunlight, and storage interact. Proper system design depends on your actual usage, roof layout, utility rate plan, and equipment selection. Any system design does not represent a quote, a guarantee of production, or a recommendation to buy. Verify every number with a detailed design before making a decision.