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 hourly usage.
Live in Arizona and want a custom design for your home? Book a free 15-minute call. Real solar guidance, without the sales pitch. Book a Call →This takes your yearly electricity use and your roof's sun exposure, applies typical Arizona seasonal patterns, and lands on a starting point. It rounds to whole panels and whole batteries, because that is how systems actually get built. Every figure is an estimate.
| Step | Logic |
|---|---|
| System size | Your annual usage times an offset target, divided by how much a kilowatt of solar produces here in a year (about 1,650 kWh on a good roof, less when shaded). Rounded to whole 400W panels so the kW figure and the panel count always agree. Savings aims for 80%, Independence for 120%. Independence aims past 100% because a system meant to carry the night has to run the house and refill the batteries during the same daylight hours, with margin for cloudy days and weaker winter sun. |
| Annual offset | The share of your yearly electricity your panels would generate. Worth knowing: 100% offset does not mean you stop buying power. It means production and usage balance out across a year, not hour by hour. Panels make nothing at night, so you still pull from the grid unless a battery covers it. |
| Batteries (Savings) | Sized to cover your peak hours, the stretch each afternoon and evening when your utility charges the most. About 22% of a summer day's electricity gets used then, and your panels are still working through part of it: output tapers after mid-afternoon but keeps producing into the evening. So the sizing assumes the battery carries about 80% of that window and live solar covers the rest. An additional unit only gets added once it would carry a meaningful share of the load, at least about 20% of its capacity. Without that rule, exceeding one battery by a sliver would add a whole second unit that sits nearly idle, and on a savings build an idle battery is just cost. One assumption to know: this strategy assumes a time-of-use style rate plan with pricier late-afternoon and evening hours. On a flat rate there is no expensive window to target, so a savings-focused battery earns less and the right count is a different conversation. |
| Batteries (Independence) | Sized to cover most of a summer night rather than just the expensive hours. Roughly half of a summer day's electricity gets used after dark while the AC keeps cycling, and this build targets about 70% of that. |
| How the battery behaves | The battery does not stop when peak hours end. It supplies your home whenever solar cannot and keeps going until it reaches the backup reserve you have set, so the two rows above describe what each strategy guarantees, not a limit on when the battery runs. Extra capacity simply carries you further into the night, which is what the coverage figure reflects. One thing this tool does not model: if you hold a backup reserve for outages, that portion is not available for everyday use, so real coverage runs shorter than shown. Reserve is a setting that ranges from 0 to 100%, so any single assumption would be wrong for most homes. |
| Battery coverage | How long the batteries could run your home after dark on a summer night. Usable storage divided by your average draw overnight, capped at the length of the night. A summer night runs about 10 hours. |
| Battery count | Two inputs cannot pin one exact number, so both strategies give a starting point rather than a firm count. An annual figure supports a starting count; only actual hourly usage data can justify adding a unit beyond it. For Savings the question that data answers is whether the system runs short during the high-cost hours; for Independence, whether it runs short overnight. Your peak window length, rate plan, usage pattern, and roof orientation all move it. |
| Battery size | Fixed at 13.5 kWh, one Tesla Powerwall 3. That 13.5 figure is Tesla’s usable rating; the sizing here applies a further ~10% real-world derate for round-trip losses and a small operating margin, so each unit contributes about 12.2 kWh in the math. Homeowners add whole units, so the tool changes the number of batteries rather than the size of each. Other brands differ in capacity, so the count would shift somewhat with different equipment. |
| Monthly chart | The yellow bars spread your annual production across the year on a typical Arizona solar curve, strongest April through June. The blue bars spread your usage on a typical cooling-driven curve, heaviest in July and August. Both are built from annual totals and typical shapes, not your real monthly data. The point is the mismatch: production peaks months before consumption does. Closing that gap is the job a battery does. |
Assumptions: 400W panels; 13.5 kWh batteries (Tesla's usable rating) with a ~10% real-world derate applied; a typical Arizona production curve; battery sizing based on a summer day rather than a yearly average. Real numbers shift with shading, panel choice, roof orientation, and your rate plan. This is not a quote and does not include pricing 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. Nothing shown here is a quote, a guarantee of production, or a recommendation to buy. Verify every number with a detailed design before making a decision.