TradesSolar Design: From Bill to Battery
A solar quote is a chain of dependent choices. Start with annual use, then make the panels, strings, inverter, wire, and battery agree.
Solar is not a panel-count decision. A useful early design makes every downstream component fit the energy target and the electrical limits.
Worked example from the live calculator
At 12,000 kWh a year, 4.5 peak sun hours, and 400 W panels, the live solar calculator recommends 9.1 kW, about 23 panels, for a 100% offset using its 0.80 derate.
Start with the bill, not the roof photograph
Annual kWh and peak sun hours set the array target. The system-size engine turns those inputs into kW and panel count for 80%, 100%, or 120% offset. A 100% target can be sensible, but future EV or heat-pump load and utility caps can move it.
Make strings and inverter agree before buying hardware
Panel count alone does not prove a design works. The string-sizing engine checks cold open-circuit voltage and operating voltage against the inverter window. The inverter-ratio engine then checks whether DC array size is sensible against AC capacity. Use manufacturer data, especially the cold-temperature voltage coefficient.
Treat wire and storage as separate constraints
A string that fits electrically can still lose too much voltage over a long DC run. Check conductor size at the actual distance. Battery reserve is a different decision: it follows critical daily kWh, autonomy days, chemistry, and usable depth of discharge, not the panel count.
Key takeaway. A credible solar concept is a compatible chain, not a panel total. Size energy first, then verify voltage, inverter ratio, conductor loss, and reserve.
Assumptions
- A planning design uses a full year of kWh, site-specific sun hours, equipment data sheets, and utility interconnection rules.
Sources
- NREL PVWatts sizing concepts
- NEC Article 690, verify local requirements
Last updated: 2026-08-27
Frequently asked questions
You can shortlist panels, but do not finalize a quantity or string layout until the inverter voltage window and current limits are known.
No. Storage shifts energy in time. It does not create annual production, so the array still has to cover the intended load.
No. It is a planning sequence. Final design needs site conditions, equipment listings, utility rules, and local code review.