Tools

Residential Rooftop Array & Home Backup Sizer

Two sizes, one tool: how much solar your roof and bill justify, and how much battery your critical loads need to ride through an outage.

Your home & usage

Or enter a monthly bill below and it will be converted using your rate.

US average roughly 3.5–5.5; desert Southwest higher, Pacific Northwest lower.

W × h

Recommended solar array8.00 kW · 20 panelsRoof area needed: 360 sq ft (18 sq ft per panel). Annual production: 11,169 kWh — 103.4% of annual use.
Recommended battery5 kWhCritical-load energy in the backup window: 2.06 kWh. Expected runtime at that battery: 23.6 hours.
Estimated solar cost$22,400
Estimated battery cost$4,250
Estimated total (before incentives)$26,650Before tax credits and installation extras.

How it works

Array size = annual usage ÷ (peak sun hours × 365 × 0.85 performance ratio), rounded up to whole panels, so predicted production covers annual usage; production applies the same 0.85 ratio for inverter, temperature, soiling and wiring losses. Battery size covers only the critical loads you ticked, for the backup window you set — whole-home backup (including AC, cooking and water heating) needs a much larger battery. Battery capacity is grossed up by 0.9 inverter efficiency and 0.9 usable depth of discharge, then rounded up to 5 kWh modules (the smallest common home-battery module).

Assumptions & sources

  • Panel footprint 18 sq ft per panel; roof check uses usable area only — setbacks, vents and shading reduce it further.
  • Fridge energy uses a ~35% duty cycle on its 150W nameplate; other loads use watts × hours directly.
  • Performance ratio 0.85 and the 0.9 / 0.9 battery factors are planning defaults, not guarantees for a specific product.
  • Battery capacity is rounded up to 5 kWh modules; gateway, backup panel and electrician installation are excluded from the battery price — get an installed quote.
  • Costs are editable planning estimates before incentives; installer quotes vary by market and roof complexity.
  • Estimates only — get a site survey before buying. Battery chemistry trade-offs (LFP vs NMC vs sodium-ion) change usable capacity and lifespan.

Related Joule Post coverage: What size home battery do you need for an outage? · Adding a battery to existing solar · Battery chemistries, explained · Is a home battery worth it without solar?