In an emergency, call 911 (US) or 999 (UK) and follow instructions from local officials first.
Ready Survival Kits

Tools

Energy and solar

List what you'll run each day. The calculator adds up your energy use, then sizes a solar array and battery bank using the standard stand-alone PV method.2

1. What you'll run

Watts are on the device label or in its manual. Labels show the maximum, so a plug-in meter gives a truer number.1

DeviceWattsHours/dayQtyWh/day

The example rows are a modest off-grid cabin. Replace them with your own. For comparison, the average U.S. home used about 899 kWh a month in 2022, roughly 30 kWh a day.6

2. Your site and system

From NREL's solar maps or PVWatts for your address.34 Use the darkest month you need power in.
Wiring, inverter, heat, dust and shading. 25% is our cautious starting assumption, not a published figure. PVWatts can model this for your site.
Days the batteries must carry you with no sun. Penn State's example: about 3 days for 95% availability, about 10 for critical loads.2
Allowable depth of discharge, typically 20–80% depending on battery type.2 Check your battery's spec sheet.
Our assumption. Use the figure on your battery's spec sheet.
Median U.S. residential price in 2024, before incentives, for grid-tied systems.5 Off-grid systems with batteries cost more.
–kWh per day
–kWh per month
–Solar array (kW)
–Battery bank (kWh rated)
–Solar array cost at that price (panels only, excludes batteries)

How it's calculated

  • Energy per device: watts × hours × quantity. Divide by 1,000 for kWh.1
  • Battery bank: daily energy × days of backup ÷ (usable depth × efficiency).2
  • Solar array: daily energy ÷ efficiency ÷ (peak sun hours × (1 − losses)). Sizing for the worst month keeps batteries charged all year.2

This is a planning estimate. Have a qualified installer design the final system, and follow your local electrical code.

Sources

  1. U.S. Department of Energy, Watt Does It Cost to Use It? (kWh = watts × hours ÷ 1,000)
  2. Penn State AE 868, Stand-alone PV system sizing
  3. NREL, Solar Resource Maps and Data
  4. NREL, PVWatts Calculator
  5. Lawrence Berkeley National Laboratory, U.S. Distributed Solar and Storage 2025 Data Update
  6. U.S. EIA, How much electricity does an American home use?