Home battery backup size calculator

Build a backup energy estimate for up to three appliance groups. Put appliances together when they need the same hours and operating share. Use separate groups for different schedules. Enter running power in watts, required hours and the share of that time when the group draws the stated power.

Add the installed battery storage, discharge and conversion efficiency, any reserve and the continuous and starting output limits. Enter the assessed simultaneous starting demand separately. The result checks stored energy, ongoing power and starting power as different requirements. Enough energy alone does not mean the battery can start or run every appliance.

Required delivered energy = sum of load watts × required hours × operating share ÷ 1000. Required stored energy = delivered energy ÷ conversion efficiency ÷ available fraction after entered reserve. Compare installed storage and output limits separately.

Use a percentage as a fraction once: for example, 50% means 50 ÷ 100 = 0.5. Use 0.5 in the energy formula; do not divide it by 100 again.

Watts measure running power. Multiplying by hours gives energy; divide by 1000 to get kilowatt-hours, written kWh. Add each included group’s delivered energy. Then allow for conversion losses and the part of storage kept in reserve to find the stored energy required.

The installed energy margin is available installed storage minus required storage. A negative margin means the entered storage falls short. For the running-power check, the tool adds the full running watts of included groups with positive hours and operating share. It does not use the lower energy average as the simultaneous power demand.

Compare the installed starting output with the assessed simultaneous starting demand. Starting demand can differ from running demand. Use the evidence for the actual equipment rather than treating the battery’s energy capacity in kWh as a power limit in kW.

These illustrative inputs are separate from your saved answers. Prices, use and performance are held constant for this worked scenario; they are not personal defaults.

Do not subtract reserve twice. If the documented usable capacity already excludes the reserve you intend to keep, enter no extra reserve for that same amount. Check whether the storage figure is before or after the losses described by your efficiency input.

Include standby use where relevant and use operating shares supported by measurements or an assessment. The calculation holds each group’s entered power and hours constant. It does not build a minute-by-minute schedule or guarantee which appliances will start together. No default load list is silently added.

The estimate does not establish safe wiring, safe separation from the grid, whole-home suitability or reliable critical medical backup. It does not include recharge during the outage. Purchase cost, electricity tariffs and payback are outside this sizing check. Display values are rounded; calculations keep full precision. Invalid edits preserve the previous valid result with a warning.

Use the US equipment documentation for the installed battery, inverter and supported loads. Keep the same units and capacity boundary across the records. A manufacturer’s maximum output is not a measurement of what your appliance groups need.

Source dates identify the stored calculation method, not approval of your home’s backup design. Check local requirements with a qualified installer. The page does not decide which circuits can be backed up or confirm that a system can operate safely during a grid outage. Your load list and starting-demand assessment remain personal inputs.

Ask a qualified installer to check the load groups, starting demand, usable storage and inverter limits. Bring appliance manuals and the battery’s capacity and output records. Ask which circuits and loads the proposed arrangement can support.

Get a separate professional assessment for critical backup needs or any wiring changes. Follow the maker instructions and local requirements. A passing arithmetic check cannot confirm safe connections or guarantee runtime in a real outage.