Backup power calculator

List the critical loads, pick a battery unit and a modelled solar day. It returns running kW, surge kW, daily kWh, units for a target and runtime.

Illustrative only. The preset loads, motor starting multipliers, array and outage start are invented assumptions. The preset solar days are modelled with pvlib (typical-year weather for Greensboro, NC; not measured). The battery numbers are from the Enphase IQ Battery 5P data sheet DSH-00010-8.0 (August 2024); the sheet gives kVA, and the tool treats 1 kVA as 1 kW. Replace every value with your own. Not a design, a quote or a code check.

Start from:

Loads (one per line)

Name; running watts; hours per day; window start hour; window end hour; motor starting multiplier

The hours are spread evenly across the window (hours 0 to 24). Use a multiplier of 1 for loads with no motor start. Decimals can be written with a full stop or a comma; thousands can be separated with a comma, so write 1,500 or 1500, and write 1.5 or 1,5.

Battery unit and outage

Usable energy per unit (kWh) Continuous power per unit (kVA, treated as kW) Peak power per unit, 3 seconds (kVA, treated as kW) AC round-trip efficiency (%) Target outage (whole hours, 1 to 168) Outage starts at hour (0 to 23) Number format of results

Solar during the outage: AC kWh in each hour starting 00:00 to 23:00 (24 values; zeros for none)

Solar is counted only if the system lets it run in an outage. Surplus above what the loads and the battery can take is thrown away.

Result	Value
Running load, all on together (kW)	-
Surge: running plus the largest extra motor start (kW)	-
Energy per day (kWh)	-
Units for the target: by energy / by running kW / by surge kW	-
Units to buy for the target (largest of the three)	-

Units	Usable kWh	Continuous kW	Peak kW (3 s)	Battery only (h)	With solar (h)
1	-	-	-	-	-
2	-	-	-	-	-
3	-	-	-	-	-
4	-	-	-	-	-

Method: running kW is the sum of the watts (a cautious all-on case). Surge kW adds the largest extra start, watts times (multiplier minus 1). Units for the target need energy = daily kWh x target hours / 24 / sqrt(round trip), divided by usable kWh per unit; the sheet does not say which side the usable kWh is measured on, so it is taken as DC battery energy and the AC round trip splits equally between charge and discharge (square root of the round trip each way). Runtime steps hour by hour from a full battery: solar serves the load first, surplus charges the battery (limited by the unit's continuous rating), the battery serves the rest. Runtime checks hourly average power only: use the running kW and surge kW results for the power check. Runtime ends when the battery cannot serve the full hour; the run stops at 168 hours. One design day repeats. Units are assumed to add; the data sheet gives no paralleling rule, so check the vendor's design rules.