Backup power size
Preview Version 0.1.0A first-pass battery and generator size for the loads you need through an outage.
Battery, usable
5.7kWh
For 12 hours, get a battery of about 7.1 kWh (5.7 kWh you can use). Or get a generator of at least 5.0 kW.
NREL REoptSelf-checked · The spec's 9.6 kWh load list, 60 kWh usable and 75 kWh nameplate at 12 hours, and 7.0 kW generator with motor start pass, each hand-computed. What this means
A first-pass size to take to an installer, not a design. · Self-checked only: no independent check has been run yet · Example values · Review pending Report a problem
Your county's outage history
Your ZIP finds your county. Its recorded outages from EAGLE-I set the outage length, and the battery for each length is shown.
How we got this
Method
Battery usable kWh = critical energy over the outage, starting at the hour with the most critical load; nameplate = usable ÷ usable share. Generator kW = peak critical kW − the largest motor's running kW + its starting kW.
Steps
- Daily critical energy: refrigerator 0.2 kW × 24 h = 4.8 kWh; well pump 1.5 kW × 2 h = 3 kWh; lights 0.3 kW × 6 h = 1.8 kWh; total 9.6 kWh.
- Critical load by hour: 0.33, 0.33, 0.33, 0.33, 0.33, 0.33, 0.33, 0.33, 0.33, 0.33, 0.33, 0.33, 0.33, 0.33, 0.33, 0.33, 0.33, 0.33, 0.63, 0.63, 0.63, 0.63, 0.63, 0.63 kW (midnight to 11 pm). A load with no start time is spread over the day.
- Battery: the outage starts at 18:00, the hour with the most critical load, and lasts 12 hours: 5.7 kWh usable ÷ 80% usable window = 7.13 kWh nameplate.
- Generator: 2 kW − 1.5 kW running + 4.5 kW starting = 5 kW. A load with no start time counts at its full kW, since it could run at any moment.
- Generator ratings are usually stated at 0.8 power factor. Check motor starting against the generator's own motor-starting data; no starting multiplier is assumed.
Assumptions
- Loads with no start time are spread evenly over the day for the battery, and counted at full kW for the generator.
- No inverter losses, battery aging, or temperature derating.
Sources
- NREL REopt, resilience analysis, REopt web tool, Critical load and outage duration inputs
Known limits
- County outage history means some customers in the county were out, not this address; durations are a typical guide, not a worst case.
- No inverter losses, battery aging, or temperature derating.