Solar Battery Size Calculator
Turn a critical-load average and outage duration into a no-solar energy reserve, then check whether modular batteries also meet peak kW. Depth of discharge, inverter loss, aging reserve, whole-unit rounding, and a separate sunny-outage scenario remain visible.
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No-solar battery recommendation
Energy requires 2 units while continuous power requires 1, so the no-solar recommendation is 2. Solar is not credited in the main result; actual outage performance depends on load shape, surge, temperature, controls, and available PV.
A battery must pass both the energy and power gates
Energy capacity, measured in kilowatt-hours, answers how long a load can run. Power, measured in kilowatts, answers how much can run at one moment. A large energy battery with a small inverter may last all night at low load but still fail to start a well pump. A powerful inverter attached to little energy may start the pump but empty quickly.
NREL illustrates autonomy with a 10-kWh battery and a 2-kW average load lasting about five hours before considering losses and operating limits. This calculator adds depth of discharge, conversion loss, aging reserve, and module rounding. It separately divides peak load by continuous power per unit.
The larger whole-unit count becomes the recommendation. Confirm that power scales with added modules for the exact product; some systems cap aggregate inverter power or use a separate inverter whose rating does not rise one-for-one.
From critical AC energy to nominal battery kWh
The default critical load is 1.2 kW for 12 hours, so the AC energy request is 14.4 kWh. The planned usable fraction is 90%, battery-to-AC efficiency is 92%, and 15% remains reserved for aging and uncertainty. Multiplying 0.90 × 0.92 × 0.85 gives 0.7038.
Dividing 14.4 by 0.7038 gives 20.46 kWh of nameplate energy. With 13.5-kWh units, the energy gate rounds up to two. The 4-kW peak divided by 5 kW per unit rounds up to one. Energy controls, so two units provide 27 kWh nameplate and a modeled conservative 19.0 kWh of AC delivery.
At the unchanged 1.2-kW average, 19.0 kWh represents about 15.8 hours. Real autonomy ends earlier if loads increase, controls reserve more capacity, the battery is cold, or the inverter shuts down.
Build a critical-load inventory before averaging
| Load category | Energy question | Power or surge question |
|---|---|---|
| Refrigeration | How many compressor hours occur over the outage? | What is measured or specified starting surge? |
| Medical equipment | Is uninterrupted operation required, and is a separate UPS specified? | Are there startup, alarm, heater, or humidifier loads? |
| Well or sump pump | How often and how long does it run? | Can the inverter start the motor at the required voltage? |
| Communications and lighting | Which devices remain continuously on? | What is the simultaneous peak when other loads start? |
| Heating or cooling | Can a smaller safe zone be conditioned? | Are compressor, blower, crankcase, and strip-heat surges compatible? |
Use plug meters, circuit monitoring, nameplates interpreted correctly, and manufacturer data. Nameplate amps can be a maximum rather than typical consumption. Average whole-home utility kWh hides which circuits are backed up and when they run.
List loads on an hourly timeline. A 1-kW appliance running 15 minutes consumes 0.25 kWh but still requires 1 kW while on. Overlapping it with a pump changes the power gate even when total daily energy remains the same.
Why the main result gives solar zero credit
Outages can coincide with storms, wildfire smoke, snow, nighttime, shading, or damaged equipment. A grid-tied solar array normally shuts down during a utility outage unless the system has compatible islanding controls. Solar modules alone are not a resilience system.
The separate solar-credit field allows a documented sunny-window scenario. With the default 6-kWh credit, remaining AC energy is 8.4 kWh and the nominal need is about 11.9 kWh, one 13.5-kWh unit by energy. The main recommendation stays at two units because dependable autonomy was defined without solar.
Solar energy may serve live loads before charging the battery, and charge rate can be limited by array output, inverter, state of charge, temperature, or controls. Do not subtract an annual-average solar day from a specific emergency night.
Depth of discharge, efficiency, and reserve are different deductions
Depth of discharge limits how much nameplate energy the operating plan uses. Product warranties and controls may define usable capacity differently, so do not apply a 90% deduction on top of a specification that already reports usable kWh without understanding the basis.
Inverter efficiency accounts for conversion from battery DC to household AC. Real efficiency varies with power level, standby operation, temperature, and power factor. Charging losses are not separately included because this autonomy calculation begins with a charged battery.
The aging reserve holds back capacity for degradation and uncertainty. It is not a prediction that the product loses exactly 15%. Warranty capacity retention, cycles, calendar age, temperature, state-of-charge history, and software limits matter. Run a larger reserve for safety-critical service and verify replacement planning.
Outage duration is a service decision, not a weather forecast
Twelve hours might cover an overnight interruption, while regional disasters can last days. NREL notes that grid-connected residential storage is often considered for one to two days and that longer outages can show decreasing marginal utility; generators or other strategies may be more economical in some situations.
A generator introduces fuel storage, ventilation, carbon-monoxide, maintenance, transfer, noise, and operational risks. Never run combustion equipment indoors, in a garage, or near openings. Battery storage introduces its own electrical and fire-safety requirements. Resilience planning should compare safe systems rather than assuming one technology solves every event.
Reduce the critical-load list before multiplying batteries. Passive cooling, insulation, safe temperature zones, efficient refrigeration, LED lighting, and load scheduling can extend autonomy. Do not shed medically necessary or life-safety equipment without a professional continuity plan.
Installation, siting, and emergency response
Local adopted codes can regulate location, separation, protection from vehicles, ventilation, alarms, maximum quantities, working clearances, and firefighter access. Outdoor temperature and flood exposure can affect placement. Utility interconnection and export settings may be required even when the goal is backup.
Household members and responders should know the shutdown procedure without approaching damaged equipment. If a battery is hot, smoking, hissing, leaking, damaged, or involved in fire or flood, move away and call emergency services. Follow manufacturer and fire-department guidance; do not touch or reuse the system.
Questions for a solar-storage proposal
Ask whether the quoted kWh is nameplate or usable, at beginning or end of warranty, and at what temperature. Request continuous and surge kW, surge duration, motor-start capability, round-trip efficiency, standby consumption, warranty throughput, outage recharge behavior, black-start capability, and PV operation when the battery is depleted.
Confirm which circuits enter the backed-up panel and whether the system can handle 120/240-volt loads, multiwire circuits, well pumps, HVAC, and generator integration. Ask for a single-line diagram, permit scope, commissioning test, monitoring access, and emergency instructions.
Run a witnessed outage test after installation. Confirm intended loads remain powered, forbidden loads are isolated, solar can restart or charge as represented, and the monitoring display uses understandable units.
Frequently asked questions
Is usable battery capacity the same as nameplate kWh?
No. Usable capacity reflects control limits and may already include a depth-of-discharge allowance. Read the exact product definition.
Why can the power gate require more batteries?
Simultaneous load or motor surge can exceed one unit’s inverter rating even when one unit stores enough total energy.
Will rooftop solar work during an outage?
Ordinary grid-tied solar usually shuts down. Outage operation requires compatible inverter, storage, controls, and safe islanding design.
Should I count expected sunshine?
Keep a no-solar reserve for dependable autonomy and show sunshine separately unless a professional resilience analysis justifies a credit.
Does the calculator size the main electrical panel?
No. Service, feeder, breaker, conductor, fault, and load calculations require code-compliant electrical design.
Can a battery support central air conditioning?
Only if energy, continuous power, surge, voltage, controls, and product compatibility all support it. Obtain a measured or engineered design.
Related household planning
It does not determine storage credit eligibility, project basis, rebate stacking, battery safety, or electrical requirements.
After selecting usable storage, test how long that capacity can support a stated load with the battery duration calculator.
References
- National Renewable Energy Laboratory: technology tips for solar plus storage
- U.S. Department of Energy: solar and resilience basics
- U.S. Department of Energy: solar energy guide for homebuilders
- U.S. Department of Energy: solar inverters and grid services
Preliminary educational sizing only. Final storage design requires exact loads, listed equipment, professional engineering or installation, permits, codes, utility approval, and emergency planning.