Battery planning

Battery Size Calculator

Estimate the battery capacity needed to power a known load for a target backup time. Enter your load and runtime, then adjust reserve, efficiency, battery health and planning margin when you know more.

Calculate required battery size

Quick Runtimes:

How to size a battery for backup power

Battery sizing starts with the energy your load needs over time. The calculator accounts for conversion losses, the usable state-of-charge window, battery health and a planning margin before showing a recommended nominal battery size.

Battery kWh vs Ah

Watt-hours describe stored energy, while amp-hours describe charge at a specified voltage. The recommended kWh does not change when voltage changes; the Ah equivalent does. That is why the calculator shows your selected system voltage separately.

Battery size formula

Load energy equals average watts multiplied by runtime hours. Conversion-adjusted energy is divided by the usable SOC window and battery health, then the planning margin is applied. AC and DC appliance rows use their own conversion efficiencies.

Common emergency backup battery sizing scenarios

Typical battery capacities needed for common residential power outage durations (assuming 12V LiFePO4 with 10% reserve and 88% AC inverter efficiency):

Battery capacity needed by load and outage duration (12V LiFePO4)
Backup Scope & Average Load	4-Hour Outage	8-Hour Outage	12-Hour Outage	24-Hour Outage
Essential Communications (Wi-Fi + Phone + LED Lights: 50W)	~250 Wh (21 Ah)	~505 Wh (42 Ah)	~758 Wh (63 Ah)	~1.52 kWh (126 Ah)
CPAP Machine + Phone Charging (70W avg)	~354 Wh (29 Ah)	~707 Wh (59 Ah)	~1.06 kWh (88 Ah)	~2.12 kWh (177 Ah)
Refrigerator + Wi-Fi + Lights (180W avg)	~909 Wh (76 Ah)	~1.82 kWh (152 Ah)	~2.73 kWh (227 Ah)	~5.45 kWh (455 Ah)
Home Office / Remote Work (Laptop + 2 Monitors + Starlink: 220W)	~1.11 kWh (93 Ah)	~2.22 kWh (185 Ah)	~3.33 kWh (278 Ah)	~6.67 kWh (556 Ah)
Critical Household Circuit (Fridge + Sump Pump + Internet: 400W)	~2.02 kWh (168 Ah)	~4.04 kWh (337 Ah)	~6.06 kWh (505 Ah)	~12.1 kWh (1,010 Ah)

Reserve, usable SOC and battery health

The usable SOC window is starting charge minus minimum remaining charge. These values are used once as one SOC-window calculation; reserve and depth of discharge are not double-counted.

How inverter efficiency affects battery size

AC loads require battery-side energy equal to device energy divided by inverter efficiency. Direct DC loads use the entered DC conversion efficiency instead. This calculator accounts for losses but does not size an inverter.

LiFePO4 and lead-acid assumptions

LiFePO4 and lithium-ion start with a 20% minimum charge preset. AGM, gel and flooded lead-acid start with 50%. These are editable planning defaults, not product specifications; a value you customize remains under your control when chemistry changes.

Battery Sizing Formulas & Capacity Math

Calculates nominal stored-energy capacity (Wh and Ah) required to sustain a continuous or cycling electrical load for a desired backup duration.

FormulaBattery_Wh = (Load_Watts × Runtime_Hours × (1 + Margin)) / (Usable_SOC × Inverter_Eff × Battery_Health)

Variable Definitions

Load_WattsContinuous Electrical Load(W): Average real-time power draw (Watts × Duty Cycle).
Runtime_HoursTarget Autonomy Duration(hours): Desired continuous operating hours without recharging.
Usable_SOCUsable DOD Window(fraction): Fraction of nominal energy available above minimum reserve (e.g., 80% for LiFePO4, 50% for Lead-Acid).
Inverter_EffInverter AC Efficiency (η)(fraction): DC-to-AC power conversion efficiency (typically 88%–93%).
MarginPlanning Design Margin(fraction): Safety buffer for temperature deratings and cable losses (typically 10%–20%).

Engineering Notes & Standards

Amp-Hour equivalent at selected voltage V: Ah = Battery_Wh / V.
For mixed loads, AC appliances use Inverter Efficiency and DC appliances use DC-DC step efficiency separately.

Frequently Asked Questions (FAQ)

What size battery do I need for a 500W load?

To run a continuous 500W AC load for 4 hours (2,000Wh of device energy), you need a battery bank of approximately 3.06 kWh (assuming 90% inverter efficiency, 80% usable depth-of-discharge, and a 10% design margin). At 12V this is ~255 Ah; at 24V this is ~127 Ah; at 48V this is ~64 Ah.

What size battery do I need to run a refrigerator during a 24-hour power outage?

A typical full-size refrigerator consumes about 1.2 to 1.8 kWh per day. Factoring in inverter standby losses, 80% depth of discharge, and safety margins, a 2.5 kWh to 3.5 kWh battery bank (or approximately 200Ah–300Ah at 12V LiFePO4) is recommended to comfortably cover 24 hours.

Why does a 12V 100Ah battery not provide 1,200 watt-hours?

While nominal capacity is 12V × 100Ah = 1,200Wh, practical usable capacity is reduced by minimum state-of-charge safety cutoffs (typically 20% for LiFePO4 or 50% for Lead-Acid) and AC inverter conversion losses (85%–92% efficiency).

Should I build a 12V, 24V, or 48V battery bank?

For systems under 1,000W of inverter load, 12V is simplest and most common. For 1,000W to 3,000W, 24V halves wire thickness and current losses. For whole-home backup systems exceeding 3,000W, 48V is the standard industry recommendation.

Worked example

With a 500 W AC load for 4 hours, 90% inverter efficiency, a 20% reserve and a 10% planning margin, the result is approximately 3.06 kWh recommended nominal capacity. At 24 V, that is about 127 Ah.

Limitations and methodology

Capacity is only one part of battery selection. Confirm that the battery, BMS and inverter can support your required continuous and peak power using manufacturer specifications. Read the methodology and sources for broader calculation and reference guidance.

Related battery calculators

After sizing a battery, use the Battery Runtime Calculator to verify how long a proposed capacity may run your load, or the Home Battery Size Calculator for whole-house backup.