Inverter Battery Calculator
Calculate the battery capacity you need for your inverter load. Determine required Ah and kWh for AC loads with DoD and inverter efficiency.
Load Parameters
Total wattage of all AC devices connected to the inverter.
How long the battery must power the AC load.
Pure sine wave: 90–95%. Use 90% for conservative sizing.
12V small, 24V mid-size, 48V whole-home or high-power.
LFP: 80–90%, Lead-Acid: 50% recommended.
Sizing Results
Battery Recommendation
For a 2,000W AC load over 2 hours at 48V, a 48V 120Ah LFP battery (5.76 kWh) provides the required backup runtime.
Formulas & Worked Examples
Mathematical Formulas
Inverter battery sizing converts AC load requirements back to DC battery capacity, accounting for conversion losses:
The inverter efficiency factor (typically 90%) accounts for DC-to-AC conversion losses. The DoD factor ensures you do not over-discharge the battery.
Example 1: Size Battery for 2000W Inverter — 12V System
Designing a 12V battery backup to run a 2000W AC load for 4 hours during grid outages.
Given Values
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Final Answer
A 12V 950Ah LFP battery bank (11.4 kWh) is required.
At 12V, the DC current draw is ~185A—thick 4/0 AWG cables are mandatory. Consider upgrading to 24V or 48V for this load level to reduce cable size and resistive losses.
Example 2: Size Battery for 5000W Inverter — 48V System
Sizing a 48V battery bank for a 5000W workshop load that needs 2 hours of runtime.
Given Values
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Final Answer
A 48V 300Ah LFP battery bank (14.4 kWh) is required.
At 48V, the DC current is ~115A—manageable with 2 AWG cables. This 5kW system benefits significantly from the higher voltage, with roughly half the DC current of a 12V equivalent.
Example 3: Compare 12V vs 48V for Same 3000W Load
Comparing two voltage configurations for a 3000W load with 2-hour runtime to understand efficiency trade-offs.
Given Values
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Final Answer
Both need 8.33 kWh of battery. 12V requires 694Ah with ~278A draw; 48V requires 174Ah with ~69A draw.
The 48V system uses 4× less current, enabling thinner cables (6 AWG vs 4/0 AWG), lower resistive losses, and more efficient power delivery. For any load above 1500W, 48V is strongly preferred.
Inverter-Battery System
DC battery power converted to AC through inverter with losses at each stage
Inverter Efficiency by Load Level
| Load Level | Typical Efficiency | Power Loss | Heat Generated | Notes |
|---|---|---|---|---|
| 10% (200W) | 82–85% | 30–35W | Moderate | Standby losses dominant |
| 25% (500W) | 88–90% | 60–70W | Moderate | Good efficiency range |
| 50% (1kW) | 90–93% | 70–100W | Normal | Optimal operating point |
| 75% (1.5kW) | 91–93% | 110–140W | Normal | Near peak efficiency |
| 100% (2kW) | 89–92% | 160–220W | High | Thermal throttling possible |
Inverter efficiency varies significantly with load — oversizing reduces efficiency at light loads
Frequently Asked Questions
What inverter efficiency should I assume?
Modern pure sine wave inverters operate at 88–95% efficiency. A conservative estimate of 90% accounts for conversion losses at typical load levels. Efficiency drops at very light loads (below 10% of rated capacity) and increases toward rated load.
What depth of discharge is safe for inverter batteries?
LFP (LiFePO4) batteries can safely discharge to 80–90% DoD. Lead-acid (AGM/Gel) should not exceed 50% DoD for acceptable cycle life. NMC lithium can use 80% DoD. Exceeding recommended DoD significantly reduces battery lifespan.
Can I run an inverter from a 12V, 24V, or 48V battery?
Yes, but battery voltage affects current draw. A 2000W load on a 12V battery draws ~185A (requiring thick cables), while the same load on 48V draws ~46A (thinner cables, lower losses). Higher voltage systems are more efficient for loads above 2000W.
How do I calculate the runtime of my battery with an inverter?
Runtime = (Battery Wh × DoD% × Inverter Efficiency) / Load Power. For example, a 12V 200Ah LFP battery (2400Wh) at 80% DoD with 90% inverter efficiency running a 1000W load: (2400 × 0.80 × 0.90) / 1000 = 1.73 hours.
What is the difference between pure sine wave and modified sine wave inverters?
Pure sine wave inverters produce clean AC power identical to grid power, compatible with all appliances including sensitive electronics. Modified sine wave inverters produce a stepped approximation that can damage motors, produce audible buzzing, and reduce efficiency. Always use pure sine wave for medical equipment, refrigerators, and electronics.
How do I account for surge current from appliances?
Motors and compressors draw 3–7× their running current for 1–3 seconds at startup. A 500W refrigerator compressor may surge to 1500–2000W briefly. Ensure your inverter's surge rating exceeds the highest startup demand, and your battery bank can supply the surge current without voltage drop below the inverter's low-voltage cutoff.
Can I use car batteries with an inverter?
<span class="text-primary font-bold">Not recommended.</span> Car starting batteries are designed for high-current, short-duration cranking—not deep discharge. Running an inverter from a car battery below 50% DoD will damage it within weeks. Use deep-cycle AGM, Gel, or LFP batteries specifically designed for sustained discharge rates.
How does battery voltage affect inverter performance?
Higher battery voltage reduces DC current for the same AC power output. At 12V, a 2000W load draws ~185A (requiring thick 4/0 AWG cables). At 48V, the same load draws ~46A (standard 6 AWG cable). Lower current means less voltage drop, smaller cables, and higher efficiency. For systems above 2000W, 48V is strongly preferred.
What happens when battery voltage drops too low?
When battery voltage sags below the inverter's low-voltage cutoff (typically 10.5V for 12V systems, 21V for 24V, 42V for 48V), the inverter shuts down to protect the battery. With lead-acid, this deep discharge causes sulfation and permanent capacity loss. LFP batteries tolerate low voltage better but still require a BMS cutoff.
Do I need a battery management system (BMS) for inverter use?
Yes, for lithium batteries (LFP/NMC) a BMS is essential—it protects against over-discharge, over-charge, over-current, and temperature extremes. Lead-acid systems use a charge controller and voltage monitor. The BMS low-voltage cutoff should be set higher than the inverter's cutoff to prevent cell damage.
Can I run an air conditioner from a battery inverter?
It depends on BTU rating and battery capacity. A 5000 BTU window AC draws ~500W running but 1000–1500W surge at startup. A 48V 200Ah LFP bank (9.6 kWh) can run it for 6–8 hours. Larger AC units require 48V systems with 400Ah+ capacity. Consider mini-split heat pumps which are more efficient.
How long does it take to recharge the battery after inverter use?
Recharge time = (Energy used / Charger power) / efficiency. If you discharge 5 kWh from a battery and charge at 1 kW (grid or solar), it takes ~5.5 hours (accounting for 90% charge efficiency). Solar recharging depends on panel wattage and sun hours—400W of panels provide ~1.6 kWh per day in average conditions.
What Is Inverter Battery?
Why This Calculation Matters
→ Undersized battery banks cause inverters to shut down under load when voltage sags below the low-voltage cutoff—often in the middle of the night during an outage.
→ Ignoring inverter efficiency (typically 10–15% loss) leads to a battery bank that appears sufficient on paper but delivers 15% less runtime than expected.
→ Failing to account for surge currents from motors and compressors causes inverter overload trips—refrigerators and pumps can draw 3–5× their running wattage at startup.
→ Using lead-acid batteries beyond 50% DoD with an inverter creates voltage sag that triggers low-battery alarms even when the inverter is nowhere near its power rating.
→ Mismatching battery voltage to inverter rating forces excessive DC current—drawing 200A from a 12V battery requires 00-gauge wire and creates significant resistive losses.
Practical Applications
Home Power Outage Backup
Size a battery bank to run essential AC appliances—refrigerator, lights, router, medical equipment—during grid outages lasting 8–48 hours.
Off-Grid Solar Storage
Calculate the battery capacity needed to store solar energy for overnight AC loads in cabins, tiny homes, or remote installations.
Workshop & Job Site Power
Size portable inverter systems for running power tools, lighting, and equipment in locations without grid access.
Emergency & Disaster Preparedness
Design a battery backup system with enough capacity to maintain critical communication, refrigeration, and medical devices for multi-day outages.
Common Mistakes to Avoid
✗ Ignoring inverter efficiency — a 90% efficient inverter wastes 10% of battery energy. Multiply load power by 1/efficiency to get the actual DC power required from the battery.
✗ Using AC load wattage directly for DC battery sizing — inverter input power is higher than output power due to conversion losses. Always account for inverter efficiency.
✗ Ignoring surge loads — motors in appliances require 3–5× running wattage during startup. The battery and inverter must handle these peaks without shutting down.
✗ Confusing Ah and Wh — a 100Ah 12V battery stores 1,280 Wh, not 100 Wh. Always convert to Wh before dividing by load power for runtime calculations.
✗ Not accounting for DoD limits — discharging lithium batteries to 0% accelerates degradation. Use 80% DoD for LFP and 80% for NMC in calculations.
✗ Ignoring cable losses between battery and inverter — undersized cables cause voltage drop that reduces the effective energy reaching the inverter input.
✗ Oversizing the inverter for the battery — a large inverter with a small battery will drain the battery very quickly. Match inverter capacity to battery capacity.
✗ Ignoring temperature derating — cold temperatures reduce battery capacity by 10–30%. Size for your worst-case operating temperature.
✗ Using peak instead of average load — runtime calculations should use continuous average load, not momentary peak loads from motor startups.
✗ Not considering round-trip efficiency — battery charging and discharging loses 5–15% of energy. Factor this into total system efficiency calculations.
Why Trust These Calculations?
This calculator applies standard DC-to-AC conversion efficiency factors (88–95% depending on load level) and chemistry-specific DoD limits. All formulas are derived from manufacturer datasheets and IEEE standards for battery discharge. The math is fully transparent—you can verify each step independently.
View our methodology and formula derivations →Runtime Calculator
Calculate discharge time from capacity and load.
Battery Sizing Calculator
Full battery bank sizing with temperature derating.
Energy Conversion
Convert between Ah and Wh at any voltage.
Parallel String Calculator
Design series/parallel cell configurations for your bank.
C-Rate Calculator
Verify battery discharge rates match inverter demands.
Home Backup Calculator
Size a complete home backup system with critical loads.
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