How do you convert Watts to Amp Hours (Ah)?

To convert Watts to Amp Hours (Ah), multiply the power in Watts by the hours of operation to obtain Watt-hours (Wh), then divide by the system nominal DC voltage (Volts): Ah = (Watts × Hours) / Volts. For real-world off-grid battery sizing, you must also factor in inverter efficiency (~90%), wiring resistance losses (~97%), and battery depth of discharge (80% for LiFePO4 vs 50% for Lead-Acid/AGM).

What is the difference between LiFePO4 and AGM usable Depth of Discharge (DoD)?

LiFePO4 (Lithium Iron Phosphate) batteries deliver 80% to 100% safe usable Depth of Discharge (DoD) for 3,000 to 5,000+ full cycles without voltage sag or damage. In contrast, Lead-Acid and AGM batteries should never be discharged past 50% DoD to avoid severe sulfation and accelerated capacity loss, meaning an AGM battery bank must be sized twice as large to deliver the same usable electrical energy.

Should I choose a 12V, 24V, or 48V electrical system?

System voltage selection depends on your total daily power load and continuous inverter wattage: 12V is standard for small campervans, boats, and RVs with loads under 1,500W; 24V is ideal for medium off-grid cabins, full-time digital nomad vans, and loads from 1,500W to 3,000W because it halves current and wire thickness; 48V is the gold standard for residential solar, induction cooking, and systems exceeding 3,000W because it keeps amperage low, safe, and highly efficient.

How many solar panel watts do I need to recharge my battery bank daily?

To calculate required solar panel wattage, divide your total daily Watt-hour consumption by your regional Peak Sun Hours (typically 4.0 to 5.0 hours) and multiply by a real-world system efficiency factor of ~76% (accounting for MPPT charge controller conversion, panel heating, tilt angle, and dust). For example, replenishing a 1,200 Wh daily load with 4.5 peak sun hours requires approximately 350W to 400W of solar panels (e.g., two 200W monocrystalline panels).

Real-Time Off-Grid Electrical Sizing

Watts to Amp Hours (Ah) & Battery Sizing Calculator

Convert appliance wattage to battery capacity (Ah) at 12V, 24V, or 48V. Accounts for inverter efficiency, battery chemistry depth of discharge (DoD), days of autonomy, and solar array replenishment.

Appliance Quick-Load Presets • 1-Click auto-fill load, run time & DC voltage

1. Power Load

Watts

Continuous or peak power draw of your appliances. ≈ 7.20 kWh/day

Run Time Per Day

24 hrs/day

30 min

Interactive Appliance Load List Itemized Math

Toggle or adjust each appliance to calculate combined daily Watt-hours & surge requirements.

On	Appliance & Type	Qty	Watts	Hrs/day	Daily Wh	Daily Ah

+ Quick-Add Common Appliance: Select preset or customize below

Continuous Load 245 W

Daily Consumed 1,420 Wh/day

Total Ah Demand 118.3 Ah

2. System Nominal Voltage

4. Battery Chemistry & Temperature 80% DoD • Max 90%

LiFePO4 Top Choice

80%–100% usable DoD, 4000+ cycles.

Lead-Acid / AGM Legacy

50% max safe DoD, AGM/GEL.

NMC (Li-ion) Compact

80% DoD (max 85%), dense power.

Operating Ambient Temperature (Cold Derate) 100% (No Derate)

5. Desired Days of Autonomy Reserve backup without solar

Inverter Conversion Efficiency: 90%

75% (Budget) 90% (Pure Sine Wave Avg) 100% (Pure DC Load)

Custom Depth of Discharge (DoD):

80% (Auto)

10% (Ultra Conservative) 50% (Lead-Acid Safe) 80-100% (LiFePO4 Safe)

Average Daily Peak Sun Hours (PSH): 4.5 hrs/day

1.5h (Extreme Winter) 4.5h (US/EU Average) 8.0h (Desert Peak)

Combined Solar System Efficiency (MPPT + Dust/Tilt): 76%

65% (Heavy shade/flat) 76% (Industry standard) 90% (Ideal conditions)

DC Cable Loss: 3% (0.97 eff.)

MPPT Tracking: 95% efficiency

DC Cable Sizing & Safety

NEC / ABYC Standards

Calculate copper conductor gauge, voltage drop, and inline DC fuse safety.

1. System Voltage DC Nominal

2. Operating Current Manual Entry

Amps

3. One-Way Run

4. Target Drop %

Recommended Copper Conductor:

6 AWG (13.3 mm² Copper)

Compliance & Safety:

✓ Meets 3.0% Drop & Ampacity

Voltage Drop 0.29 V (2.4% drop)

Delivered Volts 11.71 V at appliance terminals

Cable Heat Loss 8.8 W power loss (I²R)

Safe Ampacity 75 A 90°C rating

Inline DC Fuse 40 A (1.25× Continuous)

Calculated for a 20 ft round-trip circuit (10 ft positive + 10 ft return) at 12V. An inline DC fuse of at least 40A should be installed within 7 inches of battery positive terminal per ABYC E-11 standards.

Required Battery Capacity Real-World Sized

860 Ah @ 12V

Total nameplate battery bank capacity needed to provide continuous power with safety reserves.

Physical Stacking: ≈ 4× 200Ah (12V) LiFePO4 batteries (parallel)

Est. Bank Weight ~104 lbs (47 kg) Saves ~156 lbs vs AGM

Footprint & Volume ~1.8 cu ft (51 L) ≈ 4× Group 31 Cases

Voltage Recommendation Notice

Loads over 3,000Wh/day require extremely thick cables at lower voltages. Upgrading to a 48V system is strongly recommended to minimize heat and line loss.

Inverter Continuous vs. Surge Sizing Pure Sine Wave

Recommended Continuous 1,500W Pure Sine (100% Duty)

Inverter Peak Surge 3,000W 2.5×–3× Headroom

Continuous Running Load: 300 W (+25% buffer)

Motorized Startup Surge: 900 W (3× surge overhead)

Motor Surge Multiplier:

Pure sine wave inverter ensures full compatibility with compressor fridges, water pumps, CPAP, and sensitive electronics.

Total Bank Energy

10,309 Wh

10.31 kWh

Daily Consumed Energy

7,200 Wh/day

600.0 Ah/day (Net)

Solar Replenishment Array @ 4.5 PSH • 76% Eff.

2,412 Watts Solar

Est. Daily Generation Yield: 8,250 Wh/day

Daily Energy Debt Status: 100% Replenished

≈ 7× 400W or 13× 200W solar panels (covers MPPT + panel derating)

Continuous DC Current: 28.6 Amps

Suggested Wire Fuse: 40A DC Fuse (at least 36A rated)

Engineering Formulas

How to Calculate Watts to Amp Hours (Ah)

Electrical capacity in batteries is rated in Amp-hours (Ah), whereas solar panels and appliances are rated in Watts (W) and Watt-hours (Wh). Here is the exact physics behind the conversion.

Theoretical Conversion Formula

In an ideal circuit with zero loss, Amp-hours are directly derived by dividing total energy (Watt-hours) by the nominal DC system voltage:

Amp-Hours (Ah) = Watt-hours (Wh) / Volts (V)

Where: Watt-hours (Wh) = Power (Watts) × Operating Hours (h)

Quick Example:

A 120W DC refrigerator running 10 hours per day on a 12V system:

Daily Energy = 120W × 10h = 1,200 Wh
Theoretical Ah = 1,200 Wh / 12V = 100 Ah

Real-World Battery Bank Sizing

In reality, you must never size a battery purely on theoretical load. Real systems incur inverter inefficiencies, battery internal resistance, and chemical Depth of Discharge (DoD) limitations:

Bank Capacity (Ah) =
Daily Wh × Days of Autonomy
Voltage × η_inv × DoD × η_batt

η_inv: Inverter Efficiency (~0.90)

DoD: Depth of Discharge (0.80 LiFePO4 / 0.50 AGM)

η_batt: Chemistry Efficiency (0.95 Li / 0.85 Lead)

Autonomy: 1 to 5 Days No Sun

Quick Reference Matrix

Watts to Amp Hours (Ah) Reference Table

Direct comparison of common appliance wattages across 12V, 24V, and 48V DC systems.

Run Duration:

Appliance / Power	Wattage (W)	Energy (24h Wh)	12V Bank (Ah)	24V Bank (Ah)	48V Bank (Ah)	Action

*Values in table represent pure nominal Ah (Wh / V). Click "Load in Calc" to calculate with battery chemistry DoD and inverter buffers.

Off-Grid Systems Guide

Choosing Voltage & Battery Chemistry

Why system voltage fundamentally dictates your wiring cost, safety, and inverter performance.

12V Systems Under 1,200W

The standard for smaller camper vans, overland vehicles, and basic travel trailers. Directly compatible with automotive 12V accessories, USB sockets, and diesel heaters without DC-DC converters.

✓ Widest appliance selection
✓ Simple alternator charging
! Thick, heavy 4/0 AWG cables required for inverters over 1500W

Typical Bank: 100Ah – 400Ah

24V Systems 1,200W – 3,000W

The sweet spot for larger RVs, marine yachts, and modest off-grid cabins. Cuts current in half compared to 12V, allowing thinner wiring and running 2000W-3000W inverters with minimal voltage drop.

✓ 50% lower amperage than 12V
✓ More efficient solar MPPT controllers
! Requires 24V-to-12V DC step-down for 12V lights & USBs

Typical Bank: 100Ah – 300Ah @ 24V

48V Systems 3,000W – 15,000W+

The gold standard for full off-grid cabins, workshops, and whole-house backup running heavy AC units, heat pumps, well pumps, or EV charging. Reduces current to 25% of a 12V system.

✓ 75% lower current = ultra-thin copper wire
✓ Highest inverter efficiency (>93%)
✓ Uses cost-effective server-rack batteries

Typical Bank: 100Ah – 600Ah+ @ 48V

Expert Insights

Frequently Asked Questions

Clear, authoritative answers to common solar and battery calculations.

A 12V 100Ah lithium (LiFePO4) battery holds 1,280 Watt-hours (12V × 100Ah). At 80% safe depth of discharge, you consume roughly 1,024 Wh.

Assuming an average location with 4.5 Peak Sun Hours and factoring in 25% solar derating (losses due to heat, wire resistance, dust, and solar charge controller efficiency):


                Required Solar = (1,024 Wh × 1.25) / 4.5 PSH = 284 Watts

Therefore, you would need approximately 300 Watts of solar panels (e.g., three 100W panels or one 300W-350W residential panel) to replenish a depleted 100Ah battery in one good sun day.

Engineering Rigor & Code Compliance

Electrical Sizing Methodology & Engineering Standards

WattsToAh algorithms are built strictly upon certified electrical engineering standards, thermodynamic thermal factors, and empirical lithium discharge curves.

Reviewed for 2026 Code Cycles • ABYC & NEC Adherent

NEC

NEC Article 310 & Table 8

Direct current (DC) wire sizing adheres to National Electrical Code (NEC) Table 8 for uncoated stranded copper conductor resistance per 1,000 feet, factoring in thermal insulation ratings (75°C vs 105°C dry).

Zero Undersized Wire Hazard • True Ampacity Sizing

ABYC

ABYC E-11 DC Marine Standard

Enforces the strict 3% voltage drop threshold for critical safety circuits (inverters, bilges, navigation, battery main trunks) and 10% for non-critical loads (lighting, general auxiliary sockets).

Boat & Mobile Vehicle Compliant • 3% Drop Rule

DoD

Peukert\\'s Law & Chemistry Curves

Calculations dynamically adjust for Peukert exponent losses ($k \approx 1.25$ for lead-acid/AGM vs $k \approx 1.02$ for LiFePO4), preventing dangerous 50%+ capacity shortfalls common with traditional batteries.

Chemistry-Specific DoD • Temperature Derating

Dedicated Sizing Guides