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's nominal DC voltage (Volts): Ah = (Watts × Hours) / Volts. For real-world off-grid sizing, you must also factor in inverter efficiency (~90%) and battery depth of discharge (80% for LiFePO4, 50% for Lead-Acid).

Why does system voltage (12V vs 24V vs 48V) matter?

For the exact same wattage load, doubling the system voltage cuts current (Amperage) in half. Lower amperage reduces resistive heat loss, allows thinner and less expensive copper wiring, improves inverter efficiency, and enables higher total power capacity in off-grid solar installations.

How many solar panels do I need to charge a 100Ah battery?

A 12V 100Ah LiFePO4 battery holds roughly 1,280Wh. Assuming 80% usable depth of discharge (~1,024Wh) and 4.5 peak sun hours per day with 25% system loss factor, you need approximately 280W to 350W of solar panels (e.g., two 175W or 200W solar panels) to fully recharge it within a single sunny day.

What is the difference between LiFePO4 and AGM usable capacity?

LiFePO4 (Lithium Iron Phosphate) batteries can safely discharge to 80%-90% Depth of Discharge (DoD) for 3,000-5,000+ cycles with 95% round-trip efficiency. Lead-Acid and AGM batteries should only be discharged to 50% DoD to prevent sulfation and irreversible degradation, and they suffer from lower round-trip efficiency (80%-85%) and Peukert's effect under heavy loads.

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.

Quick Appliance Presets: (Click to load typical loads)

1. Power Load

Watts

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

2. System Nominal Voltage

3. Daily Run Time

24 hrs/day

30 min

4. Battery Chemistry (Depth of Discharge) 80% DoD • Max 90%

LiFePO4 Top Choice

80% DoD (max 90%), 4000+ cycles.

Lead-Acid Legacy

50% max safe DoD, AGM/GEL.

NMC (Li-ion) Compact

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

5. Desired Days of Autonomy Reserve backup without solar

Required Battery Capacity Real-World Sized

860 Ah @ 12V

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

Equivalent Battery Bank: ≈ 9× 100Ah or 5× 200Ah (12V) batteries

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.

Total Bank Energy

10,309 Wh

10.31 kWh

Daily Consumed Energy

7,200 Wh/day

600.0 Ah/day (Net)

Recommended Solar Array @ 4.5 PSH

2,412 Watts Solar

≈ 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.