Battery Life Calculator

Introduction

Every device that runs from a battery has a hidden design question behind it: how long will the power actually last? A 100 Ah battery sounds substantial on the label, but whether it powers a laptop for ten hours or a mini-fridge for two depends on far more than the amp-hour rating. The load you draw, the battery voltage, how deeply you discharge it, and the losses in the wiring and inverter all reshape the answer.

The Battery Life Calculator answers the two questions that matter for real battery planning. In Runtime mode it converts your battery's capacity, voltage, and your device's power draw into an estimated runtime in hours and minutes. In Capacity Needed mode it works the other way — given a load and a target runtime, it tells you the minimum battery size, including the depth-of-discharge and efficiency adjustments that prevent your "8 hour" battery from dying at hour 5.

The math rests on one relationship: energy in watt-hours equals capacity in amp-hours multiplied by voltage[powerstream][batterystuff]. From there, runtime is simply usable energy divided by the load's power draw. The calculator accounts for the two factors that make naive estimates wrong — the maximum depth of discharge recommended for your battery chemistry and the efficiency losses in the system — so the result reflects real-world usable capacity rather than the theoretical label value.

The calculator sits naturally alongside the Electricity Calculator for power and energy questions, the Ohms Law Calculator for current relationships, and the Capacitor Calculator for energy-storage components. For off-grid and solar setups, it connects directly to the Solar Panel Calculator, which sizes the generation side of the same system.

How to Use

The Formula

The calculator applies the standard runtime estimation method used by battery engineers:

Where:

• $E_{wh}$ is the stored energy in watt-hours
• $C_{ah}$ is the battery capacity in amp-hours
• $V$ is the nominal battery voltage in volts

Because a battery's amp-hour rating is a charge figure, not an energy figure, multiplying by voltage is essential before comparing batteries or computing runtime[powerstream]. A 100 Ah battery at 12 V stores 1,200 Wh; the same 100 Ah at 24 V stores 2,400 Wh — the amp-hour number alone is misleading.

Usable energy:

Where DoD is the maximum depth of discharge (as a fraction) and $\eta$ is the system efficiency. Depth of discharge matters because draining a battery to zero destroys its cycle life: lead-acid batteries are commonly limited to 50%, while lithium iron phosphate (LiFePO4) can safely reach 80%[victron]. Efficiency accounts for inverter conversion losses and wiring resistance, typically 85–95%.

Runtime:

Where $P_{load}$ is the average power draw in watts. Since power equals voltage times current[openstax], the load can also be expressed as current: a 100 W load on a 12 V battery draws about 8.3 A. The calculator's C-rate output shows how this current compares to the battery capacity — a useful sanity check, since drawing at very high C-rates reduces effective capacity in lead-acid batteries (the Peukert effect)[victron][batterystuff].

Battery Runtime Reference

The table below shows estimated runtimes for common battery sizes powering a 100 W load at 80% depth of discharge and 90% efficiency. These are the values the Runtime mode produces for typical configurations.

The pattern in the table is the same lesson from the formula section applied to real sizes. A phone-sized 3.7 V pack lasts minutes under a 100 W load, a 12 V deep-cycle battery lasts most of a workday, and a large 48 V solar bank pushes past two days. What the chart makes visible is how runtime scales with the voltage × capacity product rather than amp-hours alone.

Practical Tips

1. Never trust the amp-hour number alone. The 100 Ah rating means 100 A for one hour only in theory; in practice the same 100 Ah at 12 V stores 1,200 Wh, and at 24 V stores 2,400 Wh. Always multiply by voltage before comparing batteries[powerstream].

2. Respect the depth-of-discharge limit for your chemistry. Draining a lead-acid battery past 50% repeatedly will kill it within months. Lithium batteries tolerate 80% DoD, which is one reason they dominate modern solar and RV systems despite higher cost[victron].

3. Account for inverter losses. If you run AC devices from a battery, the inverter wastes 5–15% of the energy as heat[openstax]. Add 10–15% to your load or lower the efficiency input accordingly, or your runtime estimate will be optimistic.

4. Cold reduces capacity. Battery capacity falls as temperature drops — roughly 20% at 0 °C for many chemistries. The 25% margin the Capacity mode recommends doubles as a cold-weather buffer.

5. High discharge rates shrink lead-acid capacity. A lead-acid battery drained at 1C (full capacity in one hour) delivers far less than its rated amp-hours due to the Peukert effect; lithium is far less affected[victron][batterystuff]. If your load is heavy relative to the battery, derate the capacity further.

6. Match the calculator to your actual average load. A fridge drawing 150 W when running but only running 25% of the time averages about 37 W. Using the peak wattage gives a very pessimistic runtime; using the average gives the number you can plan around.

Limitations

Rated capacity is measured at a specific discharge rate. The amp-hour figure stamped on a battery is its capacity at a rated discharge time, typically 20 hours[batterystuff]. At higher discharge currents the delivered capacity falls, most strongly in lead-acid chemistry. The calculator models constant-power runtime with DoD and efficiency, but not the full Peukert correction, so heavy-load runtimes are optimistic.

Depth of discharge is a chemistry guideline, not a hard cap. The default 80% matches lithium iron phosphate recommendations, and 50% matches flooded lead-acid[victron], but manufacturer guidance varies. Exceeding these limits does not immediately stop the battery — it accelerates aging. Use the values appropriate to your battery's datasheet.

Efficiency is an estimate. The 90% default represents a reasonable single-inverter system, but real efficiency varies with load level, inverter quality, wiring gauge, and temperature. Very light loads convert less efficiently, and the Voltage Drop Calculator is the right tool for quantifying wiring losses.

Battery aging is not modeled. A new battery at 100% of rated capacity degrades to roughly 80% at end of life, which is why the Capacity mode offers a 25% margin[victron]. The runtime outputs assume a healthy, recently tested battery; a years-old pack will underperform the estimate.

Temperature effects are not included. The model assumes a moderate ambient temperature. Cold dramatically reduces available capacity and discharge efficiency, and extreme heat accelerates degradation. For seasonal or outdoor installations, apply an additional derate factor beyond the calculator's margin.

Frequently Asked Questions

❓ How long will a 100Ah battery last?

✅ It depends on the voltage and load. A 100 Ah 12 V battery stores 1,200 Wh. At 80% depth of discharge and 90% efficiency that is about 864 Wh usable, which powers a 100 W load for 8.6 hours, a 50 W load for 17.3 hours, or a 200 W load for 4.3 hours. Enter your actual values in Runtime mode for the precise figure.

❓ What is the formula for battery runtime?

✅ Runtime in hours = (capacity in Ah × voltage in V × depth of discharge × efficiency) / load in watts. The capacity × voltage product gives watt-hours of stored energy; DoD and efficiency reduce it to usable energy; dividing by the load converts energy to time. A 100 Ah 12 V battery on a 100 W load at 80% DoD and 90% efficiency gives 1,200 × 0.8 × 0.9 / 100 = 8.64 hours.

❓ How many amp-hours do I need for a 100W load?

✅ That depends on the runtime and voltage. For 8 hours at 12 V you need 800 Wh usable; at 80% DoD and 90% efficiency that is 1,111 Wh required, or about 93 Ah at 12 V. Halving the voltage to 6 V doubles the amp-hour requirement, and doubling the voltage to 24 V halves it. Use Capacity Needed mode for exact values.

❓ What is the difference between Ah and Wh?

✅ Amp-hours measure charge — how many amps a battery can supply for how many hours. Watt-hours measure energy — the actual work the battery can do. They are related by Wh = Ah × voltage. A 100 Ah 12 V battery stores 1,200 Wh; the same 100 Ah at 24 V stores 2,400 Wh. Wh is the honest comparison across batteries with different voltages.

❓ What is depth of discharge and why does it matter?

✅ Depth of discharge is the percentage of battery capacity you drain before recharging. Draining lead-acid batteries past 50% repeatedly shortens their life dramatically, so the recommended maximum is about 50%; lithium iron phosphate tolerates 80%. The calculator uses DoD to compute how much of the rated capacity is actually usable over the battery's lifetime.

❓ What is the Peukert effect?

✅ The Peukert effect is the observation that lead-acid batteries deliver less than their rated amp-hours when discharged quickly. A battery rated 100 Ah at the 20-hour rate may deliver only half that when drained in one hour. Lithium chemistries are far less affected. For heavy loads on lead-acid, derate the capacity beyond the standard DoD adjustment.

❓ Does voltage drop affect battery runtime?

✅ Only through energy loss in the wiring. The battery itself delivers energy at its terminal voltage, but resistance in the cables wastes power as heat, reducing efficiency. Thin or long cables on high-current loads can lose several percent. For precise wiring losses, pair this calculator with the Voltage Drop Calculator.

❓ Can I use this calculator for solar systems?

✅ Yes — the Capacity Needed mode answers the core off-grid sizing problem: how large a battery bank must be to carry the load through the night or a cloudy period. Pair the result with the Solar Panel Calculator, which sizes the panels and charging side to replenish the bank. Use a longer target runtime than your nightly load if you want multiple days of autonomy.

❓ How do I run a 120V appliance from a battery?

✅ You need an inverter to convert DC battery power to AC. Multiply the appliance wattage by the inverter's inefficiency — for example, 100 W ÷ 0.85 = 118 W drawn from the battery — then use that adjusted figure as the load in Runtime mode. The calculator's efficiency input accounts for this directly.

❓ What does the C-rate output mean?

✅ C-rate is the discharge current divided by battery capacity. A C-rate of 0.1 means the battery delivers 10% of its capacity per hour (a 10-hour discharge); 1C means full discharge in one hour. C-rates above about 0.5 reduce effective capacity in lead-acid batteries and stress lithium cells, so keeping the C-rate low extends both runtime and battery life.