Battery Load Calculator: How to Size It Right

A battery load calculator works out how long a battery can power a given load, or what battery capacity you need to run a device for a set time. You enter the battery voltage and capacity, plus the power draw of your equipment, and it returns the runtime in hours (or the required amp-hours). This is the quickest way to check whether a battery bank is big enough for a camping trip, a solar setup, or a backup power plan.

Formula

The core formula is simple: Runtime (hours) = (Battery voltage × Battery capacity in Ah) ÷ Load power in watts.

In symbols: t = (V × Ah) / P, where:

• V = battery voltage (volts, V) – typical values are 12 V for car/RV, 24 V or 48 V for solar, 3.7 V for a single lithium cell.
• Ah = battery capacity in amp-hours (Ah) – the number of amps a battery can supply for one hour.
• P = power draw of the load (watts, W) – sum of all devices connected.
• t = runtime in hours (h).

The product V × Ah is the total stored energy in watt-hours (Wh). Dividing by the load power gives the time until the battery is empty.

Worked Example

Using the calculator defaults: a 12 V battery rated 100 Ah, powering a 60 W load.

1. Stored energy = 12 V × 100 Ah = 1200 Wh.
2. Runtime = 1200 Wh ÷ 60 W = 20 hours.

The calculator shows 20.00 hours, which matches. If you halve the load to 30 W, runtime doubles to 40 hours; if you double the load to 120 W, runtime drops to 10 hours.

When to Use a Battery Load Calculator

Use it in these common situations:

• Sizing a battery for a solar system – you know your daily watt-hour usage, you can work backwards to find the Ah needed at your system voltage.
• Choosing a power station for camping – compare the watt-hour rating of a portable power station to your devices’ draw.
• Estimating backup time for a sump pump or CPAP – check if a 12 V battery will last through a power outage.
• Designing an RV or boat electrical system – sum the loads and pick a battery bank that gives you the autonomy you want.

The calculator is also useful for understanding the relationship between voltage, capacity, and power – many people confuse amp-hours with watt-hours, and this tool makes the difference visible.

How to Read the Result

The result is the theoretical runtime under perfect conditions. In practice, you should treat it as an upper limit. Real batteries lose capacity as they age, and most battery types cannot be fully discharged without damage (see the safety section below). If the calculator says 20 hours, plan for 15–18 hours of usable runtime.

For loads that are not constant (like a refrigerator that cycles on and off), use the average power draw, not the peak. A fridge might draw 150 W when running but only run 30% of the time, so the average load is about 45 W.

Common Mistakes and Misconceptions

• Mixing up Ah and Wh: A 100 Ah battery at 12 V stores 1200 Wh, but a 100 Ah battery at 24 V stores 2400 Wh. Always multiply by voltage.
• Ignoring inverter losses: If you run AC devices from a battery via an inverter, add 10–15% to the load to account for conversion losses.
• Using peak power instead of average: For intermittent loads, use the average over time, not the maximum.
• Assuming 100% discharge: Most lead-acid batteries should not be discharged below 50% for regular use. Lithium (LiFePO4) can go to 80–90% but still not 100% for long life.
• Forgetting the battery’s own self-discharge: A battery sitting for weeks loses a few percent per month – negligible for short-term calculations but relevant for long-term storage.

Choosing the Right Battery for Your Load

To choose a battery, decide how long you want to run your load. For example, you need to run a 60 W fan for 8 hours. Required energy = 60 W × 8 h = 480 Wh. At 12 V, that is 480 Wh ÷ 12 V = 40 Ah. With a 50% depth-of-discharge limit for lead-acid, you need at least 80 Ah. With lithium, 50 Ah would suffice.

Consider the battery type:

• Lead-acid (flooded, AGM, gel): Cheaper upfront, but heavier and limited to ~50% discharge for longevity.
• Lithium (LiFePO4): Lighter, longer cycle life, can discharge deeper (80–90%), but costs more.
• Nickel-based (NiMH, NiCd): Rare in large battery banks, mostly for small electronics.

Also check the battery’s maximum discharge rate (C-rate). A battery rated 100 Ah with a 0.5C rating can deliver 50 A continuously. If your load draws more, the battery may shut off or degrade.

Limits of the Simple Formula

The basic formula assumes constant voltage and constant load, which is never exactly true. As a battery discharges, its voltage drops, and some loads draw more current as voltage falls (resistive loads) or less (many electronics). The formula also ignores temperature – cold batteries deliver less capacity, and high discharge rates reduce usable capacity (Peukert’s law for lead-acid). For rough planning, the simple formula is fine; for critical systems, use a battery monitor or a more detailed simulation.

For solar systems, you also need to factor in charging input. A battery load calculator only tells you the draw side; you need to know how many amp-hours your panels put back per day. A common rule is that you need enough solar to replace what you use, plus a buffer for cloudy days.

Safety and Accuracy Notes

Always follow the manufacturer’s recommended depth-of-discharge and charge limits. Over-discharging lead-acid batteries permanently reduces capacity; over-discharging lithium batteries can cause damage or fire risk. Use a low-voltage disconnect or a battery monitor to protect your battery. When connecting batteries in series or parallel, ensure they are the same type, age, and state of charge. Never mix different chemistries.

For safety, always fuse the battery circuit close to the battery, sized for the wire, not the load. If you are unsure about any electrical installation, consult a qualified electrician.

Frequently Asked Questions

How long will a 100Ah battery run a 500W inverter?

Assuming a 12 V battery and an inverter efficiency of 85%, the load on the battery is 500 W ÷ 0.85 = 588 W. The battery stores 1200 Wh, so runtime = 1200 ÷ 588 = 2.04 hours. With a 50% discharge limit for lead-acid, you get about 1 hour of usable runtime.

What’s the difference between amp-hours and watt-hours?

Amp-hours (Ah) measure charge, while watt-hours (Wh) measure energy. To convert, multiply Ah by voltage. For example, a 12 V 100 Ah battery has 1200 Wh. Watt-hours are more useful for comparing batteries of different voltages because they represent the actual energy available.

Can I use this calculator for lithium batteries?

Yes, the formula works for any battery. The main difference is that lithium batteries can be discharged deeper (typically 80–90%) without damage, so you can use more of the rated capacity. Adjust your usable capacity accordingly.

How do I calculate the load for multiple devices?

Add up the wattage of all devices that will run at the same time. If some run only intermittently, use their average power. For example, a 40 W TV running 3 hours and a 10 W LED light running 5 hours gives (40×3) + (10×5) = 170 Wh per day.

Why does my battery run out faster than the calculator says?

Common reasons: the battery is old or not fully charged, the load is higher than you think (inverter losses, startup surges), the battery is cold, or you are discharging below the recommended depth. Also, lead-acid batteries lose capacity at high discharge rates.

What size battery do I need for a 2000W inverter?

At 12 V, a 2000 W load draws about 167 A (2000 ÷ 12). For 1 hour of runtime, you need at least 167 Ah, but with a 50% discharge limit, you’d need a 334 Ah battery bank. At 24 V, the current halves, so you’d need 167 Ah total. Consider using a higher voltage system for large loads.

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in v | Battery voltage | V | 12 | 0
in ah | Battery capacity | Ah | 100 | 0
in w | Load power | W | 60 | 0
out hours | Runtime | hours | (ah * v) / w | 2
out wh | Stored energy | Wh | ah * v | 0