Battery Runtime Calculator: How Long Will Your Battery Last?

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The Runtime Formula

Battery runtime tells you how many hours a battery will power a specific load before it reaches its minimum safe state of charge.

Runtime (hours) = Usable Battery Energy (Wh) / Load (Watts)

Where usable energy accounts for depth of discharge and inverter efficiency:

Usable Wh = Total Wh x DoD x Inverter Efficiency

And total energy is:

Total Wh = Ah x V

Putting it all together:

Runtime = (Ah x V x DoD x Inverter Efficiency) / Load Watts

For a 100Ah 12V lithium battery running a 500W AC load through a 93% efficient inverter at 90% DoD:

Runtime = (100 x 12 x 0.90 x 0.93) / 500 = 1,004 / 500 = 2.0 hours

The same battery with lead-acid chemistry at 50% DoD:

Runtime = (100 x 12 x 0.50 x 0.93) / 500 = 558 / 500 = 1.1 hours

Chemistry, depth of discharge, and inverter efficiency collectively determine whether a battery runs your load for one hour or four.

How Long Will a 100Ah 12V Battery Run Common Loads?

This is the most frequently asked version of the runtime question. A 100Ah 12V battery stores 1,200 Wh total. Here is the runtime for common household loads, assuming lithium LiFePO4 (90% DoD) with a 93% efficient inverter (1,004 Wh usable):

Key takeaway: A single 100Ah 12V battery handles low-power electronics for a full day or more, but high-wattage heating and cooling appliances drain it in under an hour. For extended backup, you need a larger battery bank or a 48V system.

Runtime At Different Battery Sizes

Here is runtime for a 300W continuous load (a moderate mix of lights, fridge, electronics) across common battery configurations, all using lithium at 90% DoD and 93% inverter efficiency:

At 300W average draw, a single 48V 200Ah server-rack battery provides over a day of runtime. Two of them would cover a weekend outage.

Inverter Efficiency: The Hidden Runtime Tax

Every time you convert DC battery power to AC for household appliances, the inverter consumes some energy as heat. This efficiency loss directly reduces runtime.

How Inverter Efficiency Varies With Load

Inverters do not run at the same efficiency across all loads. Efficiency is highest at 50-80% of rated capacity and drops at both extremes:

Practical implication: Oversizing your inverter too much hurts efficiency at low loads. A 5,000W inverter running a 200W load operates at perhaps 85% efficiency, while a 2,000W inverter running the same load hits 93%. Choose an inverter sized to your expected average load, not your absolute peak (most inverters handle 2x surge for short periods anyway).

Standby Consumption

Even with no load connected, an inverter draws power to keep its electronics running. Typical standby draw is 10-30W for a residential inverter. Over 24 hours, that is 240-720 Wh wasted -- a significant fraction of a small battery bank. If you are running intermittent loads (like a fridge that cycles on and off), the inverter draws standby power during the off cycles.

The Peukert Effect: Why Lead-Acid Fades at High Loads

If you are using lead-acid batteries (flooded, AGM, or gel), there is an additional runtime penalty at high discharge rates that the basic formula does not capture.

Peukert's law states that the effective capacity of a lead-acid battery decreases as the discharge rate increases. A battery rated at 100Ah at the C/20 rate (5A continuous for 20 hours) might deliver only:

• 100Ah at 5A (C/20 rate) -- the rated capacity
• 85-90Ah at 10A (C/10 rate)
• 75-85Ah at 20A (C/5 rate)
• 60-70Ah at 50A (C/2 rate)

This happens because lead-acid chemistry cannot convert active materials fast enough at high current. The faster you drain it, the less total energy you get.

The Peukert exponent quantifies this effect. A perfect battery has an exponent of 1.0 (no capacity loss at any rate). Typical values:

For lithium batteries, the Peukert effect is negligible. You get close to rated capacity at virtually any discharge rate within the manufacturer's specifications. This is another major advantage of lithium over lead-acid for high-power applications.

What This Means for Runtime

If you are running a 1,000W load from a 12V lead-acid battery, the current draw is about 83A -- a very high C/1.2 rate for a 100Ah battery. At this rate, Peukert losses could reduce effective capacity by 30-40%, cutting runtime significantly below what the simple formula predicts.

With lithium at the same load, you get within 2-5% of the calculated runtime.

Temperature Effects on Runtime

Battery capacity is temperature-sensitive. Manufacturers rate capacity at 25 degrees Celsius (77 degrees Fahrenheit). Deviate from this and capacity changes:

Cold Weather

A lead-acid battery in an unheated garage during a winter storm might deliver only 60% of its rated capacity -- exactly when you need it most.

Lithium charging restriction: Most LiFePO4 batteries cannot be charged below 0 degrees Celsius without risking permanent damage. Many modern lithium batteries include a heated BMS that warms the cells before charging. If yours does not, you must either insulate/heat the battery enclosure or avoid charging in freezing conditions.

Hot Weather

Heat also affects performance, though less dramatically for short-term runtime:

• Above 40 degrees Celsius (104 degrees Fahrenheit), lead-acid self-discharge rate increases significantly
• Sustained high temperatures accelerate degradation in all chemistries
• Lithium BMS systems may throttle charge/discharge to protect cells above 45 degrees Celsius

For systems installed in hot climates (garages in Arizona, outdoor sheds in Texas), ensure adequate ventilation and shade.

Sizing for Your Specific Needs

Here is a practical framework for choosing the right battery size based on runtime requirements:

Step 1 -- List your loads and their wattage. Be realistic. Use the average watts, not the peak. A fridge draws 150W when running but averages 50-70W over 24 hours.

Step 2 -- Determine how many hours of runtime you need. For grid-tied backup, 8-12 hours covers overnight. For off-grid, plan for 24-72 hours depending on your solar array size and weather patterns.

Step 3 -- Calculate total Wh needed:

Total Wh = Load (watts) x Runtime (hours)

Step 4 -- Divide by efficiency factors:

Required Battery Wh = Total Wh / (DoD x Inverter Efficiency)

Step 5 -- Convert to Ah at your system voltage:

Required Ah = Required Battery Wh / System Voltage

Example: You want to run 400W of essential loads for 12 hours on a 48V lithium system.

• Total Wh = 400 x 12 = 4,800 Wh
• Required Battery Wh = 4,800 / (0.90 x 0.93) = 5,734 Wh
• Required Ah = 5,734 / 48 = 119 Ah

A 48V 120Ah or 48V 150Ah battery covers this with a small margin.

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