Inverter Runtime Calculator

Inverter runtime equals usable battery watt-hours divided by the AC load in watts. Take a 100Ah 12V battery: rated energy is 100 × 12 = 1,200 Wh. Use 80% of it through a 90%-efficient inverter and 864 Wh remain usable. Powering a 500 W load, that battery runs 864 ÷ 500 = about 1.7 hours. The calculator above does this instantly and also shows usable watt-hours, minutes, and the DC amps drawn from the battery.

Two adjustments separate this from the naive "amp-hours × volts ÷ watts" estimate: depth of discharge (you should not drain a battery flat) and inverter efficiency (5–15% of energy becomes heat in conversion). Both shorten real runtime, and both are built into the result so your estimate matches the meter.

The equation behind the calculator is:

Reading it term by term:

• Amp-hours × Voltage converts battery charge into rated energy (watt-hours). A 100Ah 12V bank stores 1,200 Wh; a 200Ah 24V bank stores 4,800 Wh.
• Depth of Discharge (DoD) trims that to what you can safely use — 80–100% for LiFePO4 lithium, 50% for lead-acid.
• Efficiency (η) subtracts inverter conversion losses, typically leaving 85–95%.
• ÷ Load in Watts turns usable energy into time, because watt-hours ÷ watts = hours.

The tool also reports the DC draw (load ÷ efficiency ÷ voltage) so you can confirm the battery can source the current — cross-check it with the inverter size calculator. To pick the inverter's wattage, use the what size inverter do I need calculator.

Example 1 — 100Ah 12V lithium, 500 W load, 80% DoD, 90% efficiency: Usable = 100 × 12 × 0.80 × 0.90 = 864 Wh. Runtime = 864 ÷ 500 = 1.73 hours (about 104 minutes).

Example 2 — 200Ah 24V lithium, 1,000 W load, 90% DoD, 92% efficiency: Usable = 200 × 24 × 0.90 × 0.92 = 3,974 Wh. Runtime = 3,974 ÷ 1,000 = 3.97 hours. Doubling voltage let this bank store far more energy for the same amp-hours.

Example 3 — 100Ah 48V, 300 W load, 80% DoD, 95% efficiency: Usable = 100 × 48 × 0.80 × 0.95 = 3,648 Wh. Runtime = 3,648 ÷ 300 = 12.16 hours — a full overnight for lights and electronics.

Example 4 — 100Ah 12V AGM lead-acid, 500 W load, 50% DoD, 85% efficiency: Usable = 100 × 12 × 0.50 × 0.85 = 510 Wh. Runtime = 510 ÷ 500 = 1.0 hour — barely half the lithium result in Example 1 from the same nominal battery, because lead-acid should stop at 50%.

Two "100Ah" batteries can deliver very different runtimes. A LiFePO4 lithium battery routinely gives roughly double the usable runtime of lead-acid of the same rating, for three reasons:

1. Deeper discharge. Lithium uses 80–100% of capacity; lead-acid should stop near 50% to preserve cycle life.
2. Flatter voltage curve. Lithium holds voltage near nominal until nearly empty, so the inverter stays efficient and does not trip its low-voltage cutoff early. Lead-acid sags under load.
3. Peukert effect. Lead-acid capacity shrinks at high discharge rates; lithium is largely immune, so the runtime gap widens as load grows.

Voltage matters too — but only through efficiency, not stored energy. A 100Ah 12V and a 50Ah 24V bank store the same 1,200 Wh and run the same time on a given load. Higher voltage helps by lowering current and resistive losses, nudging real efficiency up. When you compare battery quotes, compare usable watt-hours, not nameplate amp-hours, and confirm runtime with the battery runtime calculator.

The formula assumes your load is the only draw, but an inverter also consumes power just being on. This no-load or idle draw — often 15–40 W for a mid-size unit — runs continuously and quietly shortens runtime, especially for small loads left on overnight.

If a 3,000 W inverter idles at 30 W and you run a 20 W night light, the inverter is drawing 50 W total, so your runtime is less than half what the light alone suggests. For long, light-load use, either pick a smaller inverter, enable the inverter's power-save/eco mode (which sleeps until a load appears), or run the device on DC directly. To be conservative for overnight scenarios, drop the efficiency input a few points to fold idle loss into the estimate.

Other real-world reducers include cold temperature (lead-acid loses 20–35% capacity near freezing), battery age (a three-year-old "100Ah" battery may effectively be 85Ah), and surge cycling from motors that repeatedly restart. Enter the battery's real measured capacity and a realistic efficiency for the most honest number.

The runtime formula uses your continuous running watts, but real loads rarely sit at a constant draw. Two effects change the picture, and both usually work in your favor for runtime while complicating inverter sizing.

Surge (inrush) does not consume much energy. When a fridge compressor or pump starts, it spikes to 2–3× its running watts for a fraction of a second. That surge matters enormously for whether the inverter can start the load — size it with the what size inverter do I need calculator — but it draws energy for such a short time that it barely dents runtime. Always calculate runtime on running watts, never surge watts, or you will badly underestimate how long the battery lasts.

Duty cycle stretches runtime. Cycling appliances run only part of each hour. A refrigerator rated 150 W running might average just 50–70 W over a full day because the compressor is off most of the time. If you enter 150 W continuous, the calculator gives a conservative (short) runtime; the real figure is often 1.5–2× longer. For an honest average, either enter the appliance's measured average watts or estimate its effective full-power hours. A good workflow: total your loads' true daily energy with the power consumption calculator, then divide usable battery watt-hours by that daily energy to get days of runtime rather than hours.

Combining these, a 100Ah lithium bank that this tool says lasts 1.7 hours on a steady 500 W might realistically carry a cycling 500 W-peak appliance mix for three or four hours, because the average draw is well below the peak. Steady loads (heaters, lights, chargers) behave exactly as calculated; motor-driven loads over-deliver.

To see the whole landscape at a glance, the table below pairs common lithium bank sizes with typical loads, using 80% depth of discharge and 90% inverter efficiency. Find the row nearest your bank and read across to your load. It is a fast planning shortcut before you fine-tune the exact numbers in the calculator above.

The pattern to internalize is that runtime scales directly with usable watt-hours and inversely with load: double the bank and you double the time, double the load and you halve it. A modest 100Ah 12V bank comfortably carries lights and electronics through a long evening but empties fast under a heater or kettle, while a 200Ah 24V bank crosses into overnight-essentials territory. For loads not shown, interpolate — a 750 W draw lands halfway between the 500 W and 1000 W columns.

1. Enter battery capacity in amp-hours. Add amp-hours for batteries in parallel; keep amp-hours the same for series.
2. Select battery voltage. Use the bank's nominal voltage — 12V, 24V, or 48V.
3. Enter the AC load in watts. Total everything running through the inverter at once.
4. Set depth of discharge. 80–100% for lithium, 50% for lead-acid.
5. Set inverter efficiency. 90% default; lower it for cold conditions, idle-heavy light loads, or budget inverters.

The result updates live in hours and minutes. Adjust the load down to see how much longer essentials would last during an outage — a fast way to plan what to keep running.

Home backup. During an outage, prioritize a fridge, a few lights, internet, and phone charging — roughly 150–300 W averaged. A 100Ah lithium bank carries those essentials for much of a day; a 200–300Ah bank plus solar extends it indefinitely.

RV and van life. Size for an evening plus overnight: lights, a 12V fridge, fans, and charging. A 100–200Ah lithium bank with 200–400 W of solar is a common baseline. A residential fridge on an inverter averages 30–60 W over a day but surges on start-up, so confirm the inverter surge rating too.

Off-grid. Runtime becomes days of autonomy — how long the bank carries the load with no charging. Layer daily consumption from the power consumption calculator on top of this single-load math, and confirm generator backup with the generator size calculator.

• Using rated instead of usable capacity. The label assumes 100% discharge most chemistries cannot sustain.
• Ignoring inverter efficiency and idle draw. Both quietly cut runtime, especially for small overnight loads.
• Forgetting the load cycles. Fridges and pumps run intermittently, so actual runtime is often longer than a continuous-load estimate.
• Cold-weather optimism. Lead-acid loses substantial capacity near freezing; derate efficiency.
• Aging batteries. Enter measured capacity, not the nameplate, for an old bank.

A reliable habit is to plan for the worst realistic case and treat the calculator's number as an optimistic ceiling. Assume the battery is a few years old and below rated capacity, assume a cold garage in winter if that is where the bank lives, and assume the inverter's idle draw is running the whole time. Dropping the efficiency input to 80–85% folds those pessimistic factors into a single conservative estimate, so the real outage does not surprise you. Then validate against reality: during a planned test, run your actual essentials off the bank and time how long it holds before the low-voltage cutoff, and compare that to the prediction. Once you know the gap between calculated and measured runtime for your specific battery, chemistry, and loads, you can size the next bank with confidence — and decide whether adding amp-hours, adding solar, or trimming the load list is the cheapest path to the runtime you actually need.