Battery Sizing Calculator
Find out exactly how many batteries you need for backup power or an off-grid system. Enter your load in watts, the runtime you want, and your battery type — the calculator accounts for depth of discharge and inverter efficiency to give you the required capacity in amp-hours and kWh.
Battery Sizing Formula
The battery sizing formula is: Ah = (Watts × Hours) ÷ (Volts × DoD × Efficiency).First multiply your load by the runtime to get the energy you need in watt-hours, then divide by the factors that determine how much of a battery's rated capacity is actually usable.
Required Ah = (W × h) ÷ (V × DoD × η)
Bank size in kWh = (Required Ah × V) ÷ 1,000
Batteries needed = Required Ah ÷ Ah per battery (round up)
- W (watts) — the total continuous load you want to power. Add up all appliances that run at the same time. Use average draw for cycling loads like refrigerators, not just nameplate wattage.
- h (hours) — how long the batteries must carry the load before recharging.
- V (volts) — your system voltage: 12V for small setups, 24V or 48V for larger banks. Higher voltage means lower current and smaller wire for the same power.
- DoD (depth of discharge) — the fraction of rated capacity you can safely use: 0.5 (50%) for lead-acid and AGM, 0.8 (80%) for LiFePO4.
- η (inverter efficiency) — the DC-to-AC conversion loss. Most inverters run 85–95% efficient; 0.9 (90%) is a good planning value. Skip this factor for pure DC loads.
Worked Example
Suppose you need to run a 500W load for 8 hours on a 24V LiFePO4 bank (80% DoD) through an inverter that is 90% efficient:
Energy needed = 500 W × 8 h = 4,000 Wh
Required Ah = 4,000 ÷ (24 × 0.8 × 0.9)
Required Ah = 4,000 ÷ 17.28 = 231.5 Ah
Batteries = 231.5 ÷ 100 Ah = 2.3 → three 100Ah batteries in parallel
Three 24V 100Ah LiFePO4 batteries in parallel give you a 300Ah (7.2 kWh) bank — comfortably above the 231.5 Ah minimum, leaving margin for cold weather, battery aging, and loads running slightly longer than planned. Not sure what your total load is? Add up your circuits with the whole house load calculator first.
Depth of Discharge by Battery Type
Depth of discharge (DoD) is the biggest single factor in battery sizing. Discharging deeper than the recommended limit shortens battery life dramatically — especially for lead-acid chemistries.
| Battery Type | Recommended DoD | Usable Ah per 100Ah | Cycle Life Notes |
|---|---|---|---|
| Flooded Lead-Acid | 50% | 50 Ah | Roughly 500–1,000 cycles at 50% DoD; deep discharges below 50% cut cycle life sharply. Requires ventilation and water maintenance. |
| AGM (Sealed Lead-Acid) | 50–60% | 50–60 Ah | Roughly 500–1,200 cycles at 50% DoD. Maintenance-free and spill-proof; tolerates occasional deeper discharge better than flooded cells. |
| Gel | 50–60% | 50–60 Ah | Similar cycle life to AGM with better deep-cycle tolerance, but sensitive to charge voltage and generally more expensive per Ah. |
| LiFePO4 (Lithium Iron Phosphate) | 80–100% | 80–100 Ah | Typically 3,000–6,000+ cycles even at 80% DoD. Built-in BMS protects against over-discharge. Highest upfront cost but lowest cost per usable kWh over its life. |
Practical takeaway: a 100Ah LiFePO4 battery delivers about as much usable energy as a 160Ah lead-acid battery. When comparing prices, always compare usable capacity, not rated capacity.
Series vs. Parallel Battery Wiring
Once you know the total capacity you need, you have to decide how to wire the batteries together. The rule is simple: series adds voltage, parallel adds amp-hours.
Series Connection
Positive terminal of one battery to the negative of the next. Voltages add; capacity in Ah stays the same.
4 × 12V 100Ah in series
= 48V, 100Ah (4.8 kWh)
Parallel Connection
All positives together and all negatives together. Voltage stays the same; capacities in Ah add.
4 × 12V 100Ah in parallel
= 12V, 400Ah (4.8 kWh)
Notice that both configurations store the same total energy — 4.8 kWh. The difference is the operating voltage. Higher-voltage banks (24V or 48V) draw less current for the same power, which means smaller wire, less voltage drop, and cheaper inverters at higher power levels. That's why most whole-home backup and off-grid systems run at 48V, while small RV and boat systems stay at 12V. You can also combine both: two series strings of two 12V batteries wired in parallel (series-parallel) gives 24V 200Ah.
When paralleling, use batteries of the same chemistry, capacity, and age, and keep interconnect cables equal length so the batteries share current evenly. Use the volts to amps calculator to see how much current your bank will deliver at each voltage.
Common Backup Scenarios
Required battery capacity for typical backup loads, calculated for a 24V LiFePO4 bank (80% DoD, 90% inverter efficiency):
| Load | Avg. Watts | 4 Hours | 8 Hours | 24 Hours |
|---|---|---|---|---|
| Refrigerator (average draw) | 150 W | 34.7 Ah | 69.4 Ah | 208.3 Ah |
| Sump pump (1/3 HP, intermittent) | 400 W | 92.6 Ah | 185.2 Ah | 555.6 Ah |
| CPAP machine (no heated humidifier) | 60 W | 13.9 Ah | 27.8 Ah | 83.3 Ah |
| Whole-home essentials | 1,500 W | 347.2 Ah | 694.4 Ah | 2083.3 Ah |
“Whole-home essentials” assumes refrigerator, sump pump, furnace blower, lights, and electronics averaging about 1,500W. Note that cycling loads like refrigerators and sump pumps draw far less on average than their nameplate rating — but their motor starting surge can be 3–5× the running wattage, so make sure your inverter can handle the surge. For outages longer than a day, pairing batteries with a generator is usually more economical — see the generator sizing calculator.
Frequently Asked Questions
How do I calculate what size battery I need?
Multiply your load in watts by the runtime in hours to get watt-hours, then divide by system voltage, depth of discharge, and inverter efficiency: Ah = (Watts × Hours) ÷ (Volts × DoD × Efficiency). For example, a 500W load for 8 hours on a 24V LiFePO4 system (80% DoD, 90% inverter efficiency) needs 4,000 ÷ (24 × 0.8 × 0.9) = 231.5 Ah of battery capacity.
How many batteries do I need to run a refrigerator overnight?
A typical refrigerator averages about 150 watts (it cycles on and off). Over a 10-hour night that is 1,500 Wh. On a 12V LiFePO4 system with 80% depth of discharge and 90% inverter efficiency, you need 1,500 ÷ (12 × 0.8 × 0.9) = 173.6 Ah — so two 12V 100Ah LiFePO4 batteries in parallel. With flooded lead-acid at 50% DoD you would need 1,500 ÷ (12 × 0.5 × 0.9) = 277.8 Ah, or three 100Ah batteries.
What is depth of discharge?
Depth of discharge (DoD) is the percentage of a battery's rated capacity that you actually use before recharging. A 100Ah battery discharged to 50% DoD delivers 50Ah. Lead-acid batteries should only be discharged to about 50% to avoid drastically shortening their life, while LiFePO4 batteries can safely use 80–100% of their capacity. This is why a 100Ah LiFePO4 battery delivers roughly as much usable energy as a 160Ah lead-acid battery.
Should I wire batteries in series or parallel?
Wire batteries in series to add voltage (Ah stays the same) and in parallel to add amp-hour capacity (voltage stays the same). Four 12V 100Ah batteries in series make a 48V 100Ah bank; the same four in parallel make a 12V 400Ah bank. Both store the same 4.8 kWh of energy. Higher-voltage banks (24V or 48V) are preferred for larger loads because they cut current, allowing smaller wire and reducing losses.
What size battery do I need to run a 1000W load for 5 hours?
A 1,000W load for 5 hours consumes 5,000 Wh. On a 24V LiFePO4 system (80% DoD, 90% inverter efficiency) you need 5,000 ÷ (24 × 0.8 × 0.9) = 289.4 Ah — three 24V 100Ah batteries in parallel. On a 48V system the same load needs 5,000 ÷ (48 × 0.8 × 0.9) = 144.7 Ah, which is why larger backup systems use 48V.
Why does inverter efficiency matter when sizing batteries?
An inverter converts DC battery power to 120V/240V AC power, and the conversion wastes energy as heat. Most inverters are 85–95% efficient, so about 90% is a safe planning figure. To deliver 900W to your AC loads, the battery must actually supply about 1,000W. Skipping the efficiency factor undersizes your battery bank by roughly 10%, which can mean losing an hour of runtime overnight.
Related Electrical Calculators
Solar System Calculator
Size the solar array and components needed to recharge your battery bank.
Size a Solar System →Generator Sizing Calculator
Compare battery backup against a generator for longer outages.
Size a Generator →Whole House Load Calculator
Add up your home's loads to find the wattage your batteries must supply.
Calculate House Load →kWh Calculator
Convert appliance wattage and runtime into kilowatt-hours of energy use.
Calculate kWh →Volts to Amps Calculator
Find the DC current your battery bank delivers at 12V, 24V, or 48V.
Convert Volts to Amps →Watts to Amps Calculator
Size inverter cables and fuses by converting load wattage to amps.
Convert Watts to Amps →