Off-Grid Battery Bank Calculator: kWh, Amp-Hours, and Days of Autonomy

Depth of discharge (DoD) is the percentage of a battery’s total capacity that can be safely discharged in normal use. Discharging a lead-acid battery below 50% state of charge (SOC) causes sulfation of the lead plates, permanently reducing capacity and shortening service life. A 100 Ah lead-acid battery operated regularly at 80% DoD (discharged to 20% SOC) may fail within 100 to 150 cycles, compared to 400 to 500 cycles when kept within the 50% DoD limit. Lithium LFP batteries have a flat discharge curve and can be routinely discharged to 20% SOC (80% DoD) without chemical damage, which is why LFP delivers more usable energy per installed kWh despite its higher purchase cost. Understanding DoD is the most important concept in off-grid battery sizing, and the reason this calculator applies chemistry-specific DoD limits rather than using a single generic factor.

For new off-grid installations in the US in 2024 and 2025, lithium LFP is the preferred choice for most applications due to its significantly longer cycle life, higher usable capacity per dollar over the system lifetime, minimal maintenance, and better cold-temperature performance compared to lead-acid chemistries. When compared on a cost per usable kWh over a 10-year period: lead-acid batteries requiring replacement every 3 to 5 years actually cost more per kWh-cycle than LFP batteries lasting 10 to 15 years in the same application. Flooded lead-acid is appropriate for very tight budgets where lower upfront cost is essential, systems in temperature-controlled environments, and users comfortable with monthly maintenance. AGM is appropriate for sealed environments (like RV compartments where off-gassing is a concern), temperature-controlled installations where the maintenance advantage over flooded is valued, and moderate-size systems where budget does not support LFP. Lithium LFP is appropriate for virtually all new installations where budget permits, particularly in cold climates (where lead-acid derating is most severe), weight-sensitive applications (RV, marine), and systems where maximum autonomy per installed kWh is the priority.

Lithium iron phosphate (LiFePO4, abbreviated LFP) is a specific lithium battery chemistry that uses an iron-phosphate cathode material. It differs from the lithium NMC (nickel manganese cobalt) and lithium NCA (nickel cobalt aluminum) chemistries found in consumer electronics and most electric vehicles in several important ways: LFP is significantly more thermally stable (substantially lower fire and thermal runaway risk), has a longer cycle life (2,000 to 5,000 cycles vs. 500 to 1,000 for NMC), has a slightly lower energy density (90-160 Wh/kg vs. 150-250 Wh/kg for NMC), and does not contain cobalt (an ethically and supply-chain problematic material). For stationary energy storage (off-grid homes, solar batteries, EV home chargers, UPS systems), LFP’s superior safety, cycle life, and temperature stability make it the preferred chemistry over other lithium types. Tesla uses NMC for their vehicle batteries but chose LFP for the Powerwall 3 home battery, specifically because the safety and longevity advantages of LFP are more important than the energy density advantage of NMC for home storage applications.

A battery management system (BMS) is an electronic circuit that monitors and protects a lithium battery pack by controlling charging and discharging to prevent conditions that damage or create safety risks with lithium cells. The BMS monitors: individual cell voltages (preventing overcharge above 3.65V per cell or over-discharge below 2.5V per cell for LFP); temperature (cutting off charging below 32F / 0C to prevent lithium plating, cutting off discharge above 140F / 60C); current (preventing short circuits and overcurrent); and state of charge estimation. For lithium LFP batteries, a BMS is not optional; it is required for safe operation. Most quality lithium battery packs designed for off-grid use (including Battle Born, Renogy, Battleborn, SOK, and brand-name LFP batteries) come with an integrated BMS. If you are building a battery bank from individual lithium cells, a separate BMS matched to your cell configuration is essential. Lead-acid batteries (flooded and AGM) do not require a BMS; their chemistry self-limits overcharge naturally through off-gassing (flooded) or pressure venting (AGM), though an appropriate charge controller with lead-acid charging profiles is still required.

The appropriate days of autonomy depends on your location’s weather patterns, whether you have a backup generator, and your tolerance for managing the system actively. For a solar-paired system: 1 to 2 days suits most US Sun Belt locations (Arizona, New Mexico, Nevada, Texas, Florida) where multi-day cloudy periods are rare. 2 to 3 days suits moderate climate regions (Midwest, Mid-Atlantic, Southeast) that experience occasional cloudy stretches. 3 to 5 days suits the Pacific Northwest and New England, where winter overcast can persist for 5 to 10 days. For battery-only backup without solar: size for the longest historical power outage in your area that you want to survive without generator use. For a generator-supplemented off-grid system: 1 to 2 days of battery autonomy is typically sufficient because the generator can recharge the bank on demand; the battery handles daily use between generator runs and avoids running the generator all night. More autonomy costs more in batteries; less autonomy costs more in generator fuel and runtime hours. The break-even point between battery capacity and generator fuel depends on your generator’s efficiency and fuel cost in your area.

System voltage selection is primarily driven by system size. 12V: appropriate for systems up to about 2 kWh of daily load, RVs, small cabins, marine, and 12V-native equipment. Simple wiring but requires heavy gauge wire for longer runs. 24V: appropriate for 2 to 6 kWh per day loads, allowing longer wire runs and lighter wire gauge than 12V. Most quality 24V inverter-chargers max out at 3,000 to 5,000 watts. 48V: recommended for any home system or any load above 3 to 4 kW peak demand. 48V allows lighter wiring over longer distances, supports larger inverter-chargers (up to 15,000 watts from a single unit), has better charging efficiency, and is compatible with virtually all high-performance battery systems. If your system will ever grow or your load might increase, designing at 48V provides the most room to expand. The exception: if you have existing 12V or 24V equipment (like an RV), matching the existing system voltage is often more practical than a wholesale upgrade. For new ground-up designs above cabin scale, 48V is almost always the right choice in the US market.

Cold temperatures significantly reduce battery capacity, especially for lead-acid and AGM chemistries. The electrochemical reactions inside lead-acid batteries slow in cold temperatures, reducing available capacity: at 32F (0C), a lead-acid battery delivers approximately 65% of its rated capacity. At 20F (-7C), approximately 50 to 55%. At 0F (-18C), 30 to 40%. This capacity reduction is temporary (the battery returns to full capacity when warmed) but very relevant for systems that operate in unheated spaces during winter. Lithium LFP batteries maintain capacity much better in cold: at 32F (0C), approximately 90% of rated capacity is available. At 20F (-7C), 75 to 80%. However, LFP batteries must not be charged below 32F (0C) because lithium plating can occur during charging at low temperatures, permanently reducing capacity and potentially creating safety risks. Most quality LFP battery packs with integrated BMS include a low-temperature charge cutoff that prevents charging when battery temperature is below 32F. For cold climate off-grid systems, either insulating the battery compartment to maintain temperatures above freezing, using self-heating LFP batteries (available from some manufacturers), or choosing AGM/lead-acid batteries with a larger bank to compensate for the reduced capacity is the standard approach.

Series wiring connects the positive terminal of one battery to the negative terminal of the next battery in line, adding the voltages together while keeping the amp-hour capacity the same. Two 12V 100Ah batteries wired in series = 24V 100Ah bank. Four 12V 100Ah batteries in series = 48V 100Ah. Parallel wiring connects all positive terminals together and all negative terminals together, keeping the voltage the same while adding amp-hours. Two 12V 100Ah batteries in parallel = 12V 200Ah. For larger banks, you combine series strings in parallel: two strings of four 12V 100Ah batteries (each string is 48V 100Ah) wired in parallel = 48V 200Ah bank. When designing a series-parallel bank: keep all batteries identical (same manufacturer, model, age, and state of charge history). Never mix old and new batteries. Balance the strings so each has the same number of batteries. Use equal-length cables between battery connections to balance current distribution. For lithium LFP batteries with built-in BMS, consult the manufacturer’s guidance on series-parallel wiring, as some BMS configurations do not support certain parallel configurations without additional balancing equipment.

No. Mixing batteries of different ages, capacities, or chemistries in a battery bank is one of the most common and damaging mistakes in off-grid system maintenance. In a parallel bank, stronger (newer or higher-capacity) batteries will discharge into weaker batteries rather than into the load, wasting energy and accelerating the degradation of both the strong and weak batteries. In a series bank, the weakest cell or battery limits the capacity of the entire string, and the stronger batteries in the string will be overworked trying to compensate, leading to premature failure. The correct approach when a battery in a bank fails: replace all batteries in the bank simultaneously with new batteries of the same model. If cost is prohibitive, isolate the failed battery by disconnecting it and reassess whether the remaining batteries can meet your capacity needs, rather than adding a single new battery to an existing aged bank. This guideline applies to all battery chemistries. Some off-grid system designers use parallel strings with individual fuses on each string specifically to allow a failed string to be isolated and replaced without taking the entire system offline.

Battery lifespan for off-grid systems: Flooded lead-acid: 3 to 7 years depending on DoD discipline, watering maintenance, temperature, and quality of charge management. Properly maintained flooded batteries kept within 50% DoD and equalized regularly can approach 7 years. Neglected batteries abused past 50% DoD regularly may fail in 2 to 3 years. AGM: 4 to 8 years under similar care, with the advantage of not requiring water addition or equalization. Lithium LFP: 10 to 15+ years in most off-grid applications, with manufacturers citing 2,000 to 5,000 cycles to 80% of original capacity. A system cycling once per day at 80% DoD would take 2,000 to 5,000 days (5 to 14 years) to reach end-of-life capacity. Most residential off-grid systems cycle less frequently (not fully discharged and recharged every day), extending LFP life further. The practical implication for system lifetime cost: over 15 years, a lead-acid system may require 3 to 4 battery replacements, while an LFP system may need none or one. The lifetime cost per kWh-cycle favors LFP significantly despite the higher upfront purchase price.

A 10 kWh total capacity battery bank at 48V using 100Ah batteries (approximately 8 batteries of 100Ah each) costs in 2024 to 2025 US pricing: Flooded lead-acid: $1,200 to $2,000 in equipment ($120 to $200 per kWh of total capacity), but requires replacement every 3 to 5 years. AGM: $1,800 to $2,500 in equipment ($180 to $250 per kWh total). Lithium LFP (from US distributors or premium brands like Battle Born, Renogy, SOK): $4,000 to $6,000 for 10 kWh total ($400 to $600 per kWh). LFP from offshore/Chinese sources (CATL-based cells in BYD, EVE, or similar battery packs): $2,500 to $4,000 for 10 kWh total, with quality variability. Note that 10 kWh of lead-acid provides only 5 kWh of usable energy (50% DoD), while 10 kWh of LFP provides 8 kWh of usable energy (80% DoD). The cost comparison on a per-usable-kWh basis: lead-acid $240 to $400 per usable kWh, AGM $300 to $417, LFP $500 to $750 upfront but $100 to $150 per usable kWh amortized over 15 years at LFP’s cycle life, versus lead-acid at $240 to $400 per kWh and 3 replacements over the same period.

Total battery capacity is the nameplate rating: 100 Ah or 10 kWh is what the manufacturer marks on the battery under ideal conditions (25C temperature, slow discharge rate). Usable battery capacity is the energy you can actually extract in normal use without damaging the battery: 50 kWh of a 100 kWh lead-acid bank (50% DoD), 60 kWh of a 100 kWh AGM bank, or 80 kWh of a 100 kWh LFP bank. The gap between total and usable is the most common source of confusion when sizing off-grid systems. A homeowner who calculates they need 6 kWh of daily backup and purchases 6 kWh of lead-acid batteries will be disappointed: they can only use 3 kWh (50% DoD), cutting their autonomy in half. This calculator always shows both total capacity and usable capacity, and sizes the battery bank so that the usable capacity meets your daily load and autonomy requirements. Usable capacity is what you plan around; total capacity is what you buy. The ratio between them is the depth of discharge, and it is the reason why battery chemistry matters so much to the final battery count and cost in your off-grid design.

The Tesla Powerwall 2 and Powerwall 3 are designed primarily for grid-tied backup applications, not pure off-grid use, though they can operate in “off-grid mode” with limitations. The Powerwall 2 provides 13.5 kWh usable capacity and requires a Tesla solar installation or a compatible third-party solar inverter (limited compatibility). It does not support connection to non-Tesla generator inputs. The Powerwall 3 (integrated solar inverter) expands off-grid capability but is still optimized for grid-interactive operation. For true off-grid systems with flexible generator integration, custom solar sizing, and full system control, purpose-built off-grid components from Victron Energy, Schneider Electric, SMA, or OutBack Power provide more flexibility, better generator integration, and compatibility with a wider range of solar charge controllers and inverters. The Powerwall’s main advantages for off-grid-adjacent use are its polished app interface, simple installation by Tesla-certified installers, and fire safety certification. Its limitations for off-grid: limited compatibility with non-Tesla inverters, no built-in generator input, and reduced control flexibility compared to purpose-built off-grid equipment.

C-rate is the rate at which a battery is charged or discharged relative to its capacity. A battery discharged at 1C is fully discharged in 1 hour. A 100Ah battery at 1C draws 100 amps. At 0.2C (C/5), the same battery is discharged over 5 hours at 20 amps. Lead-acid batteries are particularly sensitive to discharge rate: a 100Ah lead-acid battery discharged at 1C delivers only about 80 Ah of practical capacity. Discharged at 0.1C (over 10 hours), it delivers closer to 105 to 110 Ah. This is why lead-acid batteries are rated at a specific discharge rate (often C/20, meaning a 20-hour discharge), and using them at faster rates produces less usable energy than the nameplate suggests. For off-grid sizing purposes, the practical implication: if your system will draw high currents (starting a well pump or air compressor from a small battery bank), the instantaneous current demand may exceed what your battery bank can sustainably supply. Oversizing the battery bank by 20 to 30% beyond the energy calculation helps ensure adequate peak current delivery for motor-start loads. Lithium LFP batteries have a flatter discharge curve and less capacity reduction at high discharge rates, making them better suited to systems with variable peak demands.

To calculate daily load in kWh for battery sizing: (1) List every electrical appliance in your off-grid home or cabin. (2) For each one, find the wattage (from the nameplate or user manual) and estimate the hours of daily use. (3) Multiply wattage by hours to get watt-hours per day for each load. (4) Sum all load watt-hours and divide by 1,000 to convert to kWh. Example for a small off-grid cabin: LED lighting (6 fixtures at 10W for 5 hours = 300 Wh), propane refrigerator (no electrical load), laptop (50W for 4 hours = 200 Wh), phone charging (20W for 2 hours = 40 Wh), water pump (500W running for 0.5 hours per day = 250 Wh), fans (40W for 6 hours = 240 Wh). Total: 1,030 Wh = 1.03 kWh per day. For loads with variable use patterns, use average daily consumption rather than peak use. For appliances that cycle (refrigerator, water heater), the average running draw is typically 30 to 50% of the rated wattage. The appliance electricity cost calculator in this Energy Hub can help you build this list systematically with running watt data for common appliances.

Inverter sizing for off-grid systems is based on peak power demand (the maximum instantaneous wattage all your loads might draw simultaneously), not your daily energy consumption. The inverter must be able to supply the peak surge demand including motor starting surges. If you run a 1,500W microwave and a 750W well pump simultaneously, plus 300W of background loads, the inverter must supply at least 2,550W continuously. If the well pump surges to 2x running on startup, the peak demand spikes to 4,800W for 1 to 2 seconds. A 3,000W inverter would trip; a 5,000W inverter handles the surge comfortably. General sizing guidance: small cabin (lights, devices, no motors): a 2,000W pure sine wave inverter is typically adequate. Medium cabin or RV (refrigerator, small pump, kitchen appliances): 3,000 to 5,000W. Full off-grid home with well pump, AC or heat pump, kitchen: 8,000 to 15,000W. Inverter and battery bank must be compatible in system voltage and matched to each other’s current capacity. Use the inverter size calculator in this Energy Hub to find the correct inverter rating for your specific load list. See the Inverter Size Calculator for detailed load analysis.