Figure out exactly how many batteries you need for your off-grid solar system. Enter your daily energy use (or use the appliance estimator), pick your battery type, and get total storage, battery count, series/parallel wiring, and estimated cost.
Every battery has a rated capacity — 100Ah, 200Ah, 300Ah — but that number is the total capacity from fully charged to completely dead. In practice, you should never discharge most batteries anywhere near 100%, and how deep you regularly discharge them is the single biggest factor determining how many charge cycles — and how many years — you get out of them.
Depth of discharge (DoD) is the percentage of a battery's capacity you actually use before recharging. A 100Ah battery discharged to 50% DoD has used 50Ah. Flooded lead-acid and AGM batteries are the most sensitive to deep discharges. A flooded lead-acid battery cycled to 50% DoD consistently might give you 400–600 cycles. The same battery cycled to 80% DoD regularly might give you only 150–200 cycles before its capacity degrades to the point of replacement. In real-world terms, that's the difference between a battery bank lasting 3–5 years versus 1–2 years. This is why this calculator defaults lead-acid and AGM to 50% DoD — it's the sweet spot that balances usable capacity with long battery life.
Lithium iron phosphate (LiFePO4) batteries are a fundamentally different chemistry. They can be cycled to 80–90% DoD with far less damage to the cathode structure. A quality LiFePO4 battery cycled to 80% DoD daily will often deliver 2,000–4,000+ cycles — potentially 10–15 years of daily cycling. This longer cycle life, combined with higher energy density and lighter weight, is why lithium is increasingly the preferred choice for serious off-grid systems despite its higher upfront cost. Over a 10-year system lifespan, the total cost of ownership for lithium often comes out lower than lead-acid once you factor in replacement battery costs.
The practical implication: if you size a lead-acid bank for 50% DoD and then regularly discharge to 70% or 80% because your panels didn't fully recharge the bank, you're dramatically shortening battery life. Never size a lead-acid or AGM bank so tight that you're routinely hitting deep discharges — always add margin.
Voltage and amperage are inversely related for the same amount of power (Watts = Volts × Amps). A 2,400W load on a 12V system draws 200 amps. The same 2,400W load on a 48V system draws only 50 amps. This matters enormously for wire sizing, heat losses, and inverter efficiency.
Copper wire resistance causes power loss proportional to the square of current — double the current means four times the resistive losses. A 12V system carrying 200 amps needs extremely heavy wire (2/0 or 4/0 AWG) over even short distances to keep losses acceptable. A 48V system carrying 50 amps can use much lighter wire (4 AWG or 6 AWG) with less resistance loss. For any system above about 2,000W continuous load, 48V is the practical standard because the wire costs and power losses at lower voltages become prohibitive.
Charge controllers and inverters also operate more efficiently at higher system voltages. MPPT solar charge controllers — the type used in virtually all modern systems — convert the high-voltage output of solar panels down to battery voltage. At 48V, there's less voltage step-down required, and the controller runs cooler and more efficiently. Most quality inverters also achieve their peak efficiency at 48V input rather than 12V or 24V.
The main argument for 12V is simplicity: 12V components are universally available, and a small 12V system (cabin lights, phone charging, a small fridge) can be built very cheaply. If your total load is under about 500W and you're not running an inverter for AC appliances, 12V is completely reasonable. For a full homestead with a refrigerator, chest freezer, lights, charging, and a water pump, 48V will cost you less in wire, run cooler, and deliver better inverter efficiency.
Batteries in series add voltage while keeping capacity (Ah) the same. Two 12V 100Ah batteries wired in series produce 24V at 100Ah — 2,400Wh total. Batteries in parallel add capacity (Ah) while keeping voltage the same. Two 12V 100Ah batteries in parallel produce 12V at 200Ah — also 2,400Wh total. To reach both a higher voltage and higher capacity, you combine series strings wired in parallel.
A common 48V lithium bank using 12V batteries might have 4 batteries in series per string (4 × 12V = 48V) and 3 strings in parallel — 12 batteries total. Each string contributes the same voltage (48V) and the parallel wiring sums their amp-hour capacity. This calculator tells you exactly how many batteries go in series per string and how many strings you need in parallel.
When wiring batteries in parallel, it's critical to use identical batteries — same brand, same age, same capacity, same state of charge when you connect them. Mixing old and new batteries in parallel causes the new batteries to constantly try to charge the old ones, dramatically shortening the life of both. If you're adding capacity to an existing bank, the safest approach is to match the exact same battery model and replace the whole bank at once rather than mixing old and new.
For series wiring, all batteries must also be identical. A weak battery in a series string becomes the limiting factor for the entire string, like the weakest link in a chain. Always balance-charge your batteries before building a series string, and monitor individual battery voltages periodically to catch a weak cell before it drags down your whole system.
Use the appliance estimator at the top of this calculator as a starting point. The key is being honest about run time. A refrigerator rated at 150W doesn't run 24 hours a day — the compressor cycles on and off, so actual daily energy is more like 150W × 8 hours = 1,200Wh on a moderate day. A chest freezer is more efficient, running around 100W × 6–8 hours per day. Lights, phone chargers, and laptops are easy to estimate by actual hours of use. For pumps and motors, count the actual run time, not the time they're plugged in. Once you have a rough daily total, add 15–20% as a safety margin — it's always better to have slightly more battery than you need. You can also get a precise reading by plugging your actual loads into a Kill-A-Watt meter for a week to measure real consumption before investing in a battery bank.
Both are lead-acid batteries with the same basic electrochemistry — lead plates in a sulfuric acid electrolyte — but the physical construction differs. Flooded lead-acid batteries have liquid electrolyte that can evaporate and must be periodically topped off with distilled water. They vent hydrogen gas during charging and must be installed in a well-ventilated enclosure. They're the cheapest option per Wh and have been used in off-grid systems for decades. AGM (Absorbed Glass Mat) batteries have the electrolyte absorbed into fiberglass mat separators — they're sealed, maintenance-free, can be mounted in any orientation, and don't vent gases during normal charging. They're more forgiving of installation conditions, perform better in cold temperatures, and accept higher charge rates. AGM costs roughly 20–40% more than flooded lead-acid per Wh but saves you the maintenance headache. Both are limited to 50% DoD for long life, and both are sensitive to chronic undercharging (sulfation) if they sit in a partial state of charge for extended periods.
Battery life is measured in charge cycles and depends almost entirely on how you use them. Flooded lead-acid and AGM at 50% DoD: 400–600 cycles, roughly 1–3 years of daily cycling. At 30% DoD (shallower): 800–1,200 cycles, 3–5 years. LiFePO4 at 80% DoD: 2,000–4,000 cycles, 7–15 years depending on brand quality. Temperature also matters significantly — heat above 40°C (104°F) accelerates degradation in all battery chemistries, and chronic undercharging causes sulfation in lead-acid that permanently reduces capacity. Install batteries in a temperature-controlled space when possible, keep them fully charged whenever solar production allows, and you'll get the maximum life from whichever chemistry you choose.
Lithium LiFePO4 batteries absolutely require a BMS — it's not optional. The BMS protects each cell from overcharge, over-discharge, overcurrent, and thermal runaway. Most commercially sold lithium battery banks have a built-in BMS, but if you're building a DIY lithium bank from cells, you must add a BMS rated for your system voltage and maximum charge/discharge current. Without a BMS, a lithium bank can experience cell imbalance that causes one cell to go to dangerous voltages while others remain normal, potentially causing a fire or permanent cell damage. Lead-acid batteries are more forgiving and don't require a BMS, though a good MPPT charge controller with temperature compensation is still essential for long life. For any system over about 1,000Wh, a battery monitor that tracks state of charge, charge/discharge rates, and cumulative cycles is a worthwhile investment regardless of battery type.
No. Never mix lithium and lead-acid batteries, and never mix batteries with different capacities, ages, or brands in the same series string or parallel bank. Each battery type has a different voltage curve, charging profile, and capacity — a charge controller can only optimize for one chemistry at a time. Mixing chemistries means one type will always be incorrectly charged, dramatically shortening its life and potentially causing safety issues. If you're upgrading from lead-acid to lithium, replace the entire bank at once and reprogram your charge controller to the lithium charging profile. The only acceptable mixing is identical batteries wired in parallel strings, and even then, they should be the same age and from the same manufacturing batch if possible.
Cold is the enemy of battery capacity — particularly for lead-acid. At 0°C (32°F), a flooded lead-acid battery delivers about 80% of its rated capacity. At -20°C (-4°F), that drops to around 50%. This means a battery bank sized for summer use may leave you short in winter. LiFePO4 handles cold better than lead-acid but still loses capacity below freezing — most LiFePO4 batteries also have charging protection that prevents charging at temperatures below 0°C to prevent lithium plating on the anode, which causes permanent damage. If your battery bank will be in an unheated space in a cold climate, use the cold-temperature derating factor for your chemistry and size up accordingly, or insulate the battery enclosure. In extreme cold, a small heating pad on a thermostat inside the battery enclosure can keep lead-acid batteries warm enough to deliver full rated capacity.