This tool uses industry-standard electrical formulas to determine the optimal battery capacity for your solar installation. The Advanced Technical Model unlocks additional parameters used by engineers — including ambient thermal derating and conduit fill adjustments — to produce a more accurate estimated configuration.
Calculation based on standard electrical formulas (Ref: NEC Chapter 9 Tables for copper resistance and thermal derating). PROVISIONAL ESTIMATE: For educational reference only. Not for construction.
Depth of Discharge (DoD) is the single most important variable that separates lithium and lead-acid battery technologies in an off-grid solar context. It defines what percentage of a battery's rated capacity you can safely use before recharging — without causing accelerated degradation or permanent capacity loss.
Lithium iron phosphate (LiFePO4) batteries tolerate a 90% DoD with minimal long-term impact on cycle life, which is why they are increasingly preferred for solar storage despite a higher initial cost. In contrast, traditional lead-acid (AGM/Gel) batteries should never be routinely discharged below 50% — doing so dramatically shortens their lifespan from a potential 800 cycles down to fewer than 300.
Why Lead-Acid Requires ~2× the Capacity of Lithium: A 200Ah AGM battery bank provides only 100Ah of usable energy at 50% DoD — the same as a 100Ah LiFePO4 bank at 90% DoD. This difference in usable capacity is why the calculator outputs significantly higher Ah figures when lead-acid is selected. Over a 10-year system lifetime, the higher cycle life of lithium typically offsets the upfront cost premium.
One of the most overlooked factors in DIY solar installations is the effect of ambient temperature on electrical wiring. The National Electrical Code (NEC) mandates that conductor ampacity — the maximum safe current-carrying capacity — must be derated when wires operate in elevated temperature environments or when multiple current-carrying conductors are bundled together in conduit.
The underlying physics are straightforward: higher ambient temperatures reduce a conductor's ability to dissipate the heat generated by resistive losses (I²R heating). If a wire cannot shed heat fast enough, insulation degrades, increasing resistance, which further increases heating — a cascade that can lead to insulation failure and fire.
When multiple current-carrying conductors share a conduit, each conductor's heat output adds to the total thermal environment inside the conduit. NEC Table 310.15(C)(1) specifies:
The wire sizing tool on this page applies both thermal and conduit fill corrections simultaneously when Advanced Technical Settings are enabled, allowing you to see the compound effect on each AWG size's derated ampacity limit.
Not all electrical loads are equal. The inverter sizing calculator on this page distinguishes between three fundamental load types, each with different implications for inverter selection:
Resistive loads — such as electric heaters, toasters, and incandescent light bulbs — convert electrical energy directly into heat. Their current draw is predictable and steady, with no startup surge. Resistive loads are the most inverter-friendly, and either pure sine wave or modified sine wave inverters will work reliably.
Capacitive loads — such as LED televisions, laptop computers, and most battery chargers — use internal power supplies with capacitors. They create a brief current inrush at startup, but the surge is typically only 2–3× the running wattage and lasts just milliseconds. Modified sine wave inverters usually operate these loads, though efficiency and compatibility varies by device.
Inductive loads are the most demanding for inverter sizing. Motors — found in refrigerators, air conditioners, well pumps, power tools, and washing machines — require a large startup surge current that can be 3–7× their continuous running wattage. This surge occurs because the motor's coils initially act as a short circuit, requiring the inverter to deliver its peak rated power instantaneously.
Pure Sine Wave Inverters Are Required for Inductive Loads: Modified sine wave inverters produce a stepped approximation of AC power that causes induction motors to run hotter, less efficiently, and with greater vibration. Over time this degrades motor windings and dramatically shortens equipment life. If any appliance in your system contains a motor or compressor — a refrigerator, AC unit, or water pump — select a pure sine wave inverter rated at or above the highest surge_watts value shown in the appliance calculator above. Always add a 25% continuous buffer above total running watts to allow for thermal headroom.
The appliance library tool above automates this calculation, but the formula is:
Your inverter must satisfy both conditions simultaneously. For example, if your total running load is 2,000W (requiring a 2,500W continuous inverter) but your well pump surges to 3,000W, you need an inverter rated for at least 3,000W peak — while also handling 2,500W continuous. This often means selecting the next inverter size tier up.
All calculations on this page are estimated configurations for educational reference only. Results are based on standard electrical formulas (Ref: NEC Chapter 9 Tables). Always verify with a licensed electrical professional before installation.
Divide your total required capacity (shown in Ah by the calculator) by 100. For example, a 400Ah result means four 100Ah batteries wired in parallel (parallel adds Ah, keeps voltage constant). Always round up. The wiring visualizer tool on this page illustrates series vs. parallel configurations.
The wire sizing calculator uses the standard copper conductor formula: V_Drop = 2 × Distance × Amps × (Resistance_per_1000ft ÷ 1000). The factor of 2 accounts for both the outgoing and return conductors. NEC recommends keeping total voltage drop at or below 3% for branch circuits.
Higher voltage systems are more efficient for larger installations:
Higher voltage systems carry the same power at lower current, reducing resistive losses and allowing smaller wire gauge. A 48V system running 2,400W draws only 50A — the same system at 12V would draw 200A, requiring much heavier wiring.
The safety margin adds a percentage buffer to your adjusted load before calculating required capacity. A 20% margin accounts for real-world inefficiencies, unexpected loads, and gradual battery aging over the first few years. This prevents chronic over-cycling and significantly extends battery lifespan.