Amp Hours to kWh Calculator

kWh = (Ah x Voltage) / 1000. For a 100 Ah 12V battery: (100 x 12) / 1000 = 1.2 kWh. The voltage is absolutely essential -- without it, Ah alone tells you nothing about stored energy. A 100 Ah 12V battery stores 1.2 kWh; a 100 Ah 48V battery stores 4.8 kWh -- four times the energy for the same amp-hour rating. The formula comes directly from the electrical energy equation: Energy (joules) = Charge (coulombs) x Voltage (volts). Since 1 Ah = 3,600 coulombs (1 A x 3,600 s), and 1 kWh = 3,600,000 joules, the conversion is: kWh = (Ah x 3,600 x V) / 3,600,000 = (Ah x V) / 1000. The factor of 1000 converts watt-hours (Wh) to kilowatt-hours (kWh).

At 12V: 1.2 kWh. At 24V: 2.4 kWh. At 48V: 4.8 kWh. The most common interpretation is at 12V (car, RV, marine batteries). For a 100 Ah deep-cycle lead-acid battery at 12V: the total stored energy is 1.2 kWh, but the USABLE capacity at 50% depth of discharge (the limit to avoid permanent capacity loss per IEEE 485) is only 0.6 kWh -- enough to run a 100W load for 6 hours. For LiFePO4 (lithium iron phosphate) at 100% DoD, the full 1.2 kWh is usable. This is why lithium batteries deliver roughly twice the usable energy of lead-acid for the same nameplate Ah. At the system level: a 48V, 100 Ah server rack battery (EG4, SOK, etc.) stores 4.8 kWh total, with about 4.3 kWh usable at 90% recommended DoD for maximum cycle life (typically 6,000+ cycles to 80% capacity).

First, calculate your daily kWh usage from your electric bill. The US average is approximately 30 kWh/day, but whole-home backup typically targets essential loads only (fridge, lights, internet, well pump, furnace blower, medical equipment) which is typically 8-15 kWh/day. Step-by-step: (1) List essential loads and sum their daily kWh. Example: refrigerator 3 kWh, LED lights 1 kWh, internet/router 0.5 kWh, well pump 1.5 kWh, furnace blower 2 kWh, microwave 0.5 kWh = 8.5 kWh/day. (2) Choose battery voltage: 48V is standard for residential. (3) Convert daily kWh to Ah: 8,500 / 48 = 177 Ah usable. (4) Adjust for DoD: LiFePO4 at 90% DoD needs 177 / 0.9 = 197 Ah rated. (5) Select batteries: one 48V 200 Ah server rack battery (9.6 kWh) covers one day plus margin. Add a second for 2-day autonomy during extended cloudy periods. With 2 x 200 Ah = 400 Ah at 48V = 19.2 kWh, the system provides over 2 full days of backup without solar recharge. (6) Size the solar array to fully recharge the battery bank in one sunny day: 19.2 kWh / 5 peak sun hours / 0.75 system efficiency = 5.1 kW of solar panels needed.

Energy (kWh) = Charge (Ah) x Voltage (V) / 1000. The amp-hours measure how many electrons the battery can move (current integrated over time), while the voltage measures the energy each electron carries (electrical potential difference, in joules per coulomb). Multiplying them gives total energy. This is identical to gravitational potential: the energy stored in a raised water tank depends on both the mass of water (analogous to charge) AND the height it is raised (analogous to voltage). A small mass raised very high can store as much energy as a large mass raised a small amount. Similarly, a 50 Ah 400V EV battery (20 kWh) stores 167 times the energy of a 50 Ah 12V car battery (0.6 kWh). Higher voltage systems deliver more energy for the same current, which is why EVs use 400V and 800V architectures -- more kWh per Ah, faster charging at the same current limit, and smaller/lighter copper wiring for the same power. The same principle drives the solar industry's migration from 12V to 24V to 48V over the past 20 years. At 48V, a 200 Ah battery (9.6 kWh) replaces a 12V, 800 Ah battery bank (also 9.6 kWh) but with far simpler wiring, no massive parallel strings, and lower I^2R losses.

The Peukert effect describes how a lead-acid battery's usable Ah capacity decreases as the discharge rate increases. The Peukert equation: Cp = I^k x t, where Cp is the Peukert capacity (a constant for the battery), I is the discharge current, t is the discharge time, and k is the Peukert exponent (typically 1.2-1.5 for flooded lead-acid, 1.05-1.15 for AGM, ~1.01-1.03 for lithium). A battery rated at 100 Ah at C/20 (5 amps for 20 hours) with k = 1.3 will deliver only about 63 Ah at C/5 (20 amps), and roughly 40 Ah at C/1 (100 amps). The kWh equivalent drops accordingly: at C/20, the battery delivers 1.2 kWh (at 12V); at C/5, only 0.76 kWh. This matters enormously for UPS systems (short-duration, high-rate discharge) and EV conversions. IEEE 485 provides detailed methodology for sizing lead-acid batteries accounting for the Peukert effect, including temperature correction factors, aging factors (typically 1.25 for end-of-life capacity), and design margin (10-15%). Lithium batteries (LiFePO4, NMC) have such a low Peukert exponent that the effect is negligible for practical discharge rates -- a 100 Ah lithium battery delivers very close to 100 Ah whether discharged over 1 hour or 20 hours. This is a major advantage of lithium chemistry for variable-load applications like solar storage, where daytime loads are low and nighttime loads include a microwave or induction cooktop at high burst currents.

The mathematical conversion kWh = (Ah x V) / 1000 does not change with temperature -- it is a fixed unit relationship. However, the battery's actual Ah capacity is strongly temperature-dependent, especially for lead-acid chemistries. At 0 degrees C (32 degrees F), a flooded lead-acid battery delivers approximately 75-80% of its rated 25 degrees C capacity. At -20 degrees C (-4 degrees F), capacity drops to 50-60% of rated. Lithium iron phosphate (LiFePO4) performs better: 85-90% capacity at 0 degrees C, but cannot be charged below 0 degrees C without cell damage (lithium plating) unless a battery heating system is active. The practical implication: a 100 Ah lead-acid battery rated at 25 degrees C that converts to 1.2 kWh on paper may only deliver 0.9 kWh at 0 degrees C. For cold-climate solar installations, battery enclosures with insulation and heating pads are essential. IEEE 485 requires applying a temperature correction factor to the design Ah based on the lowest expected electrolyte temperature during the battery's service life. For outdoor telecom cabinets in northern climates, the design temperature might be -20 degrees C, requiring a temperature correction factor of 1.43 for VRLA batteries -- effectively oversizing the rated Ah by 43% to meet the actual energy requirement at low temperature. The nameplate kWh is at 25 degrees C standard test conditions; field-deployed kWh at temperature extremes is always lower.