⚡ LiFePO4 Lithium Battery Calculator — What Size Do You Need?

How this calculator works

Inverter sizing (motor-surge safe): Inverter W = max(largest motor × 1.8, connected load + largest motor, connected load × 1.15), rounded up to the next 100 W. Fixed-speed AC compressors, pumps and refrigerators pull 3–5× their rated watts for a moment at start. The 1.8× rule works because Indian inverters can themselves deliver roughly 3× their continuous rating for the surge moment — so an inverter sized at 1.8× the largest motor can briefly supply about 5.4× that motor's rated watts (1.8 × 3), safely above the worst starting surge. Market-verified: a 1.5 ton AC runs on a 5 kVA inverter alongside other loads. BEE 5-star ACs and fridges with inverter-compressor technology are different: their compressors soft-start and modulate, so the surge is very small — select "BEE 5-star AC/fridge" and the calculator sizes with a 1.2× factor instead, which can mean a whole inverter class smaller. Note the star rating applies to appliances (AC, fridge, TV) — there is no BEE star rating for power inverters in the Indian market; an inverter is judged by its BIS nameplate wattage.

Battery sizing: Ah needed = (average W × target minutes) ÷ (48 V × 60 × 0.95 usable × 0.82 inverter efficiency). Lithium capacity is rated at C1 — the full Ah is deliverable in one hour, unlike tubular batteries whose labels are C10/C20 ratings that collapse under real load. LiFePO4 delivers about 95% of its rated Ah (against 60–70% for lead-acid), and standard Indian home inverters and solar PCUs convert battery DC to AC at roughly 82% efficiency — both are counted, which is why these numbers are lower than brochure math and closer to what your stopwatch will say. The calculator picks the smallest standard single 48V battery (50/100/150/200 Ah) that meets your backup target — and tells you honestly if even 200Ah cannot. Why a single battery only? The inverter's built-in charger has a limited charging current. A battery bank bigger than the charger can recharge will never refill between power cuts — so unless the charging current increases, the battery capacity cannot be increased. Bigger backup needs a higher-voltage system with a larger charger, not more parallel batteries.

Cycling appliances are counted at their real average draw: AC at 55%, geyser at 60%, refrigerator at 35% of nameplate — compressors and thermostats switch off after reaching temperature. Continuous loads (fans, lights, TV, pumps while running) count in full. In rare extreme conditions of continuous running — a heatwave with the AC never resting — expect less backup than shown.

C-rate health zones: C-rate = (average W ÷ 0.82) ÷ (48 V × Ah) — measured on the battery side, because the battery also supplies the inverter's losses. Up to 0.3C the bank is relaxed (best life), up to 0.5C healthy, up to 1C working hard, and beyond 1C you must confirm the battery's BMS continuous-discharge limit — many Indian LiFePO4 packs are capped at 0.5–1C.

BMS surge-current limitation: unlike lead-acid, a lithium battery cannot deliver whatever the load demands — its BMS gates the output, and every BMS has a continuous rating plus a short-duration peak/surge setting. When a motor or compressor starts, the surge passes through the inverter to the battery; if it exceeds the BMS surge setting, the BMS disconnects and the whole system trips — even when the inverter itself could carry the surge. The calculator shows the approximate surge amps your selection will pull so you can match it against the battery's BMS specification before buying.

Assumptions & disclaimer

The 82% efficiency figure applies to standard Indian home inverters/UPS and solar PCUs — this calculator is built for those. Hybrid solar inverters, and high-frequency inverters with PFC (power factor correction), convert more efficiently — on those you can expect somewhat longer backup than shown. All recommendations assume new, healthy LiFePO4 batteries with honest C1 Ah labels and a 48V system. Appliance wattages are typical Indian market values — check your nameplate. Motor surge is assumed to occur at startup only; your inverter's surge rating must still be confirmed with the manufacturer. Temperature matters less for lithium than lead-acid, but extreme cold reduces output. This is guidance for planning, not a guarantee — final selection and installation must be done by a qualified professional. I name no brands and earn nothing from any purchase: zero affiliates, zero sponsorships, zero bias.

The principles this calculator follows

Lithium is flat where lead-acid collapses. A tubular battery gives less total energy the harder you load it — my discharge tests show a 150Ah delivering only 52Ah at 1000W. LiFePO4 holds its voltage and delivers nearly the same energy at light and heavy loads, which is why this calculator can use simple energy math where the tubular calculator needs measured curves.

Efficiency is where honest math wins. Brochures multiply Ah × volts and stop. Real backup passes through an Indian home inverter at about 82% efficiency, and a LiFePO4 BMS reserves ~5% at the bottom. Counting both is the difference between a number that impresses and a number that matches your stopwatch.

Buy inverters by watts, not VA. VA has no fixed relationship to real output in the Indian market — BIS makes it compulsory to print continuous watts on the nameplate. Ask the dealer to show it before you pay.

Higher voltage, better performance. For lithium, the higher the system voltage, the better the performance — lower current at the same watts means lower losses, cooler running and thinner cables — and the same goes for the inverter itself.