Guide, formulas, and engineering reference
How to use the calculator
Start with the inverter nameplate ratings. Add each appliance and enter realistic running and starting power. Set scheduled hours, duty cycle, and days per week. Complete battery, solar, tariff, and cable assumptions. Submit the form and review every warning before changing equipment sizes.
Appliance energy formula
Daily appliance energy equals quantity multiplied by rated watts, scheduled hours, duty-cycle fraction, and days-per-week fraction. This converts occasional loads into an average daily value. Use measured wattage where available because labels may show maximum rather than typical consumption.
Inverter input formula
Battery-side energy for AC loads equals AC output energy divided by effective inverter efficiency. Conversion loss is the difference between battery-side input and delivered AC output. Efficiency varies with load, voltage, temperature, waveform, and product design.
Inverter self-consumption
An inverter may consume power even when appliances are off. Active idle, standby, sleep, fans, displays, communication modules, and transformer magnetizing current can create meaningful daily demand. Datasheet standby figures should be preferred over guesses.
Connected and simultaneous loads
Connected load is the sum of all ratings. Simultaneous load estimates how much operates together. Critical designs should use measured peaks or a formal load schedule. Diversity factors should never hide a load that may actually start during another large load.
Starting surge
Compressors, pumps, motors, transformers, and some power supplies can briefly demand several times normal power. Starting watts and duration both matter. Confirm inverter surge duration, battery peak-current capacity, protective-device behavior, and voltage drop during startup.
Battery nominal energy
Nominal battery energy equals bank voltage multiplied by amp-hour capacity. Series batteries raise voltage. Parallel strings raise amp-hour capacity. Nominal energy is not fully usable because depth of discharge, efficiency, health, reserve, temperature, and current limits reduce availability.
Battery usable energy
Usable battery energy equals nominal energy multiplied by allowed depth of discharge, battery efficiency, state-of-health factor, and the remaining fraction after reserve. Conservative limits usually improve reliability and battery life.
Peukert effect
Lead-acid batteries deliver less capacity at high discharge rates. The Peukert exponent approximates this behavior. Lithium batteries usually use an exponent near one. Manufacturer discharge curves provide better estimates than a generic exponent.
Battery C-rate
C-rate equals discharge current divided by battery amp-hour capacity. A 100 Ah bank delivering 50 A operates at 0.5 C. Battery cells, interconnects, fuses, contactors, and battery management systems must support continuous and peak current.
Solar generation formula
Daily solar energy equals array watts multiplied by peak sun hours and all production factors. Temperature, shade, wiring, dirt, controller efficiency, orientation, weather, and seasonal conditions reduce real output.
Peak sun hours
Peak sun hours represent the equivalent number of hours at full standard irradiance. They are not daylight hours. Use location-specific monthly data, especially when winter reliability matters.
Solar derating
Losses are multiplied as sequential factors. This avoids overstating reductions by simply adding percentages. Detailed models may also include mismatch, degradation, clipping, availability, snow, curtailment, and battery charge acceptance.
Grid and export pricing
Imported energy may use peak and off-peak rates. Exported energy may receive a lower credit. Fixed charges and taxes can remain even when annual generation equals annual consumption. Utility rules should be entered from the current tariff schedule.
Simple payback
Simple payback divides installed cost by annual savings. It ignores financing, maintenance, component replacement, degradation, discount rates, rebates, taxes, insurance, and residual value. Use a discounted cash-flow model for investment decisions.
Power factor
Watts describe real power while volt-amperes describe apparent power. Poor power factor increases current and may require a larger VA rating. Confirm both watt and VA limits for the inverter and large loads.
System voltage
Higher DC voltage reduces current for the same power. Lower current can reduce conductor size and voltage drop. Equipment voltage compatibility, insulation, protection, battery configuration, and local rules determine the appropriate system voltage.
Cable sizing
The cable estimate controls voltage drop using copper resistivity. It does not prove safe ampacity. Temperature, insulation, conduit, grouping, terminals, fault current, fuses, breakers, installation method, and electrical codes must also be checked.
Voltage drop
Excess DC voltage drop can trigger low-voltage shutdown, weaken motor starting, waste energy, and heat terminals. Keep battery cables short, use approved lugs, apply correct crimping and torque, and install appropriate overcurrent protection.
Essential load planning
A separate essential-load scenario can reduce inverter and battery requirements. Refrigeration, lighting, communication, medical devices, and security equipment are common priorities. Optional heating and cooking loads often dominate system size.
Hybrid operation
Hybrid inverters can combine solar, batteries, and grid power. Operating priorities, charge limits, time-of-use schedules, export rules, generator support, and reserve settings strongly affect energy flow. Model the actual control strategy.
Off-grid reliability
Off-grid systems normally require winter solar data, autonomy for poor weather, backup generation, conservative battery limits, and load management. A single average-day result is not enough for a final off-grid design.
Grid-tied systems
Grid-tied systems may prioritize annual production, self-consumption, export limits, and utility interconnection requirements. Battery backup capability depends on hardware topology and protected-load circuits.
Modified sine wave caution
Modified sine wave output can increase noise, heat, and losses in some motors, transformers, audio equipment, clocks, chargers, and medical devices. Pure sine wave equipment is usually preferred for mixed residential loads.
Efficiency at low load
A large inverter operating at a tiny fraction of capacity may have poor efficiency because fixed overhead becomes significant. Correct sizing and reliable search or sleep modes can reduce energy waste.
Temperature effects
High temperatures can reduce inverter output, battery life, and panel voltage. Cold temperatures can reduce lead-acid capacity and may restrict lithium charging. Use manufacturer temperature limits and derating curves.
Battery aging
Battery health declines with cycles, calendar age, temperature, charge voltage, storage state, and operating depth. Enter a reduced health percentage when evaluating an older bank.
Panel degradation
Solar modules slowly lose output over time. Long-term projections should include manufacturer degradation rates. Soiling and shading are separate operational losses that may change seasonally.
Scenario comparison
Save a baseline, then change one major assumption at a time. Compare 24 V versus 48 V, lead-acid versus lithium, old versus efficient inverters, different panel counts, and normal versus essential-only loads.
Measurement recommendations
A plug-in energy meter can measure small AC appliances. Clamp meters and logging power analyzers can measure larger circuits. DC shunts can measure battery current. Use properly rated instruments and qualified personnel.
Data quality
Calculator accuracy depends on input quality. Replace generic presets with appliance labels, datasheets, measured duty cycles, inverter efficiency curves, battery discharge charts, and local solar data whenever possible.
Safety
Solar arrays and battery banks can produce dangerous voltage, extreme fault current, fire risk, toxic gases, and arc hazards. Final design and installation require qualified professionals, approved components, protective devices, isolation, earthing, ventilation, and inspection.