Make the assumptions visible.
Free planning calculators that show assumptions for solar sizing, battery runtime, inverter sizing, appliance load and simple payback.
Every calculator explains what the result leaves out.
Solar system size
From daily energy, peak sun and backup targets
kWh/day ÷ effective sun→02Battery runtime
From usable battery energy and average load
usable kWh ÷ kW load→03Inverter size
From simultaneous load, headroom and surge
continuous kW + surge check→04Appliance load
Build an estimate appliance by appliance
watts × hours × quantity→05Solar payback
Simple cost-and-savings planning
system cost ÷ annual savings→Battery runtime is easy to estimate—and easy to misunderstand.
Energy capacity does not prove that a battery can carry the load. Use runtime as one check, then verify continuous power and current limits separately.
Battery runtime calculator
Runtime changes as loads cycle. This estimate does not validate battery power/current limits or motor-starting surge.
Planning tools, not black boxes
Every calculator on Melnti is intended to make a first estimate easier to challenge. If a result changes because of solar resource, efficiency, backup duration or load, the input should be visible so you can see why.
Choose the calculator that matches the question
- Solar system size calculator — a starting PV array, battery and inverter class from daily energy, peak-sun-hours and backup assumptions.
- Battery runtime calculator — runtime from capacity, average load, usable depth of discharge and conversion efficiency.
- Inverter size calculator — continuous-load headroom plus a separate motor-surge check.
- Appliance load calculator — build daily kWh and connected-load estimates appliance by appliance.
- Solar payback calculator — a simple cost-and-savings estimate with maintenance shown separately.
Why the result is only a starting point
Real system design also depends on roof orientation and shade, PV string voltage, inverter MPPT windows, temperature, cable lengths, protective devices, battery current limits, motor starting, local electrical rules and the actual load profile. NREL’s PVWatts similarly treats solar production as an estimate with assumptions and uncertainty rather than a guaranteed output.
These calculators are planning aids. Final PV string design, conductor sizing, protective devices, battery current limits, structural loading and code compliance require site- and equipment-specific design.
Get better inputs
Use recent electricity bills or meter data when available. For appliances with variable or cycling demand, measured energy is more useful than a generic wattage label. The better the inputs, the more useful the estimate becomes.