Backup Hours
Lithium: (Ah x V x DoD x inverter eff.) / load. Lead-acid: runtime is Peukert-adjusted from battery-side current.
Lead-acid runtime shrinks faster at high current, so the calculator sizes it differently from LiFePO4.
Solar Calculator
Size backup runtime, charging, and full solar systems from one page.
Enter battery amp-hours, load watts, panel watts, and sun hours to estimate runtime, charge amps, array size, inverter size, and backup storage.
Battery AH to runtimeSolar watts to charge ampsOff-grid and on-grid sizing
What You Can Calculate
Battery Runtime
Solar Charging
System Design
Charge Current
(Solar W x controller efficiency) / battery V
Recharge time then applies chemistry-aware charge efficiency: about 99% for lithium and 95% for lead-acid.
Array Sizing
Daily AC load / (sun hours x derate x inverter eff. x battery charge eff.)
Off-grid array sizing now includes inverter and battery charging losses instead of only panel derate.
Battery Runtime
How long will this battery bank run my load?
Adjust AH, voltage, and usable depth to estimate true backup hours.
Stored battery energy
20.5 kWh
400Ah at 51.2V (LiFePO4 48V class)
Usable energy at this load
17.0 kWh
90% DoD x 92% inverter efficiency
Estimated backup time
17.0 hr
At 1,000W continuous load
Battery needed for target time
283.1 Ah
14.5 kWh installed bank for 12.0 hours
Solar Charge
How many amps will solar charge my battery with?
Estimate bulk charging amps and recharge time from panel wattage and sun hours.
Bulk charging current
44.5 A
From 2,400W into 51.2V (LiFePO4 48V class)
Amp-hours to replace
320.0 Ah
80% state-of-charge swing
Ideal bulk charge time
7.3 hr
Lithium recharge uses a near-linear charge-efficiency model.
Daily solar harvest
9.9 kWh
1.6 solar days from start to target
Charge rate
0.11C
Useful for checking whether charge current is conservative or aggressive for the battery.
Recommended charge current
120.0 A ideal
Below Victron's 0.3C optimal rate, which is gentle but slower. Victron NG lithium supports up to 400.0A continuous (1C).
Off-Grid Sizing
Size a full off-grid solar and battery system.
Translate daily load into battery bank, panel count, controller amps, and inverter size.
Daily energy use
9.6 kWh
1,200W x 8.0 hours
Battery bank required
23.2 kWh
452.9Ah at 51.2V (LiFePO4 48V class) to deliver 16.0 runtime hours
Solar array required
2.4 kW
5 panels at 550W each
Charge controller size
58.5 A
Includes 25% controller headroom.
Recommended inverter
2.5 kW
Sized from continuous load and starting surge.
On-Grid + Hybrid
Size a grid-tied solar system and optional outage backup battery.
Use utility consumption for net-metering solar and add a battery for critical loads during outages.
Required solar array
11.4 kW
21 panels at 550W each
Expected monthly production
1,530 kWh
85% offset target
Expected annual production
18,360 kWh
Before local shading, snow, and utility export limits.
Estimated monthly savings
$245
At $0.16/kWh
Installed backup battery bank
18.1 kWh
15.0 kWh usable outage energy requires 353.8Ah at 51.2V (LiFePO4 48V class)
Hybrid inverter target
9.7 kW
LiFePO4 backup bank sized for 10.0 hours at 1,500W critical load.
Engineering Note
Use these numbers as a fast sizing estimate, then finalize with equipment and site specifics.
Need the actual equipment matched to these calculations?