What it is
The in‑verter sizing and battery calculator is an instant, browser‑only tool that applies classic energy‑budget equations to give installers and owners the minimum inverter rating and battery bank size required for a given load profile. It takes into account both instantaneous peak demand and continuous average use, then adds a user‑chosen safety margin to avoid under‑rating the inverter. Battery sizing is based on daily kWh consumption multiplied by desired days of autonomy, divided by the depth‑of‑discharge allowed by the chemistry used. Converting this nominal energy into ampere‑hours at the chosen system voltage completes the specification that installers typically hand to manufacturers or local utility authorities when proposing a design. Knowing these parameters accurately is critical: an inverter that is too small can cause brownouts, whereas over‑specifying both inverter and batteries drives up capital unnecessarily.
The calculator also provides quick “what‑if” capability for adjusting system voltage or discharge depth, making it useful not only during initial design but also when retrofitting or scaling a system to meet seasonal demand changes. Because all calculations run client‑side, the tool stays fast, reliable and always offline—exactly what field installers need.
How to use it
Enter your site’s peak power in watts—the amount your largest appliance will draw at startup. Fill in the continuous average load (sum of all running appliances). Then provide daily energy use (kWh) averaged over a month to capture typical consumption, followed by how many days you want grid‑outage autonomy. Select the depth‑of‑discharge percentage that matches your chosen battery chemistry: 50 % for standard lead‑acid, 70 % or 90 % for newer Li‑ion cells. Pick the nominal system voltage (12 V, 24 V or 48 V). Finally choose a safety margin for inverter sizing—10–25 %. Click calculate to see an instant rating of your inverter in kW and battery bank energy in both usable kWh and nominal Ah.
Worked example
1. Peak power is 5 kW; continuous load is 3 kW. 2. Choosing a 20 % margin means the inverter must support x 5 kW × (1+0.20) = 6 kW. Since that exceeds the continuous load, the required rating is 6 kW. 3. Daily energy consumption is 10 kWh and autonomy desired is 3 days → usable battery energy needed: 10 × 3 = 30 kWh. 4. With a depth‑of‑discharge of 50 %, the nominal (maximum) battery capacity must be x 30 kWh ÷ 0.5 = 60 kWh to provide those 30 kWh when discharged fully. 5. For a 12 V system, converting that energy into ampere‑hours gives: (60 kWh × 1000 Wh/kWh)/12 V = 5000 Ah. 6. The calculator therefore reports an inverter rating of 6.000 kW, usable battery capacity of 30.000 kWh, nominal battery capacity of 60.000 kWh and a required bank size of 5 000.000 Ah.
Inputs
- Peak Power (W): 5000
- Continuous Load (W): 3000
- Daily Energy Consumption (kWh): 10
- Desired Autonomy (days): 3
- Depth‑of‑Discharge (%): 50
- System Voltage (V): 12
- Inverter Margin (%): 20
Result
- Inverter Capacity (kW): 6
- Battery Nominal Energy (kWh): 60
- Battery Usable Energy (kWh): 30
- Battery Capacity (Ah): 5000
Frequently asked questions
Do I need to include the inverter margin for every calculation?
The margin protects against short‑duration spikes and aging. For most residential systems a 20–25 % margin is recommended; installers can lower this if the inverter provides built‑in surge capability.
Is the battery capacity output in Ah always realistic for Li‑ion batteries?
Yes, because the calculation uses nominal voltage. Lithium‑ion cells are typically rated at 3.7–3.8 V each; when wired to achieve the system voltage (e.g., 48 V) you multiply by the number of cells in series and the Ah value follows from this formula.
What if I use a modular battery bank with multiple strings?
Treat each string as an individual module; add their Ah capacities together at the selected nominal voltage. The calculator’s total Ah output is already a sum, so you can split it later across your chosen configuration.
Can I run the tool offline?
All logic sits in the browser and requires no external requests—once loaded, the tool works fully offline, which is ideal for remote installations or field testing.