Calculator Inputs
Example Data Table
Use this style of appliance breakdown to estimate daily energy use.
| Appliance | Watts | Hours/Day | Wh/Day | kWh/Day |
|---|---|---|---|---|
| LED Lights (8 bulbs) | 80 | 5 | 400 | 0.40 |
| Refrigerator | 150 | 10 | 1500 | 1.50 |
| Fans (3 units) | 210 | 8 | 1680 | 1.68 |
| TV + Router | 140 | 6 | 840 | 0.84 |
| Total | — | — | 4420 | 4.42 |
Formula Used
This calculator estimates energy capacity. Confirm current limits, surge ratings, and battery manufacturer guidelines before purchasing hardware.
How to Use This Calculator
- Estimate daily energy in kWh from bills or an appliance table.
- Pick autonomy days based on outage risk and solar availability.
- Choose system voltage to match your inverter and charge hardware.
- Set depth of discharge and efficiencies using battery datasheets.
- Apply temperature derate if your battery room gets cold.
- Enter a battery unit voltage and Ah to see wiring suggestions.
- Press Submit to view results above the form.
- Download CSV or PDF reports for sharing and recordkeeping.
Load-Driven Sizing Benchmarks
Typical homes using 8–15 kWh per day often target 1.5–3.0 days of autonomy, creating 12–45 kWh of usable backup demand. If reserve margin is 10%, the usable target becomes 13.2–49.5 kWh. A modest essential-load profile of 4–6 kWh per day can cut that to 6–20 kWh, improving affordability and recharge speed.
Depth of Discharge and Cycle Planning
Usable energy depends heavily on depth of discharge. At 80% DoD, a 20 kWh nominal bank yields 16 kWh before losses. At 60% DoD, the same bank yields 12 kWh but typically supports more cycles. For long service life, many designs keep routine cycling between 60–85% DoD and reserve the lower range for rare outages. For critical loads, target 20% headroom beyond calculated nominal capacity.
Efficiency and Temperature Impacts
Round-trip battery efficiency and inverter efficiency compound. A battery at 92% with an inverter at 90% produces a combined 82.8% before temperature effects. If temperature derate is 95%, the combined factor falls to 78.7%. That means a 10 kWh usable goal requires about 12.7 kWh nominal at 80% DoD, and about 15.9 kWh nominal at 60% DoD. In warm climates, derate may remain near 100%, reducing oversizing.
Voltage, Amp-Hours, and Wiring Practicalities
Amp-hours scale inversely with system voltage. For example, 20 kWh nominal equals about 417 Ah at 48 V, but about 833 Ah at 24 V. Higher voltage can reduce current, cable size, and resistive losses. The series and parallel counts shown here translate capacity into a buildable bank using your chosen unit voltage and Ah rating.
Budget Signals and Upgrade Path
If battery unit pricing is entered, total battery count becomes a fast budget proxy. As a planning rule, add 15–30% contingency for balance-of-system parts, enclosures, monitoring, and commissioning. If your required nominal capacity is close to a standard rack size, consider leaving space for one parallel string to support future load growth or seasonal autonomy increases. Tracking monthly kWh and adjusting autonomy by season can prevent costly overbuilds.
FAQs
1) Why does the calculator show both usable and nominal kWh?
Usable kWh is what your loads can consume. Nominal kWh is the battery nameplate energy required to deliver that usable amount after discharge limits, conversion losses, and temperature effects.
2) What depth of discharge should I choose?
Many systems use 70–90% for lithium and 40–60% for lead-acid. Lower DoD generally improves cycle life, but it increases the nominal capacity you must purchase.
3) How do I estimate daily kWh accurately?
Start with your utility bill average daily kWh, then subtract loads you will not back up. For off-grid or partial backup, build an appliance table using watts × hours.
4) Why does voltage change the amp-hour result?
Amp-hours depend on voltage because energy equals voltage times amp-hours. The same kWh at 48 V requires about half the amp-hours of a 24 V bank.
5) Should I include inverter efficiency if I run DC loads?
If most loads are AC, include inverter efficiency. If significant loads are DC and bypass the inverter, you can raise the inverter efficiency input or reduce daily kWh to reflect DC delivery.
6) Does this calculator size for surge power?
It sizes energy capacity, not peak current. Use your inverter surge rating and battery current limits to validate motor starts. The optional peak kW window is only an energy check.