Calculator Inputs
Fields marked * are required. The layout adapts to screen size automatically.
Formula Used
The calculator estimates required storage energy using an autonomy target and adjusts for losses and derating.
- Daily energy: Eday = (P × h) / 1000, in kWh/day (if using power mode).
- Growth-adjusted energy: E'day = Eday × (1 + g).
- Autonomy energy: Eauto = E'day × D.
- Loss factor: η = ηinv × ηbat.
- Usable required: Euse = (Eauto / η) × (1 + a) / T.
- Nominal capacity: Enom = Euse / DoD.
- Amp-hour estimate: Ah = (Enom × 1000) / V.
Where g is load growth, a is aging margin, T is temperature factor, D is autonomy days, and V is system voltage.
How to Use This Calculator
- Pick an input mode: daily energy, or power × hours.
- Set autonomy days to match your backup requirement.
- Choose a system voltage used by your inverter and charger.
- Enter a safe depth of discharge based on chemistry and life goals.
- Fill efficiencies and temperature derating for realistic sizing.
- Add aging and growth margins if planning for multi-year use.
- Optionally enter peak load and surge factor for inverter guidance.
- Press Calculate to view results and download reports.
Example Data Table
| Scenario | Daily energy (kWh/day) | Autonomy (days) | DoD (%) | Eff. inv × bat (%) | Temp factor | Estimated nominal capacity (kWh) |
|---|---|---|---|---|---|---|
| Small cabin | 6 | 2 | 80 | 90 | 0.90 | ~18.5 |
| Home backup | 12 | 1 | 80 | 86 | 1.00 | ~17.4 |
| Critical loads | 20 | 3 | 70 | 88 | 0.85 | ~115 |
Values are illustrative and depend on chosen margins and efficiencies.
Battery Storage Sizing Guide
Battery capacity planning converts real energy demand into a practical storage target. This guide explains how the calculator translates daily use, autonomy, and losses into a bank size you can build and budget confidently.
1) Start with realistic daily energy
Daily energy is the foundation: sum appliance kWh, or use average watts times hours. For example, a 600 W average load running 20 hours uses 12 kWh/day. If you expect new devices, add 5–20% growth to avoid undersizing.
2) Choose autonomy that matches risk
Autonomy days represent how long you can operate without charging. One day suits occasional outages; two to three days is common for off-grid cabins. Critical loads may need three to five days, especially where weather limits solar input.
3) Account for conversion and storage losses
Energy passes through conversion stages before reaching loads. Typical inverter efficiency is 90–96% at moderate load, while battery path efficiency can be 88–95% depending on chemistry and current. Combined losses can reduce delivered energy by 8–20%, so correcting for them matters.
4) Use safe depth of discharge
Depth of discharge limits how much stored energy you routinely use. Many lithium systems target 70–90% for good balance, while lead-acid often targets 40–60% for longevity. A lower depth means a larger bank, but more cycles over its life.
5) Apply temperature derating
Cold weather reduces usable capacity and power. A factor of 0.80–0.95 is typical for winter performance if the bank is unheated. Use 1.00 for controlled indoor rooms. This adjustment prevents a bank that looks adequate on paper from failing in real conditions.
6) Add aging margin for multi-year design
Capacity fades with cycling and time. A 10–25% aging margin is common when planning for several years of service. High cycle counts, warm storage, or heavy discharge rates can justify higher margin. Planning margin now avoids costly mid-life expansion.
7) Convert energy to amp-hours
The calculator converts required kWh to amp-hours using system voltage. Higher voltage reduces current for the same power and can simplify wiring on larger systems. As a quick check, 10 kWh at 48 V corresponds to about 208 Ah of nominal capacity.
8) Interpret outputs for build decisions
Focus first on nominal kWh capacity, then verify amp-hour sizing at your chosen voltage. If you enter peak load, the calculator estimates peak DC current to support cable and protection checks. Optional module sizing helps translate capacity into an approximate module count and configuration.
Frequently Asked Questions
1) What does “nominal capacity” mean?
Nominal capacity is the total stored energy rating. Usable energy is lower because of depth of discharge limits, temperature effects, and efficiency losses. The calculator converts your usable need into a nominal bank target.
2) How should I pick depth of discharge?
Choose a value aligned with your battery chemistry and life goals. Many lithium banks use 70–90% for regular cycling, while lead-acid often uses 40–60% to extend lifespan and reduce sulfation risk.
3) Why are inverter and battery efficiencies separate?
Inverters lose energy converting DC to AC, and batteries lose energy internally during discharge. Treating them separately lets you model realistic end-to-end delivery, especially when loads vary over the day.
4) What temperature factor should I use?
Use 1.00 for climate-controlled rooms. Use 0.90 for mild winter exposure and 0.80–0.85 for cold conditions with limited heating. If your manufacturer provides curves, match them as closely as possible.
5) Do I need both aging and growth margins?
Aging margin covers capacity fade over time. Growth margin covers future load additions. If your loads and replacement timeline are certain, you can reduce them; otherwise, modest margins often prevent expensive redesigns.
6) How accurate is the module count?
It is a planning estimate based on nominal module energy. Real designs must consider series voltage limits, allowable parallel strings, discharge rates, and manufacturer rules. Use the count to compare options, not finalize procurement.
7) Can I use this for solar-only systems?
Yes. Size storage from your daily energy and autonomy needs, then size charging sources separately. For solar, verify that array and charge controller can refill the bank during typical weather windows.