Formula used
- Battery nominal energy (Wh) = V × Ah
- Usable DC energy (Wh) = nominal × DoD%
- Usable AC energy (Wh) = usable DC × inverter efficiency%
- Effective load (W) = load × (1 + system losses%)
- Battery-only runtime (h) = usable AC Wh ÷ effective load W
- Average solar power (W) = array W × derate% × controller efficiency%
- Daily solar energy (Wh/day) = average solar power × sun hours
- Recharge time (sun-hours) = usable DC Wh ÷ (avg solar W × charge efficiency%)
How to use this calculator
- Enter your average load in watts, then add expected system losses.
- Fill in battery voltage, amp-hours, and depth of discharge you allow.
- Set inverter efficiency to reflect AC conversion (or 100% for DC).
- Add solar array watts, sun hours, and a realistic derate percentage.
- Review battery-only runtime, battery+solar runtime, and recharge time.
- Adjust inputs to test upgrades, seasonal sun, or different loads.
Example data table
| Scenario | Load (W) | Battery (V×Ah) | DoD | Solar (W) | Sun Hours | Expected Outcome |
|---|---|---|---|---|---|---|
| Cabin lights + router | 120 | 12×200 | 60% | 400 | 5.0 | Long runtime; solar often covers daytime load. |
| Small fridge + fans | 350 | 24×200 | 80% | 800 | 4.5 | Solar helps; battery covers evenings and clouds. |
| Workstation backup | 600 | 48×100 | 70% | 1000 | 3.5 | Good daytime support; size battery for nights. |
Operational notes
Load profiling
Most small off‑grid systems run a mixed load: lighting, fans, networking, and a refrigerator cycling. For planning, convert everything to an average watt figure. A 350 W average load uses 8.4 kWh per day, so even a large battery bank may cover only a fraction of a day without solar assistance.
Battery energy and limits
Battery energy is voltage times amp‑hours, reported in watt‑hours. A 24 V, 200 Ah bank is 4,800 Wh nominal. If you cap depth of discharge at 80% and assume 90% inverter efficiency, usable AC energy becomes about 3,456 Wh. That single constraint often doubles required capacity compared with “nameplate” thinking.
Solar contribution during daylight
Solar helps in two ways: it reduces net battery draw while the sun is available, and it can recharge the battery for later use. Derating is important; dust, heat, wiring, and sub‑optimal tilt commonly reduce output by 15–30%. With a 600 W array, 75% derate, and 95% controller efficiency, average delivered power is roughly 428 W.
Recharge and resilience
Recharge time matters for consecutive cloudy days. If the battery must recover 3,840 Wh of usable DC energy and charging efficiency is 92%, the solar charging power becomes the average solar power times 92%. When charging power is 394 W, refilling that usable portion takes about 9.7 sun‑hours, often spread across two days.
Decision signals
Use the runtime numbers to decide whether to add panels, batteries, or reduce load. If battery‑only runtime is below your overnight need, prioritize battery capacity. If recharge time is too long, prioritize panel wattage or reduce losses. The graph highlights sensitivity to load growth. For many users, a practical target is one full night of autonomy plus 30% buffer. Compare sun hours by season; winter sun may be 40% lower. If your load is mostly daytime, extra panels can outperform extra batteries. If loads are nighttime, shifting usage earlier helps. With minimal expense.
FAQs
Q: What load value should I enter?
Use the average watts your system draws over time. For cycling devices, estimate duty cycle. Example: a 120 W fridge running 40% of the time averages about 48 W.
Q: Why is depth of discharge important?
Depth of discharge limits how much energy you safely use. Higher DoD increases runtime but can reduce battery life. Match the DoD to your battery chemistry and warranty guidance.
Q: What does solar derate represent?
Derate reduces nameplate panel watts to a realistic delivered value. It covers temperature, dust, shading, wiring, tilt, and mismatch. Conservative derate values produce more reliable sizing decisions.
Q: Why is battery+solar runtime only an estimate?
Solar output varies minute to minute with clouds and angle, while loads fluctuate. The calculator uses average solar power during sun hours to approximate net draw and remaining battery energy.
Q: How do I improve runtime fastest?
First reduce the load and losses: efficient appliances, better wiring, and fewer conversions. Then add battery capacity for overnight autonomy. Add solar wattage when recharge time is too long.
Q: Can this handle multiple batteries and panels?
Yes. Enter total system voltage and combined amp‑hours for the battery bank. For panels, enter total rated array watts. Use realistic efficiencies that match your controller and battery type.