What this calculator does
This planner estimates how long your home battery can support critical loads during a grid outage. It converts your battery’s nameplate capacity into usable energy (respecting depth-of-discharge limits and inverter efficiency), then runs an hour-by-hour state-of-charge simulation across your chosen outage horizon. You can also compare a baseline plan to two common resilience upgrades: more load shedding and more solar recharge.
How to use it (practical workflow)
- Enter battery and inverter details (capacity, depth of discharge, efficiency, inverter rating).
- Enter your critical load as an average kW you want to keep running during the outage.
- Enter surge load (largest short-term kW draw you expect). The tool flags when surges may exceed the inverter’s continuous rating.
- Enter solar recharge per day (kWh/day) if you have PV or portable panels available during the outage.
- Set the planned outage horizon (days) and your planned load shedding (% reduction).
- Click Simulate Autonomy to update the results, daily SoC table, and scenario comparison.
Inputs and guidance (what to measure and what to estimate)
The most common planning mistake is mixing up power (kW) and energy (kWh). Your battery is sized in kWh (how much energy is stored). Your loads are typically described in kW (how fast energy is used). If you run a 2 kW average load for 5 hours, that consumes about 10 kWh.
- Battery bank nameplate capacity (kWh): total stored energy. If you have multiple batteries, add their kWh.
- Maximum depth of discharge allowed (%): the portion you are willing to use (often 70–90% for longevity, depending on chemistry and warranty).
- Inverter efficiency (%): conversion losses from DC battery to AC household power. Typical values are ~88–96% depending on load.
- Inverter continuous power rating (kW): the sustained power limit. This is separate from battery energy.
- Average critical load (kW): your “keep-on” circuits averaged over time (fridge, lights, router, blower, well pump cycling, etc.).
- Largest short-term surge load (kW): the biggest momentary overlap (pump start, microwave + pump, compressor start). Use a conservative estimate if unsure.
- Expected solar recharge per day (kWh): energy you expect to harvest during the outage. Use a low value for winter/clouds/shading.
- Planned load shedding (%): how much you will reduce usage versus “normal.” This tool applies it to the average critical load.
Model and formulas (what the simulation assumes)
The simulation uses a simple but useful planning model:
- Usable energy is computed from nameplate capacity, depth-of-discharge limit, and inverter efficiency.
- Hourly load is your average critical load adjusted by your load shedding percentage.
- Solar recharge (kWh/day) is spread evenly across six midday hours (10:00–15:00) to approximate typical PV production.
- The battery starts the outage at 100% of usable energy (fully charged within the usable window).
- The simulation stops when the battery reaches zero usable energy or when the horizon ends (up to 30 days).
Usable energy (kWh):
where C is capacity (kWh), DoD is depth of discharge as a fraction (e.g., 0.8), and η is inverter efficiency as a fraction (e.g., 0.92).
Effective hourly load (kWh per hour):
where P is average critical load (kW) and s is load shedding as a fraction.
Worked example (realistic, end-to-end)
Suppose you have a 13.5 kWh battery, you limit discharge to 80%, and your inverter efficiency is 92%. Your average critical load is 2.4 kW, you plan to shed 15%, you expect 5 kWh/day of solar recharge, and you want to cover a 3-day outage.
- Usable energy: 13.5 × 0.80 × 0.92 ≈ 9.94 kWh
- Effective hourly load: 2.4 × (1 − 0.15) ≈ 2.04 kWh/hour
- Without solar, a rough runtime estimate: 9.94 ÷ 2.04 ≈ 4.9 hours
The hour-by-hour simulation can show longer autonomy than the rough estimate when solar is present because it adds energy during midday hours. Use the Daily state of charge summary table to see whether you are cutting it close (low minimum SoC) or have buffer.
Interpreting results (what “good” looks like)
- Modeled autonomy (hours/days): the simulated time until the battery reaches zero usable energy (or the horizon ends).
- Meets target horizon: “Yes” means the modeled autonomy is at least your planned outage horizon.
- Surge note: if surge exceeds the inverter’s continuous rating (or 125% of it), consider sequencing appliances or verifying surge specs.
Limitations and assumptions
This is a planning model, not a full electrical design tool. It intentionally simplifies several real-world effects so you can compare scenarios quickly.
- Constant average load: real loads vary by hour; HVAC and pumps cycle. If your load is spiky, increase the average load to be conservative.
- Temperature and battery behavior: cold weather can reduce available energy and power; some systems reserve energy for self-protection.
- Solar variability: clouds, snow, shading, and panel orientation can reduce kWh/day significantly. Use conservative solar inputs for storm scenarios.
- Starting charge: the model assumes the battery starts fully charged within the usable window.
- Inverter limits: the surge check is a heuristic; always confirm your inverter’s surge rating and duration limits.
If you need code compliance, life-safety backup, or medical-grade reliability, confirm your plan with qualified professionals and manufacturer documentation.
Planning notes: making outage autonomy more realistic
Battery autonomy planning is most useful when you treat it as a scenario exercise rather than a single “answer.” Outages are messy: loads change as you cook, pump water, or heat/cool a room; solar harvest changes with weather; and batteries may not be full when the grid goes down. The goal is to understand your margin—how close you are to depletion—and which lever (load, solar, or storage) improves that margin the most.
A good starting point is to list your critical circuits and estimate their average draw. For example, a refrigerator may average a few hundred watts over time but can draw more when the compressor starts. A well pump may run intermittently but can have a high starting surge. If you don’t have measured data, you can still get value by using conservative estimates and then testing “what if” cases.
Tips for choosing a conservative load
- If you only know appliance wattage, convert to kW by dividing by 1000 (e.g., 600 W ≈ 0.6 kW).
- If you expect cycling loads (HVAC, pumps), consider adding a buffer to the average load.
- When in doubt, run two scenarios: one optimistic and one conservative. Planning is about the range.
Tips for solar recharge during outages
If you have rooftop PV, your ability to recharge during an outage depends on your inverter/backup configuration (many grid-tied systems shut down without a battery or special hardware). If you use portable panels, your kWh/day depends on panel wattage, sun hours, shading, and how efficiently you can route power into the battery system. For storm planning, it’s reasonable to use a lower kWh/day than your best-day production.
Why the surge check matters
Many homes have modest average loads but occasional high-demand moments (microwave, kettle, pump start, compressor start). Even if your battery has plenty of energy, an inverter can shut down if instantaneous demand is too high. This tool compares your surge estimate to the inverter’s continuous rating and provides a warning note. Always verify your specific inverter’s surge rating and allowable duration.
