Residential Battery Backup Autonomy Planner

Introduction

In a blackout, the practical question is not simply how large your battery looks on a spec sheet. What matters is whether the usable energy left after depth-of-discharge limits and inverter losses is large enough to support the circuits you truly need. A battery that appears generous on paper can still empty quickly if the house keeps pulling a couple of kilowatts around the clock. On the other hand, a realistic shedding plan and even modest daytime solar can stretch a backup system much farther than a quick nameplate division would suggest. This planner is designed for that real-world decision. It turns the assumptions you already think about during outage planning—capacity, inverter efficiency, average critical load, surge demand, shedding, and solar recharge—into a concrete hour-by-hour reserve model.

The goal is not to replace a licensed designer or your equipment documentation. The goal is to give you a better planning conversation. You can test whether your current battery is enough for a short outage, whether a tighter load list would buy meaningful autonomy, or whether a little daytime recharge changes the picture from risky to workable. Because the tool simulates state of charge one hour at a time instead of only doing one rough division, it helps you see when reserve fades, when solar helps, and whether your plan has comfortable margin or only barely survives.

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 by 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. That makes the tool useful both for quick screening and for comparing backup strategies before you buy more hardware or rewire a panel.

How to use it (practical workflow)

A good way to approach the form is to work from the battery outward. Start with the storage you already own or are considering, then estimate the load you actually intend to keep alive during an outage. After that, pressure-test the plan with surge demand and realistic solar. If you move through the inputs in that order, the result usually feels intuitive instead of abstract.

1. Enter battery and inverter details such as capacity, depth of discharge, efficiency, and continuous inverter rating.
2. Enter your critical load as an average kW value for the circuits you want to keep on.
3. Enter the largest short-term surge you expect, such as a pump or compressor starting while another load is already running.
4. Enter expected solar recharge per day if panels or other charging sources can replenish the battery during the outage.
5. Set the outage horizon and your shedding plan so the model knows how long you want to last and how much you are willing to trim usage.
6. Click Simulate Autonomy to update the runtime estimate, daily state-of-charge table, and scenario comparison.

Inputs and guidance (what to measure and what to estimate)

The most common planning mistake is mixing up power and energy. Your battery is sized in kWh, which tells you how much energy is stored. Your appliances and circuits draw kW, which tells you how fast that energy is being used. If you run a 2 kW average load for 5 hours, you consume roughly 10 kWh. That simple distinction makes the rest of the calculator much easier to interpret.

• Battery bank nameplate capacity (kWh): total stored energy. If you have multiple batteries, add their kWh values together.
• Maximum depth of discharge allowed (%): the share of the battery you are willing to use. Many systems are operated below 100% discharge to protect longevity and stay inside warranty settings.
• Inverter efficiency (%): conversion losses from DC battery energy to AC household power. Higher efficiency means more of the stored energy reaches your loads.
• Inverter continuous power rating (kW): the sustained power limit. This tells you how much load the inverter can support at once, which is different from how long the battery can run.
• Average critical load (kW): the long-run average of the circuits you plan to keep alive, such as refrigeration, internet, lights, fans, controls, or a well pump that cycles.
• Largest short-term surge load (kW): the biggest temporary overlap you expect, such as a pump start plus microwave or a compressor kicking on.
• Expected solar recharge per day (kWh): the energy your system may add during an outage. Conservative planning is best here because clouds, shading, and winter conditions can cut production sharply.
• Planned load shedding (%): how much lower your outage usage will be compared with the average critical load you entered. A higher shedding percentage reduces the modeled 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 chosen shedding percentage. Solar recharge is spread across six midday hours, from 10:00 through 15:00, to approximate a typical production block rather than giving all of the solar at once. The battery is assumed to start the outage fully charged within its usable window, and the simulation stops when reserve reaches zero or when your chosen horizon ends.

Usable energy (kWh):

$U=C\times \mathrm{DoD}\times \eta$ where C is capacity (kWh), DoD is depth of discharge as a fraction, and η is inverter efficiency as a fraction.

Effective hourly load (kWh per hour):

$L=P\times (1-s)$ where P is average critical load (kW) and s is load shedding as a fraction.

A quick mental shortcut is useful here. If you have usable energy in kWh and an average load in kW, dividing one by the other gives a rough number of hours. The full planner goes further because it adds solar at specific times of day and preserves the battery state from one hour to the next, which is why the simulated answer can differ from the rough estimate.

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 is 9.94 ÷ 2.04 ≈ 4.9 hours

That rough runtime is a helpful first check, but it is not the whole story. In the simulation, any solar recharge is added during the midday block, so the battery can recover some reserve before the next overnight period. The Daily state of charge summary table is especially useful here because it shows whether you are barely scraping through each day or finishing with healthy cushion.

Interpreting results (what the output really means)

When the calculation finishes, the main result gives you a concise summary, but the supporting tables tell the more useful planning story. Think of the runtime figure as the headline, then use the tables to understand why the plan succeeds or fails.

• Modeled autonomy (hours/days): the simulated time until the battery reaches zero usable energy or until the horizon ends.
• Meets target horizon: a yes/no check that compares modeled runtime with your planned outage duration.
• Daily minimum SoC: a stress indicator. A very low minimum state of charge means you have little margin for colder weather, worse solar, or a slightly higher load than expected.
• Surge note: a warning that compares your surge estimate with inverter rating. A battery can have plenty of energy and still fail if the inverter cannot deliver the short-term power demand.

Limitations and assumptions

This is a planning model, not a full electrical design package. It simplifies several real-world effects so you can compare scenarios quickly and understand the direction of the tradeoffs. That makes it useful, but it also means the answer should be treated as a model rather than a promise.

• Constant average load: real loads vary by hour. If your home has spiky or weather-driven demand, use a slightly conservative average.
• Temperature and chemistry effects: cold weather can reduce available energy and power. Some systems also reserve energy for internal protection.
• Solar variability: clouds, snow, smoke, shading, and panel orientation can reduce daily recharge far below a good-weather estimate.
• Starting charge: the calculation assumes the battery starts fully charged within the usable range when the outage begins.
• Inverter limits: the surge check is a heuristic. Manufacturer surge ratings and allowed duration vary by product.

If the backup system must support life safety, medical equipment, code-required loads, or a high-confidence whole-home design, confirm the plan with qualified professionals and your equipment documentation. The calculator is best used as a scenario planner that helps you ask sharper questions and choose safer assumptions.