GRID COMPASSA home battery lasts as long as the energy stored in it (measured in kilowatt-hours) divided by how fast your home is using power (measured in kilowatts). Back up just the essentials and a common home battery can run for a day or more; try to run central air conditioning and the same battery might last only a few hours. There's no single number, runtime is a math problem, and this guide shows you how to do it, including how smart load management changes the equation.
The simple formula behind runtime
Two numbers decide everything:
- Capacity (kWh), how much energy the battery holds. This is the same unit on your electric bill.
- Draw (kW), how much power your home is pulling at any moment.
Runtime (hours) ≈ usable capacity (kWh) ÷ average draw (kW).
A 13 kWh battery running a 1 kW load lasts about 13 hours. Running a 4 kW load, it lasts a little over 3 hours. Same battery, the difference is entirely what you choose to keep on.
Definition, kWh vs. kW: A kilowatt (kW) is a rate of power, like speed. A kilowatt-hour (kWh) is an amount of energy, like distance. A battery's kWh is the size of the tank; your kW draw is how hard you're pressing the gas.
Real Florida examples
These are illustrative estimates to show how the math moves, your home's real draw depends on your specific appliances.
| What you're running | Rough draw | ~Runtime on 13 kWh |
|---|---|---|
| Essentials only (fridge, Wi-Fi, lights, phones) | ~0.5 to 0.8 kW | ~16 to 24+ hours |
| Essentials + one efficient A/C zone / mini-split | ~1.5 to 2 kW | ~6 to 9 hours |
| Whole home including central A/C | ~3 to 5 kW | ~2.5 to 4 hours |
Notice the pattern: central A/C is the deciding factor. In Florida, that one choice can swing your runtime from a full day to an afternoon.
Why air conditioning changes the answer so much
A central A/C compressor is both a large steady load and a surge load, it spikes hard every time it starts. That does two things: it drains capacity fast, and it demands high instantaneous power output. A battery that's big enough on paper can still struggle if its power output (kW) can't handle the A/C's startup surge. This is why "how long will it last?" and "can it even run my A/C?" are the same conversation. (More in our whole-home battery backup guide and Can a home battery power my whole house?)
How to make a battery last longer
- Prioritize loads. Run the fridge, medical devices, internet, and a fan or single A/C zone, not the whole house. This is the single biggest lever.
- Create one cool room instead of cooling the entire home.
- Add capacity. Multiple battery units stack to extend runtime for larger loads.
- Pair with solar. This is the only option that recharges the battery during the outage. On a sunny Florida day, solar can refill the battery while you use it, turning "a few hours" into "indefinite, as long as the sun comes up." For a multi-day hurricane outage, that's the difference that matters. (See hurricane & outage resilience.)
Smart load management: automating what actually stretches runtime
Everything above has a catch: manually prioritizing loads means you deciding, in the dark, during a storm, what to switch off. Newer setups automate that with intelligent load management, an electrical panel or controller that monitors every circuit and powers them by priority during an outage.
For battery runtime, that matters in three ways:
- Circuit-level prioritization. Set which circuits are "must-run" (fridge, medical, internet), "nice-to-have" (a room's A/C), and "can-drop" (water heater, dryer, EV charger). In an outage, power flows in that order.
- Automatic load shedding. As the battery drains, the system can drop lower-priority circuits to protect the essentials, so a load spike doesn't quietly burn your reserve.
- Flexibility over a fixed backup subpanel. Instead of an electrician hard-wiring your essentials once at install, every circuit stays manageable and re-prioritizable.
The honest framing: load management doesn't add a single kilowatt-hour of storage. What it does is make the kWh you have go further, by matching your draw to your reserve intelligently instead of hoping you sized for the worst case. Whether it's worth it depends on your home, a house with big, spiky loads (central A/C, EV, electric water heat) benefits far more than a small, simple one. It's worth asking about, not a default yes.
The honest part
Any advisor who gives you a single runtime number without asking what you want to run is guessing. Today, your runtime is a function of your battery size, your loads, and how intelligently those loads are managed, that's just physics plus controls. What you should build depends on your outage history and what you can't afford to lose. The smart first step isn't buying the biggest battery; it's knowing your own numbers: what must stay on, and for how long.
The bottom line
A home battery doesn't last "X hours", it lasts as long as your choices let it. Decide what's essential, size to that, let smart load management handle the rest, and if multi-day resilience is the goal, pair it with solar so it can recharge. That's how you go from hoping the power comes back to knowing your home stays on.