Many Texans have experienced power outages following big thunderstorms or winter storms. These outages can last from a few minutes to a few weeks! Those lasting a few minutes are minor nuisances, but when an outage goes into hours or weeks, it gets progressively more distressing.
The quick answer to how many kWh of battery power a person needs is: "It depends!" If you've read the previous article, "Grid-Tied Solar vs. Grid-Tied Solar with Battery Backup: What's the Difference?", you may already know that if you want power during an outage, you'll need a system that includes batteries. Batteries can be charged up using solar modules, a generator, or even just the grid. To figure out the number and size of batteries you need, calculate your total loads, then calculate the loads of just the appliances you'd like to back up and for how long you'd like that backup to last. You can use our handy calculator to roughly determine what those loads are.
Most people want emergency power for their internet router, the refrigerator and freezer, some outlets and lights, and maybe the coffee pot! Many people want to keep their air conditioner running, but without a significant investment, that's not going to happen in Texas. The fan may have to suffice!
For me, I'd want to run my refrigerator and freezer, coffee maker, router and TV, some lights and outlets, and my attic fan, and I'd like to know I could last three days. For comparison, check out my normal usage below, and then my critical loads backup.
What My House Actually Uses Day to Day
Here's where all 2,038 kWh a month actually goes in our house, appliance by appliance:
What We'd Actually Run During an Outage
Full household use is one number. What I'd actually need during an outage is a much shorter list, built around the things I mentioned above, refrigerator and freezer, coffee maker, router and TV, a few lights and outlets, and the attic fan.
*Attic fan hours are an estimate; adjust to match how long you'd realistically run it during an outage.
Our Own Setup: What 6 kW and a Battery Bank Would Actually Buy Us
We're not just running these numbers on paper, either. Right now we have 6 kW of grid-tied solar producing at our own house. What we're working on next is pairing that with a hybrid inverter (sometimes called a grid-tied battery-backup inverter) and a battery bank, so we can actually answer the question this whole article is asking: how long could we really keep the lights on?
Our battery bank is rated at 400 amp-hours at 48 volts. Multiply those together and you get the battery's total energy storage: 400 Ah × 48V = 19,200 watt-hours, or about 19.2 kWh.
Stack that against our real critical-loads number from above (roughly 11.5 kWh a day to run the fridge, keep a couple of fans going, stay lit, and stay connected) and the battery bank alone, with zero help from the sun, would carry us for about a day and a half before running dry.
That's really the piece that matters for outage planning. The battery is what gets you through the night, no matter what the sky is doing. The solar is what recharges it the next day. On a clear day, our 6 kW array produces far more than 11.5 kWh, so it tops the batteries right back off. But it's worth being honest about the kind of outage we're actually planning for. Thunderstorms and straight-line winds cause most of the outages around here, and those are exactly the conditions where solar output drops off a cliff. You can see that for yourself in the cloudy-day chart from our other article, where production barely registered. So the real answer isn't "solar keeps up in real time during the storm." It's "the battery gets you through the storm itself, and the solar earns its keep once the weather clears."
Curious what this would look like at your house?
Get a free, no-pressure quote from Hill Country Solar to see what critical-loads or whole-home backup would cost for your home.
The Real Number to Plan Around
Our full household draws over 2,000 kWh a month. What we'd actually need to get through an outage is about a tenth of that: roughly 11.5 kWh a day. That gap is really the whole point of this exercise. Sizing a battery backup system around everything you own gets expensive fast, and it's usually not necessary. Sizing it around what you'd genuinely use during an outage is a lot more affordable, and it's the number that should drive the conversation with whoever's designing your system.
Want to run this same exercise for your own house, full usage versus critical loads? Our calculator will walk you through it in a few minutes.