The Question Every Solar Owner Eventually Asks
When I started seriously researching battery backup for my Ohio home, one of my first stops was a hands-on EcoFlow DELTA Pro review to understand the portable battery option. Then, I asked my solar installer a simple question: “How much storage do I need?” His answer was a 13.5 kWh Powerwall, because that’s what he sold. (I eventually did a detailed comparison — see Powerwall vs. EcoFlow DELTA Pro: the real price difference.) I pushed for the math. He gave me a vague answer about “covering your critical loads for a night.” I went home and did the math myself.
This is what I found — and how you can run the same calculation for your house.
Step One: Define What “Backup” Means to You
Before you calculate storage size, you need to decide what you’re trying to back up. There are three common scenarios, and they have wildly different storage requirements:
Scenario A: Keep the Lights On During an Outage
Lights, phone chargers, WiFi router, a few small appliances. This is what most people actually need during a typical outage. A fridge. Maybe a TV. Not running your HVAC, not your electric water heater, not your EV charger.
Scenario B: Critical Comfort Loads
Everything above, plus a window AC unit or limited heat pump operation, your refrigerator and freezer, and maybe a medical device. You’re comfortable but not running the whole house normally.
Scenario C: Whole-Home Backup
Everything runs exactly as normal. HVAC, water heater, oven, EV charger — all of it. This requires a lot more storage than most people realize.
When my installer said “critical loads for a night,” he meant Scenario A or B. That’s legitimate for most power outages in Ohio, which typically last 2–8 hours. But if you want Scenario C during an ice storm that knocks out power for 48 hours, you’re looking at a very different system.
My House’s Critical Load Calculation
Here’s my actual math. I went through my home circuit by circuit using data from my Emporia Vue energy monitor, which showed me real-world consumption for every circuit.
Scenario A Loads (my house, 24-hour period)
- Refrigerator: 1.2 kWh/day
- Chest freezer: 0.8 kWh/day
- Lights (LED throughout): 0.6 kWh/day
- WiFi router + devices: 0.3 kWh/day
- Phone/laptop charging: 0.2 kWh/day
- Total Scenario A: ~3.1 kWh/day
With 3.1 kWh/day for Scenario A loads, a 10 kWh battery gets me about 3 days of outage coverage — and that’s without any solar recharging during the day.
Scenario B Loads (adding comfort)
- Everything in Scenario A: 3.1 kWh/day
- Window AC unit (6,000 BTU, running 8 hours): 2.4 kWh/day
- Electric water heater (limited usage): 2.0 kWh/day
- Total Scenario B: ~7.5 kWh/day
At 7.5 kWh/day for Scenario B, a 13.5 kWh Powerwall covers about 1.8 days without solar input — or nearly indefinitely if my panels can recharge it each day (which, in summer, they can).
Scenario C Loads (whole home)
- Everything in Scenario B: 7.5 kWh/day
- Central HVAC (heat pump, Ohio winter): 25–40 kWh/day during heating season
- Electric dryer: 2.5 kWh per use
- EV charger (if applicable): 10–40 kWh/day
- Total Scenario C: 35–80+ kWh/day in winter
This is where people get shocked. Whole-home backup in a cold Ohio winter can require 35–80 kWh per day. A single 13.5 kWh Powerwall lasts maybe 4–6 hours on a cold day if you’re running your heat pump. Even a 27 kWh system (two Powerwalls) might only give you 8–12 hours of true whole-home coverage without solar recharging.
The 10 kWh vs. 13.5 kWh vs. 27 kWh Decision
Here’s how I think about these three common system sizes:
10 kWh (smaller systems like Franklin iFlex or entry EcoFlow DELTA Pro)
Good for: Scenario A loads for 2–3 days, or Scenario B loads for 1–1.5 days. Great for the typical Ohio outage (under 12 hours). Also excellent if you have a gas furnace and only need electricity backup, not heating backup.
Not good for: Running HVAC during a multi-day winter storm without significant solar recharging.
13.5 kWh (Tesla Powerwall 2, Generac PWRcell 9 kWh, etc.)
Good for: Scenario B loads for about 2 days. This is the sweet spot for most Ohio homeowners who want real backup without massive cost. With solar recharging during the day, this system can sustain indefinitely during spring/fall outages.
My installer’s recommendation was right for the wrong reasons — it happened to be the right size for my needs, but I didn’t know that until I ran the numbers myself.
27 kWh (two Powerwalls, or expandable systems at larger capacity)
Good for: Homes with whole-home backup aspirations, cold climates where HVAC backup is critical, or homes with medical equipment that needs 24/7 power reliability. Also makes sense if you have or plan to get an EV.
Not good for: Your budget if you don’t actually need that coverage. Two Powerwalls are roughly $26,000–$32,000 installed. That’s a significant premium over a single unit.
The Solar Recharging Variable
Here’s the math most installers don’t walk you through carefully: what does your system actually generate during winter in your area?
In Ohio, my 10.2 kW system generates an average of:
- January: ~6 kWh/day
- February: ~9 kWh/day
- December: ~5 kWh/day
- July: ~42 kWh/day
In summer, my panels can fully recharge a 13.5 kWh battery every single day with plenty of power left over. In a January storm, I might generate 6 kWh while consuming 7.5 kWh/day on Scenario B loads — meaning I’d drain about 1.5 kWh net per day. A 13.5 kWh battery would last about 9 days at that rate.
In a true worst-case winter scenario with heavy overcast — generating 2–3 kWh while consuming 7.5 kWh on Scenario B — I’d drain about 4.5–5.5 kWh/day. A 13.5 kWh battery would still last 2.5–3 days. For Ohio power outages, that covers the vast majority of events.
What I Actually Bought (And Why I’m Waiting)
Here’s my honest answer: I haven’t bought a battery yet. After running these numbers, I realized that for my specific situation — gas furnace for heat, solar panels that generate enough in summer to recharge daily, and a history of outages under 12 hours — I’m in the Scenario A camp. A 10–13.5 kWh system would serve me fine.
But I’m also watching the market. Battery prices have dropped significantly in the last two years. The EcoFlow DELTA Pro series offers modular expandability — start with one unit at ~10 kWh and add capacity later. That flexibility matters when you’re not sure exactly how your usage patterns will evolve (especially if an EV is in your future).
The 30% federal tax credit (through 2032) applies to battery storage paired with solar, which brings the effective cost down considerably. A $10,000 battery system becomes $7,000 after the credit.
Run Your Own Numbers First
Don’t let an installer tell you what size battery you need without doing this calculation yourself:
- List your critical loads and their daily kWh consumption
- Decide which scenario (A, B, or C) you’re actually planning for
- Look up average solar production for your location by month
- Calculate the net daily drain: consumption minus production
- Decide how many days of coverage you want at that net drain rate
Storage size = (daily net drain) × (days of coverage desired)
For most Ohio homeowners targeting Scenario B with 2 days of coverage, that math points to 13–15 kWh. For Scenario A, 10 kWh is more than enough. For Scenario C in winter, you’re looking at 30+ kWh and the cost math gets harder to justify.
Know your scenario before you sign anything. If you’re also weighing whether to get a battery vs. a traditional generator, I covered that decision in depth in my whole-home standby generator vs. battery backup comparison.