TL;DR: Most Utah homeowners overestimate the battery they need. The right size depends on three things: do you have central AC, do you have a well pump or EV, and how long do you actually need to run during an outage. Here's the four-tier framework we use on every BYOP install — from 9–16 kWh essentials backup (one battery) to 48+ kWh virtual off-grid (four or more). For the typical Utah home that wants whole-home backup including AC, the answer is two batteries (18–32 kWh).
The four-tier framework
| Tier | What you get | Tesla PW3 | Sigenergy | EG4 | Ruixu |
|---|---|---|---|---|---|
| Tier 1 — Essentials backup | Fridge, lights, Wi-Fi, outlets. 3–4 hours of evening coverage. | 1 × 13.5 kWh | 1 × 9 kWh | 1 × 14.3 kWh | 1 × 16 kWh |
| Tier 2 — Overnight backup | Whole-home including AC for 6–8 hours. Solar runs you through 5–6 PM, battery covers until 11 PM–2 AM. | 2 × 13.5 (27 kWh) | 2 × 9 (18 kWh) | 2 × 14.3 (28.6 kWh) | 2 × 16 (32 kWh) |
| Tier 3 — Near-off-grid | Rarely touch the grid. Solar covers daytime, batteries cover most nights without grid import. | 3 × 13.5 (40.5 kWh) | 3 × 9 (27 kWh) | 3 × 14.3 (42.9 kWh) | 3 × 16 (48 kWh) |
| Tier 4 — Virtual off-grid | Never touch the grid in normal weather. Multi-day outage survival. | 4+ × 13.5 | 4+ × 9 | 4+ × 14.3 | 4+ × 16 |
The "essentials backup" tier is where most retrofit customers land — they want power security through summer thunderstorm outages and the occasional winter ice event, not energy independence. The "overnight backup" tier is the sweet spot for households with AC, well pumps, and EVs. Tier 3 and 4 are for customers actively buying down their grid dependence.
The five questions we ask to sort you into a tier
- What's your typical Utah monthly electric bill? Sub-$100: probably Tier 1. $150–$250: Tier 2 sweet spot. $300+: Tier 2 or 3 with bigger solar.
- Do you have central AC? If yes, you need at least Tier 2 power capacity (Tesla 11.5 kW or higher continuous output) or AC won't run on battery.
- Well pump? Same rule — Tier 2 minimum on continuous power. Most well pumps want 5–7 kW startup surge.
- EV(s)? Charging an EV from battery during a multi-day outage is real backup planning. Tier 3+ recommended, or Sigenergy with V2H so the car becomes the battery.
- How long is your worst-case outage? Utah's record was a 5-day winter outage in 2021. Tier 2 covers most. Tier 3 covers everything short of a multi-week event.
The "kWh from your bill" shortcut
Look at your last utility bill. Find total kWh used this month. Divide by 30 = your daily usage. Roughly half of daily usage happens after sunset (evening + night) — that's what your battery needs to cover, minus any solar carryover.
Example: 900 kWh/month ÷ 30 = 30 kWh/day. Half = 15 kWh evening/night usage. One 13.5 kWh Tesla Powerwall ≈ one night of typical usage in that house.
AC sizing — the question that catches customers off-guard
Running central AC on battery requires:
- Continuous power: at least 4–6 kW (one zone, mid-summer)
- Startup surge: 1.5× continuous draw at compressor start
- Energy budget: ~3 kWh/hour of runtime in mid-summer Utah heat
Translation: a single 4.5 kW Enphase battery can't run your AC. A 5 kW SolarEdge Home Battery can barely. A Tesla Powerwall 3 (11.5 kW continuous) runs AC plus the rest of your house easily. This is the single biggest reason 200A pass-through and 11.5+ kW continuous output matter. Full brand comparison here.
Well pumps
Submersible well pumps (1–2 HP) draw 1.5–3 kW continuously but surge to 5–7 kW at startup. Any battery on our bench (Tesla, Sigenergy, EG4, Ruixu) handles a well pump. Older 4.5–5 kW essentials batteries do not.
Two-Powerwall vs one — when does the second pay off?
Add the second when:
- You have AC + EV + electric heat (winter heating loads will drain one PW3 by 2 AM).
- You have more than 7.68 kW of existing AC solar (single PW3 caps at 7.68 kW AC-coupled solar input).
- You want to survive a multi-day outage without rationing.
Don't add the second when:
- Your bill is under $150/mo and you don't have AC. You're paying for capacity you'll never use.
- You're chasing "off-grid feel" but only have 4 kW of solar. The battery can't recharge fast enough — bigger solar should come first.
Customers who think they need way more than they actually do
We routinely talk customers down from "three Powerwalls" to one. The typical mismatch: they're sizing for "what if I never want to depend on the grid" but they don't have enough solar to recharge three batteries daily. A 7 kW solar array producing ~25 kWh/day can recharge one battery overnight. It can't recharge three. Three batteries with a 7 kW array = you have storage you never refill.
The right correction conversation: "Start with one battery. Add a second next year if you want it. We make expansion easy on every brand we install." Sigenergy 9 kWh modules expand at $2,700 installed. Ruixu at $3,500 per 16 kWh. EG4 at $4,000 per 14.3 kWh. Full pricing here.
Frequently Asked Questions
How many kWh of battery do I need for whole-home backup in Utah?
For whole-home backup including AC, plan on 18–32 kWh of battery (Tier 2 — two batteries on most BYOP installs). A single Tesla Powerwall 3 at 13.5 kWh covers essentials plus AC for about 3–4 hours; two Powerwalls or equivalent gets you through an entire summer night.
Can one Tesla Powerwall 3 run my central AC during an outage?
Yes. A Tesla Powerwall 3's 11.5 kW continuous output is enough to start a central AC compressor and run it along with the rest of your house simultaneously. The constraint isn't power — it's runtime. AC draws roughly 3 kWh per hour in mid-summer Utah, so a single 13.5 kWh Powerwall runs AC for about 3–4 hours before it depletes. Two Powerwalls double that runtime.
How do I calculate how much battery I need from my electric bill?
Find your monthly kWh on your last bill. Divide by 30 = daily usage. About half of daily usage happens after sunset, so that's what the battery needs to cover (minus any solar carryover). Example: 900 kWh/month ÷ 30 = 30 kWh/day. Half = 15 kWh evening usage. One 13.5 kWh battery ≈ one typical night in that house.
When is it worth adding a second Powerwall?
Add the second when (1) you have AC + EV + electric heat, (2) you have more than 7.68 kW of existing AC solar (single Powerwall caps there), or (3) you want to survive multi-day outages without rationing. Don't add the second if your bill is under $150/mo without AC, or if you're chasing "off-grid feel" without enough solar to recharge multiple batteries.
About the author
Batsaikhan(Bat) Ariun-Erdene is the owner of BYOP Electric, a licensed Utah Master Electrician (E200). More about Bat and BYOP Electric.