Whole-home battery: usable kWh vs the sticker

The sales deck says 13.5 kWh, which is the size of a Tesla Powerwall 3-class pack, and it sounds like a whole house. Kilowatt-hours are a tank of energy. Kilowatts are how hard the tank can pour. A pack can have a big tank and a small hose, or a small tank and a fat hose. If you mix those two numbers, you will buy a silent box that still cannot start the well.

Tesla lists Powerwall 3 at 13.5 kWh of energy and 11.5 kW continuous on-grid and backup power, with a 10-year warranty. Their spec library also lists solar-to-battery-to-grid round-trip efficiency at 89% (beginning of life, 25°C). That 89% is the trip through the inverter and the cells. You do not get 13.5 kWh out of a 13.5 kWh charge.

Tesla: Powerwall

Tesla: Powerwall 3 specifications

If you already know you want gas on a pad instead, the battery vs standby generator page is the fork. This page is how to read the battery half of that quote.

What does EnergySage say a pack costs?

EnergySage’s 2026 storage cost page puts a typical install at $15,647 before incentives for about 13.5 kWh, the size they associate with a Powerwall 3. Brand-level dollars per kWh from their first-half 2026 quotes:

• Tesla: $967/kWh, typical 13.5 kWh, about $13,055
• Enphase: $1,429/kWh, typical 10 kWh, about $14,290
• FranklinWH: $1,232/kWh, typical 15 kWh, about $18,480
• Sigenergy: $1,028/kWh, typical 15.4 kWh, about $15,831

EnergySage: solar battery cost

Their storage overview also notes that whole-home backup is a different order of money: about $34,000 in their “whole home backup” example, and much more if you are trying to go off-grid. One 13.5 kWh pack is the essentials-overnight product, not two days of electric heat.

EnergySage: home batteries in 2026

The federal storage credit followed the same end-of-2025 cliff as the other IRA home credits. EnergySage says the federal tax credit is no longer available for residential batteries. Don’t let a 2026 bid subtract 30%. State and utility programs may still exist.

Usable vs nameplate vs what the fridge actually gets

Nameplate / nominal is the chemistry in the box. Usable is what the inverter will let you take before it protects the cells. Tesla’s 13.5 kWh figure is the energy they want you to compare. Other brands still print a larger nominal number and a smaller usable number. If Bid A is “16 kWh” and Bid B is “13.5 kWh usable,” ask both for usable kWh at the AC terminals.

Then apply the round trip. Tesla’s 89% solar-to-battery-to-grid number means a full 13.5 kWh charge is not 13.5 kWh of lights. Call it on the order of 12 kWh out of that pack on a good day, before a cold garage and a ten-year-old pack. Tesla’s warranty language on their learn page is at least 70% of original capacity at year 10. Budget the decade, not the brochure day.

Power (kW) is the other axis. 11.5 kW continuous will run a lot of a typical house at once. A 3.8 kW module (Enphase IQ Battery 5P-class, 5 kWh usable per unit in EnergySage’s brand table as a 10 kWh typical Enphase size at the 5P/10C family) will not start a well and a range together no matter how many hours are in the tank. Size the surge and the simultaneous loads first, then the hours. The critical-load panel list is how you stop guessing.

Do the hours on a napkin before you argue about brands. A fridge that averages 150 watts is about 3.6 kWh in a day. Add 100 watts of internet and lights for 16 hours and you are near 5 kWh. One 13.5 kWh pack, after Tesla’s 89% round trip, still covers that with margin, which is why EnergySage calls 13.5 kWh enough for essentials.

A 2 kW well pump for 10 hours is 20 kWh, which is already more than one pack even before the fridge. A heat-pump air handler at 500 watts for 12 hours is another 6 kWh. Whole-home on one pack only works if “whole-home” means you turned most of it off. Two packs are not automatically two times the power, either. Tesla’s 11.5 kW is per Powerwall 3 inverter. Expansion units add kWh. Extra inverters add kW. The quote has to say which.

What do three real-looking bids mean?

Bid A, about $13,000 to $16,000. One 13.5 kWh Powerwall-class pack, gateway, critical-load panel, no solar. EnergySage’s Tesla line and their $15,647 typical. This covers fridge, internet, a few lights, and maybe the air handler if you shed everything else. It is not a 30-hour ice storm with the heat pump on.

Bid B, about $18,000 to $25,000. Same pack plus solar coupling, or two smaller AC-coupled modules (Enphase’s 10 kWh typical at $14,290 is one module-family step; two get you hours, not a free lunch on power). Retrofit on existing solar costs more labor than pairing at first install, which EnergySage calls out as extra wiring and equipment.

Bid C, about $34,000. EnergySage’s whole-home backup example. Several packs, or a large stack, plus a gateway that can island the main panel. Compare that number to a standby generator on the sizing page, because runtime is now the product and gas still wins on days, not hours.

If Bid A is $9,000, find the missing gateway, the missing critical-load panel, or a usable kWh that is smaller than the sticker.

When is one pack the right call?

Outages where you live are hours, not days. You will put the fridge, the heat’s air handler or a couple of window loads, internet, and phones on a labeled critical-load panel. You can live without the range and the dryer until the utility is back. You like quiet and you already have, or will have, solar to refill the tank.

One pack also fits as a bridge: it covers the common four-hour switching event, and you keep a portable generator inlet for the rare long storm. That inlet is the portable generator inlet job, not a second battery.

When is the sticker lying?

“Whole-home backup” on a 13.5 kWh quote. The gateway may be able to pick the whole panel, and then the pack empties in a couple of hours with the heat pump and the water heater on. Whole-home is a wiring topology. Hours of runtime are math: loads in kW times hours, compared with usable kWh after efficiency.

A cold garage. Inverters derate when it is freezing. Tesla’s environmental range exists, but winter output is not the 25°C spec sheet. Ask where the pack will hang, and what they claim at 20°F.

A bid that still shows a 30% federal credit on a 2026 install. Cross it off.

A “16 kWh” module quoted against a “13.5 kWh usable” Tesla without converting both to AC kWh at the same depth of discharge.

What should be on the quote?

1. Usable kWh at the AC terminals, not only nominal.
2. Continuous kW and surge / locked-rotor amps if you have a well or a compressor.
3. Round-trip efficiency, and whether that is DC-side or AC-side.
4. Warranty years and capacity remaining (Tesla’s 10 years / 70% is the comparison point).
5. Gateway, critical-load panel vs whole-panel islanding, and which circuits are in.
6. Solar: DC-coupled vs AC-coupled, and whether existing inverters stay.
7. Location, operating temperature, and who owns a cold-weather derate.
8. No 25D/storage federal dollar on a 2026 placed-in-service date.
9. What is excluded: panel upgrade, trench, extra packs.

How do I decide in one afternoon?

1. Write the outage you actually get: four hours or two days.
2. Write the loads you refuse to lose, in watts, including motor start.
3. Divide those watts into hours of backup you need, then compare to usable kWh after ~10% round-trip loss, not to the sticker.
4. Get two bids that use the same usable-kWh and kW lines. If they will not write usable, you have a brochure.

One 13.5 kWh pack is a real product for a short outage on a short list of circuits. It becomes a disappointment when you asked it to be a generator.