Home storage is priced per kilowatt-hour of capacity, and the honest headline is that on energy arithmetic alone it is the worst-value part of a solar installation. It costs more per kWh delivered over its life than the grid charges for the same kWh in most of the country.
That does not make it a bad purchase. It makes it a purchase that has to be justified by something other than the energy bill.
What it costs
Installed, alongside a solar system, typical US pricing at the time of writing:
| Capacity (usable) | Typical installed | Per usable kWh |
|---|---|---|
| 5 kWh | $6,000 – 9,000 | $1,200 – 1,800 |
| 10 kWh | $10,000 – 15,000 | $1,000 – 1,500 |
| 13.5 kWh | $12,000 – 18,000 | $890 – 1,330 |
| 20 kWh | $17,000 – 25,000 | $850 – 1,250 |
| 27 kWh | $22,000 – 32,000 | $815 – 1,185 |
Retrofitting storage to an existing solar system costs more than including it at install — often $1,500–4,000 more, because the inverter may need replacing and the electrical work happens twice.
The federal credit applies to storage as well as to panels, on the same terms. That is a material part of the case for a battery and is worth confirming against your own tax position.
Usable is not nameplate
Every battery is sold on a nameplate capacity and delivers less:
- Depth of discharge. Lithium iron phosphate is typically rated to 90–100% usable; older chemistries less. Lead-acid should not go below 50%, which is why a 200 Ah lead bank is a 100 Ah bank in practice — see LiFePO4 vs lead-acid solar battery.
- Round-trip efficiency. 86–95% for lithium. Store 10 kWh, get 9 back.
- Reserve for backup. If the battery holds a reserve for outages, that portion is not available for daily cycling.
Price everything on usable kWh after those three deductions. A quote comparing a nameplate figure against a competitor's usable figure is not a comparison.
Why the energy arithmetic does not work
Take a 13.5 kWh battery at $15,000 installed, $10,500 after the federal credit. Cycled once daily at 90% round trip, it moves about 12 kWh a day, or roughly 4,400 kWh a year.
The battery does not generate that energy — it time-shifts it. So the saving is not the value of 4,400 kWh; it is the difference between what you would have been paid for exporting it and what you would have paid to import it later. On a full-retail net metering tariff that difference is close to zero, and the battery saves almost nothing.
That is the key point most battery sales skip: under full-retail net metering, the grid already is your battery, and it is free.
The three cases that do hold up
1. Your utility no longer offers full-retail export. Where exports are credited at a wholesale rate and imports are billed at retail, the gap is real — often 15–25 cents per kWh. That gap is what the battery harvests, and it is the strongest financial case there is. This is the situation an increasing number of homeowners are in, and it is the reason storage economics differ so much by state.
2. Time-of-use rates with a steep peak. Charge off-peak or off the array, discharge through the expensive window. The saving is the spread, which needs to be several cents per kWh to be worth the capital.
3. Outages you actually care about. This is the honest reason most people buy storage, and it does not need to pencil out against a utility bill. A battery that keeps the fridge, the well pump and a few circuits running through a multi-day outage is bought the way a generator is bought — for the outcome, not the return.
If none of the three applies, the money buys more return as more panels. See solar cost per watt.
Sizing it
Size to the loads you want to carry and the hours you want to carry them, not to the array. A fridge, a well pump, some lights and internet is typically 3–6 kWh overnight. Adding air conditioning or electric heat changes the answer by an order of magnitude and usually needs a conversation about what you will let run.
For off-grid, where the battery carries several days of autonomy rather than one night, the arithmetic is entirely different — see how many batteries for off-grid.
The short version
Budget $850 – 1,500 per usable kWh installed. Price on usable capacity, not nameplate. Buy storage for export-rate loss, a steep time-of-use spread, or backup you genuinely want — and if none of those is your situation, buy panels instead.