- Where a price gap can exist
- Usable energy and the round trip
- A hypothetical day, labeled as arithmetic
- What a savings sentence on a quote is hiding
- Backup can be worth buying when the bill is not
- Frequently asked questions
- Is a battery worth it if my export credit equals my retail rate?
- Can I use the 59 cent example as my savings?
A home solar battery changes an electric bill only when it moves energy across a real price gap, and only after its losses. The gap is either a time-of-use difference, an export credit that is lower than the retail rate, or a demand charge that punishes a short peak. If every kilowatt-hour is worth the same amount at every hour, the battery returns less energy than it absorbed and there is no rate difference to pay for the missing part. Wanting lights during an outage is a separate reason to buy storage. It does not, by itself, show that the pack will pay for itself.
No national percentage survives that test. Tariffs are local. Usable energy is whatever this datasheet allows at this temperature and this power, not the number on the cabinet door. Round-trip efficiency is a property of this system, not a slogan. The rest of this page is how to line those three facts up, including a hypothetical month that is arithmetic only, and how to read a quote that skips them.
Where a price gap can exist
Net metering, in the sense of a credit that offsets later purchases at or near the retail rate, shrinks the bill reason for a battery. The kilowatt-hour you export is already doing much of the work a battery would do, which is why the mechanism belongs in what net metering means and should not be re-derived here. A time-of-use rate does the opposite: the evening kilowatt-hour costs more than the midday one. Whether a battery can live on that spread is the subject of time-of-use rates and a home battery. A commercial demand charge is a third instrument. It prices the highest short draw, not the energy under the curve, and what a demand charge does keeps those units apart.
Self-consumption is the share of solar used on site instead of exported. Raising it saves money only when the exported kilowatt-hour was worth less than the imported one it replaces. That comparison is in what self-consumption changes. A household that already uses most of its solar while the sun is up has less exported energy left for a battery to rescue.
Usable energy and the round trip
Nameplate kilowatt-hours are the size of the tank before the datasheet takes a cut. Depth of discharge, continuous power, and temperature all shrink what you can move in the window that matters. A pack that can store a lot and deliver only a small continuous power will not cover a short, high evening peak. A pack with high power and little energy will start the load and then be empty. How to read those lines, without treating a warranty cycle count as measured life, is in how to read a battery datasheet and cycle life and calendar life.
Round-trip loss is the second cut. Energy put in does not all come out. Heat, conversion, and standby take a share. The price gap has to be wide enough that the kilowatt-hours you get back are worth more than the kilowatt-hours you gave up, including that share. A battery charged from solar that would have been exported gives up an export credit and avoids a later retail purchase. If those two prices are close, the loss dominates. If retail is much higher than export, the same loss can still leave a positive difference. The prices have to be yours.
A hypothetical day, labeled as arithmetic
The figures in this section are hypothetical. They are not a tariff, not a national average, and not a quote. Suppose export credit is 8 cents per kilowatt-hour and evening retail is 22 cents. Suppose the battery absorbs 5 kWh that would otherwise have been exported, and suppose this example's round trip returns 90 percent, so 4.5 kWh comes back. The export credit given up is 5 times 8 cents, or 40 cents. The retail purchase avoided is 4.5 times 22 cents, or 99 cents. The difference is 59 cents on that day, before any standing charge, degradation, or finance cost. Change the 8 cents, the 22 cents, the 5 kWh, or the 90 percent and the 59 cents moves. Discard these stand-ins when you have your own rate card and your own sheet.
Fifty-nine cents a day is not an annual savings claim. A year of identical days would be a fiction, because solar, occupancy, and seasons change. The point of the arithmetic is the shape: benefit equals avoided purchases minus sacrificed export credits, after losses. How a solar payback period is calculated shows how a larger solar example has to be built, and why a single payback year without the rate is unfinished. Do not add the hypothetical 59 cents to that other example. They were built on different assumptions.
What a savings sentence on a quote is hiding
| If the quote says | Ask for |
|---|---|
| The battery pays for itself in a stated number of years | The rate schedule, the usable kilowatt-hours, the round-trip assumption, and the load shape |
| You will use 80 percent of your solar | Whether that percentage is this building's interval data or a default in the software |
| Backup is included, so the savings are higher | Which circuits transfer, for how many hours, and whether a reserve is held back from the bill-shifting cycles |
| Losses are negligible | The round-trip figure on this inverter and battery combination, from their sheets |
A proposal that cannot fill that table is not yet a bill analysis. It may still be a reasonable backup purchase. Those are different yes-or-no questions. The split between a pack sized for expensive hours and a pack sized for a dark house is in a battery for the bill versus a battery for an outage.
Backup can be worth buying when the bill is not
Some households will pay for a defined outage even when the tariff is flat. That is a preference about inconvenience, food, medical equipment, or work, and it does not need a fake payback to be legitimate. It does need a circuit list and an hour count, which sizing backup for critical loads walks through. It also needs equipment that is listed to run when the grid is gone. Panels on a grid-tied inverter will not do that job, which can solar panels run a house in an outage states directly.
Buy the battery for the reason you can document. If the reason is the bill, document the tariff gap and the usable energy. If the reason is the outage, document the loads and the hours. A quote that blends both into one heroic payback year is asking one pile of kilowatt-hours to be spent twice.
Frequently asked questions
Is a battery worth it if my export credit equals my retail rate?
Not for bill shifting. When the exported kilowatt-hour already offsets a purchased one at the same price, the battery's losses are a cost without a price gap. An outage reserve can still be a reason. It will not show up as bill savings.
Can I use the 59 cent example as my savings?
No. Those cents, the 90 percent round trip, and the 5 kWh are stand-in arithmetic. Replace every figure with your tariff and your datasheet before you compare the result with a purchase price.
