- The bill counts a tariff window
- A hypothetical evening, labeled as arithmetic
- The outage counts a circuit list
- A reserve is the setting that stops the double spend
- What to ask for on the proposal
- Frequently asked questions
- Can a small battery do both jobs if outages are rare?
- Is the 90 percent round trip my battery's efficiency?
A home battery is asked two questions, and they count different hours. The bill question is which expensive hours the pack should cover, and whether the price gap survives the round trip. The outage question is which circuits should stay up after the utility fails, and for how long. A pack scheduled to empty itself across the evening rate can be at its floor when the street goes dark. A pack held full for a storm does almost no bill shifting. One cabinet can be asked to do both. It will do both only if someone sets a reserve and sizes the rest on purpose.
The bill counts a tariff window
Time-of-use storage charges when energy is cheap or when the array would otherwise export, and discharges across the expensive hours. The rate has to exist. A flat rate leaves the losses with nothing to beat, which is why when a home solar battery is worth it refuses a national payback. The shape of the rate itself is in time-of-use rates and a home battery. A commercial demand charge is a different instrument: it prices the height of a short peak, not only the energy under it. What a demand charge does keeps those units apart. Do not size a demand battery with a time-of-use hour count.
Usable energy is the cut the datasheet takes before any of this arithmetic. Nameplate kilowatt-hours, depth of discharge, continuous power, and temperature all change how many of those hours you can actually cover. How to read a battery datasheet is the order of those lines. A warranty cycle count is still not a measured life. Cycle life and calendar life holds that line so a savings spreadsheet does not treat the pack as immortal.
A hypothetical evening, labeled as arithmetic
Everything in this section is hypothetical. It is not a tariff, not an average house, and not a quote. Suppose the expensive window is four hours, the loads you want to shift draw 1.5 kW continuously, and you ignore surge. Energy is 4 times 1.5, or 6 kWh, delivered from the battery. Suppose this example's round trip is 90 percent, so the battery has to absorb about 6.7 kWh to return those 6. If the datasheet's usable energy is 5 kWh, this window does not fit, even though a brochure might call 5 kWh "enough for an evening." Change the hours, the 1.5 kW, or the 90 percent and the 6.7 moves. The 90 percent is a teaching assumption, not a product rating.
The bill value of that shift is a separate multiplication, and it belongs to your rate card. This page does not put a dollar on the 6 kWh. How a solar payback period is calculated shows how a payback has to be built when the figures are real, and why a single year without the rate is unfinished. Do not paste the hypothetical 6.7 kWh into that other example.
The outage counts a circuit list
Outage storage starts from the circuits that transfer, their running watts, and the hours you are willing to carry them. A refrigerator cycling, a router, and a few lights are one list. Adding a well pump adds surge the inverter and the battery both have to carry, which is a different limit from the energy. The method is in sizing backup for critical loads. Whole-house backup is the same method with a longer list, and it fails when the list was never written. Home battery backup for outages develops that choice.
Another hypothetical, again not a quote: three circuits at 400 W combined, carried for 10 hours, need 4 kWh delivered, before inverter losses and before any solar that might arrive the next day. A pack whose usable energy is 10 kWh can hold that outage and still have energy left. The same pack cannot also give the 6 kWh evening shift above unless the evening shift is taken from the leftover, not from the reserve. The numbers are stand-ins so the subtraction is visible. Your circuits and your hours replace them.
A reserve is the setting that stops the double spend
The reserve, backup buffer, or minimum state of charge is the slice the bill schedule is not allowed to touch. Set it at the outage energy you refuse to gamble, and the tariff window can use only what remains. If the screen has no such setting, or it is set to zero, the evening discharge spends the outage kilowatt-hours. The lights then depend on the grid you were trying to survive without. If the reserve is the entire pack, the bill never moves and you have bought outage cover only. Both outcomes are coherent. The incoherent one is a brochure that claims bill savings and multi-day backup from the same kilowatt-hours without showing the split.
| Job | What you count | What a shared pack needs |
|---|---|---|
| Shift an expensive window | Hours on the tariff, times the load, grossed up for round-trip loss | Usable energy above the reserve |
| Ride through an outage | Circuits that transfer, times hours, plus surge the inverter can start | A reserve the schedule cannot empty, and a listed backup path |
| Both | The two sums added, not the larger one alone | A setpoint written down, and a pack that still fits after the addition |
The backup path is a listing, not a side effect of having a battery. Grid-tied panels stop when the utility opens, even if the pack is full. Can solar panels run a house in an outage is that result. A reserve inside an inverter that cannot form an island holds energy you still cannot use when the street is dead.
What to ask for on the proposal
Ask which hours are the bill hours, which circuits are the outage circuits, what usable kilowatt-hours the sheet allows, and what state of charge is held back. Ask for those four on one page. A payback year that does not mention a reserve has assumed the pack is free to empty. An outage promise that does not mention a transfer has assumed the energy can reach the lights. What an inverter installation has to include is the rest of the commissioning record those settings belong in.
Frequently asked questions
Can a small battery do both jobs if outages are rare?
Rare does not shrink the outage you decided to cover. It only changes how often you are glad the reserve was there. If the usable energy is smaller than the outage sum, there is no leftover for the tariff, however rare the storms are. Size the reserve first. Then see if anything remains to shift.
Is the 90 percent round trip my battery's efficiency?
No. It is a teaching assumption in the hypothetical evening above. Use the round-trip figure on this inverter and battery combination, from their datasheets, and label any example you build with it as your own arithmetic.
