CellWise

Home Battery Tariff Savings Calculator

Charge cheap, discharge dear — see the battery size and yearly saving your time-of-use tariff makes possible.

Daily peak-window use (kWh)

Peak rate ($/kWh)

Off-peak rate ($/kWh)

Round-trip efficiency (%)

Usable depth of discharge (%)

Daily arbitrage saving

$2.17

This models tariff arbitrage only — charging cheap off-peak and discharging at peak. Add solar self-consumption and backup value on top. Battery degradation and the upfront price decide whether the annual saving actually pays back.

Annual saving: $791
Usable capacity needed: 10.0 kWh
Nameplate battery size: 11.1 kWh

What you entered

Daily peak-window use (kWh): 10Peak rate ($/kWh): 0.35Off-peak rate ($/kWh): 0.12Round-trip efficiency (%): 90Usable depth of discharge (%): 90

1
Usable capacity needed
covers 10 kWh peak-window load= 10 kWh
2
Nameplate size (÷ depth of discharge)
10 ÷ 90%= 11.1111 kWh
3
Off-peak charge energy (incl. losses)
10 ÷ 90%= 11.1111 kWh
4
Daily saving
(10 × $0.35) − (11.1111 × $0.12)= $2
5
Annual saving
$2 × 365= $791

Result

Daily arbitrage saving: $2.17

To shift 10 kWh out of the peak window you need about 10.0 kWh usable (11.1 kWh nameplate). The arbitrage saves roughly $2.17/day — about $791 a year before degradation and upfront cost.

Battery size and yearly arbitrage saving by daily peak-window use

Each row sizes a battery to cover one day's peak-window consumption and prices the arbitrage from charging off-peak and discharging at peak. Nameplate capacity assumes a 90% usable depth of discharge, and the charge energy assumes a 90% round-trip efficiency, so you buy slightly more than you deliver. The wide-spread column uses $0.35 peak against $0.12 off-peak; the narrow-spread column uses $0.25 against $0.10.

Daily peak-window use (kWh)	Nameplate battery (kWh)	Daily saving at $0.35 / $0.12	Annual saving at $0.35 / $0.12	Annual saving at $0.25 / $0.10
4.0	4.4	$0.87	$316	$203
6.0	6.7	$1.30	$474	$304
8.0	8.9	$1.73	$633	$406
10.0	11.1	$2.17	$791	$507
12.0	13.3	$2.60	$949	$608
15.0	16.7	$3.25	$1,186	$760
20.0	22.2	$4.33	$1,582	$1,014

Savings scale straight in line with the peak-to-off-peak spread, which is why the narrow-spread column is roughly a third lower across every row. Round-trip losses mean you always buy more than you deliver: covering the 10 kWh row takes 11.1 kWh of off-peak charging at 90% efficiency, and at 80% efficiency it takes 12.5 kWh, which cuts that row's annual saving from $791 to $730. Depth of discharge sets the size rather than the saving - the same 10 kWh row needs only a 10.0 kWh nameplate if the battery is rated for a 100% usable depth of discharge instead of 90%. Every row assumes the battery is fully cycled all 365 days, which no household achieves perfectly, and none of these figures deduct the battery's purchase and installation cost or its gradual capacity fade. Arbitrage is also only one value stream - solar self-consumption and backup power are usually worth more. Illustrative only: run your own numbers against your actual tariff, usage pattern, and an installed quote before buying.

A home battery tariff savings calculator sizes a home storage battery for time-of-use arbitrage and estimates what that arbitrage is worth per day and per year. On a time-of-use electricity tariff, power costs far more during peak hours than off-peak — often three times as much. A home battery lets you buy energy cheaply overnight, store it, and discharge it during the expensive peak window, pocketing the difference. This tool works out how big a battery you need to cover your peak-window usage and how much the daily and annual savings come to, so you can judge the value before layering in solar or backup benefits.

Sizing starts from the amount of energy you actually draw during the peak-rate window each day — the load you want the battery to cover. That figure is the usable capacity the battery must deliver. Because batteries should not be fully drained every cycle, the calculator divides by a depth-of-discharge percentage to get the nameplate capacity you'd actually buy: covering 10 usable kWh at 90% depth of discharge means an 11.1 kWh battery. The savings side accounts for the fact that storage is lossy. To deliver a kilowatt-hour at peak you must have charged more than a kilowatt-hour off-peak, because some energy is lost in the round trip through the inverter and cells. Dividing the peak load by the round-trip efficiency gives the off-peak energy you must buy; multiplying by the off-peak rate gives the charging cost. The peak energy you avoid buying, valued at the peak rate, minus that off-peak charging cost, is your daily arbitrage saving — scaled up to an annual figure.

The honest framing, which the source research insists on, is that tariff arbitrage is only one of a battery's value streams and rarely the whole story. Round-trip losses and the gap between peak and off-peak rates cap the arbitrage saving, and a battery that only does arbitrage often has a long payback once its upfront price and gradual capacity degradation are included. The other value streams — maximizing self-consumption of your own solar instead of exporting it cheaply, and providing backup power during outages — frequently matter as much or more. Use this calculator to quantify the arbitrage piece precisely, then add solar self-consumption and backup value on top, and compare the total against the installed cost and warranty cycle life. It estimates arbitrage savings and sizing only, at the rates and efficiencies you enter; real-world results depend on your usage pattern, tariff, and how the battery is controlled.

Round-trip losses eat into the spread

A battery never returns all the energy you put in — 8-15% is lost in the inverter and cells each cycle. So to avoid buying 10 kWh at the peak rate, you might buy 11 kWh off-peak. The arbitrage saving is the peak energy avoided minus the (slightly larger) off-peak energy bought, which is why a big peak/off-peak price gap matters more than the raw battery size.

Arbitrage is only one value stream

Peak/off-peak arbitrage rarely justifies a battery on its own once you include upfront cost and degradation. The bigger wins are usually maximizing self-consumption of your own solar (instead of exporting it for a low feed-in rate) and backup power during outages. Size for arbitrage here, then add those benefits before judging whether a battery pays back.

Frequently asked questions

I use 10 kWh during peak hours, at $0.35 peak and $0.12 off-peak — what does a battery save?

You'd need about 10 kWh usable (an ~11.1 kWh battery at 90% depth of discharge). Charging 11.1 kWh off-peak (after 90% round-trip losses) costs ~$1.33; the 10 kWh of peak energy avoided is worth $3.50. That's about $2.17/day of arbitrage — roughly $790 a year before degradation and upfront cost.

What's the difference between usable and nameplate capacity?

Nameplate is the battery's rated size; usable is how much you can actually draw without over-discharging and shortening its life. Manufacturers specify a depth of discharge (often 90-100%). Size to your usable need, then divide by depth of discharge to find the nameplate capacity you have to buy.

Will tariff arbitrage alone pay for a battery?

Usually not by itself. The peak/off-peak spread and round-trip losses cap the annual saving, and battery cost plus degradation often make arbitrage-only payback long. Batteries pay back fastest when arbitrage is combined with high solar self-consumption and valued backup power — model all three, not just this one.

How is this different from the solar plus battery ROI calculator?

This isolates the time-of-use arbitrage value and the battery size needed for it. A solar-plus-battery ROI calculator combines solar generation, self-consumption, export, and battery costs into a full return figure. Use this to nail the arbitrage component, then feed it into the broader ROI picture.