Energy
Assess whether backup needs, solar, outages, and electricity pricing support adding a battery.
Enter your numbers to see the answer.
Should I buy a home battery? guide
A home-battery decision depends on what you want the battery to do: keep essential loads running during outages, shift energy from one time of day to another, use existing solar, or reduce exposure to changing electricity prices. This calculator combines outage duration, essential load, solar availability, and outage frequency into a transparent fit assessment. It multiplies the outage hours by the essential load to show the energy a battery would have to deliver, then returns “worth investigating” when outages happen at least twice a month, when you already have solar, or when a single outage needs 5 kWh or more; otherwise it suggests comparing alternatives first. It is a screening tool, not a quote or a guarantee of savings. Battery chemistry, usable capacity, inverter limits, round-trip efficiency, tariffs, incentives, installation cost, warranty, and local electrical rules still need to be checked.
Backup energy (kWh) = outage duration (h) × essential load (kW). Verdict = “worth investigating” if outages per month ≥ 2, or solar is available, or backup energy ≥ 5 kWh; otherwise “compare alternatives first”.
Worked example with the defaults: a 6-hour outage with a 1 kW essential load requires 6 × 1 = 6 kWh of delivered energy before reserve and efficiency losses, which the calculator lists as the energy for the stated outage. The default 2 outages per month meets the frequency threshold, solar is marked as available, and 6 kWh is above the 5 kWh energy threshold, so all three tests pass and the calculator displays “worth investigating”. Reducing the scenario to 1 outage per month, no solar, and a 3-hour outage would give 3 × 1 = 3 kWh and the verdict “compare alternatives first”.
Use hours for outage duration and kW for essential load; a refrigerator, router, some lights, and a phone charger together draw roughly 0.3–0.5 kW, while adding a well pump or space heater pushes the figure past 1 kW. Enter 1 for solar available and 0 for none. Jurisdiction, effective date, source, update owner, and review date are included so tariff or market assumptions can be traced rather than treated as universal facts.
A “worth investigating” verdict means the stated backup and energy scenario may justify further investigation; it does not prove a positive financial return. Compare a battery-size estimate, backup-runtime estimate, tariff scenario, installation quote, and warranty before deciding.
Do not size a battery from total household load when only essential circuits need backup; a whole-house average of 1.2 kW is very different from the 0.4 kW a fridge and router actually need. Do not count solar as available during every outage, because most grid-tied inverters shut down without a battery and solar produces nothing overnight. Do not compare a battery’s nameplate capacity with the delivered energy shown here without accounting for reserve and round-trip losses, and remember that the verdict reflects your inputs, so an optimistic outage count produces an optimistic answer.
Verify tariffs, incentives, equipment specifications, transfer equipment, installation requirements, and electrical decisions with the utility, manufacturer, and qualified installer.
Sources
How it works
Assess whether backup needs, solar, outages, and electricity pricing support adding a battery. This tool explains the calculation so you can adjust the assumptions to match your situation.
Worked example
With Typical outage duration = 6 hours · Essential load = 1 kW · Solar already available = 1 · Outages per month = 2 → worth investigating (based on backup needs and energy context). Change an input above and this example updates with your numbers.
Common questions
Start by estimating backup needs, outage frequency, tariff savings, installed cost, and the value of resilience. This page screens the scenario using outage hours, essential load, outage frequency, and solar availability; it is not a financial guarantee, so follow it with a payback calculation and real quotes.
Multiply essential load in kW by expected outage hours to estimate delivered kWh, then allow for reserve, inverter limits, and efficiency losses. The default 1 kW for 6 hours needs 6 kWh delivered, which typically means a 7–8 kWh usable battery.
Only when the system has appropriate backup hardware, supported circuits, and an installation configured for islanded operation. Many batteries installed purely for time-of-use savings do not provide backup, so confirm the equipment design before buying.
It can improve backup and energy-shifting options, which is why this screen treats existing solar as a point in favor, but value depends on outage behavior, solar production, export credits, tariffs, and installation cost. Low export credits strengthen the case; generous net metering weakens it.
No. It is a fit assessment that returns one of two verdicts. Use the battery ROI or payback calculator with current quotes and tariff assumptions for a financial scenario that includes degradation, financing, and replacement.