Tool 08 · Storage

What size battery, honestly?

Backup essentials or shift evening load — size for the job you want, then the honest economics. Assumptions labeled; real quotes beat this page.

ModesBackup or shift

CalculationsRun in your browser

Hardware $In review

Battery vs no battery

Average monthly electricity bill ($)

Your rate ($/kWh) · enter a positive rate from your bill

Your export-credit situation

What the battery is for

Capacity and energy assumptions

Blank required fields do not imply zero. Prefilled values are examples, not a battery recommendation. Use measured daily loads and realistic solar surplus; bill-derived usage is only a reference.

Monthly fixed charges ($)

Installed usable capacity (kWh)

Capacity withheld from this scenario (%)

Round-trip efficiency (%) — shifting

Available surplus solar to charge (kWh/day) — shifting

Evening load to supply (kWh/day) — shifting

Export opportunity value ($/kWh) — non-retail shifting

Optional net installed battery cost ($) — shifting

Essential loads (kWh per 24 hours) — backup

Requested backup duration (hours)

Backup uses entered usable capacity after the withheld reserve, at a full starting charge, with no solar recharge assumed. It is an energy-duration estimate, not a guarantee of appliance starting power or circuit support. Shift mode is a repeated daily flat-rate scenario, not seasonal dispatch or time-of-use optimization.

Use this free solar battery size calculator to estimate practical home storage for outage backup or shifting solar energy into the evening. Enter your bill, electricity rate, export-credit situation and goal for an instant planning frame.

No email required. Instant browser-based planning math with clear assumptions.

When a battery earns, when it doesn't

Retail credit: on a flat tariff, storage losses can make shifting less valuable than exporting. This is not a claim about every utility tariff.
Weak export credit: potential savings still depend on charge availability, load, efficiency and installed cost. Bigger is not automatically better.
Time-of-use: this planner does not model hourly dispatch or seasonal rates. Use a tariff-specific study before investing.
Backup: verify appliance demand, starting power, circuits and outage duration with an installer. Energy capacity alone cannot guarantee whole-home backup.

The quick frame: battery vs no battery, in plain language.

Questions homeowners ask

Why does the planner ask about my export credit?

Solar used to charge a battery cannot also earn export credit. The shifting scenario subtracts that lost export value from avoided imports and includes round-trip losses. Real tariff settlement and hourly pricing need separate verification.

What battery size do most homes need?

There is no universal size. Enter your essential daily load and requested outage duration, then compare required energy with usable capacity after the reserve you withhold. Appliance starting power, circuits and recharging require a separate design.

Is a battery ever a good investment?

That depends on actual energy flows, tariff and cost. This planner shows a repeated-day flat-rate shifting scenario and optional simple payback, not a lifetime investment return. Backup value is considered separately; minimum bills and hourly tariffs are not modeled here.

Why does the planner ask for my battery cost?

No market price is assumed. Enter an installed net cost after only confirmed incentives to see simple payback. Leave it blank to see energy and operating-value estimates without a payback claim.

What this tool estimates

No email required. Results are calculated in your browser; use them as a planning estimate, not a guaranteed quote or savings forecast.

How the math works

Shifting output is the minimum of entered evening load, usable capacity after reserve, and available surplus solar times round-trip efficiency. Charging input is output divided by efficiency. Avoided import cost minus forgone export value gives daily benefit; the annual scenario repeats that day 365 times. Optional installed cost gives simple payback, not lifetime ROI. Backup uses explicit essential loads and requested hours, assumes a full initial charge and no recharge, and cannot guarantee starting power.

Key variables that change your result

Daily electricity use

A bill and rate create a planning estimate; actual appliance loads matter for an outage-backup design.

Export credit

Retail net metering, net billing and low export compensation can change the economics of storing solar energy.

Backup goal

Essentials-only backup usually needs much less storage than whole-home or long-duration outage coverage.

Questions homeowners ask

What size solar battery does my home need?

Enter essential loads and outage duration for backup, or surplus solar and evening load for shifting. Compare those requirements with entered usable capacity after reserve. There is no universal module count or electrical-design guarantee.

Is a solar battery worth it with net metering?

With full retail net metering, storage often adds resilience more than bill savings. Weak export credit or time-of-use price gaps can change that math.

Does this battery planner estimate backup time?

It gives a storage planning frame. Actual backup time depends on the appliances you run, battery usable capacity and outage conditions.

State context matters

Find your state solar guide

Costs, incentives, export rules and payback inputs vary by state. Open your state guide before treating any calculator result as a buying decision.

Browse all 50 state guides