Solar Battery Calculator

How the calculator computes the battery figures

The battery math is simple enough to check by hand. The calculator sizes your solar system from your bill, applies the 30% federal credit to the solar cost, and divides the net cost by yearly savings for the solar-only payback. It then adds a fixed battery price and divides again:

payback with battery = (net solar cost + battery cost) ÷ yearly savings

Two price points are built in: $9,000 for a small backup battery and $18,000 for whole-home. Yearly savings stay the same either way, because the model assumes the battery does not change how much of your bill the solar system offsets. That is exactly right under retail net metering and conservative everywhere else.

Worked example: a $150 monthly bill at $0.17/kWh is about 10,600 kWh per year. With 4.5 sun hours and the calculator’s 78% performance ratio, that needs an 8.3 kW system. At $3.00 per watt the gross cost is about $24,800, the credit removes $7,440, and the net cost is roughly $17,360. Yearly savings are $1,800, so the solar-only payback is 9.6 years. The $9,000 battery raises it to 14.6 years; the $18,000 option pushes it to 19.6. Those extra five to ten years are the price of backup power. Run the same inputs in the solar payback calculator to confirm the baseline.

Why the tax credit is applied to solar only

The federal Residential Clean Energy Credit does cover home batteries of 3 kWh or more, with or without solar. This calculator still applies the 30% credit to the solar system only and adds the battery at full price, which keeps the battery figures conservative. If you expect to claim the credit on the battery too, reduce the battery cost by 30% ($9,000 becomes $6,300; $18,000 becomes $12,600) or check the federal tax credit calculator. Every constant behind this site’s calculators is listed on the methodology page.

Small backup vs. whole-home battery

Two ratings matter: kilowatt-hours (kWh) of stored energy and kilowatts (kW) of continuous output. The calculator’s options map to these typical configurations:

• Small backup (~$9,000): one unit of 10–13.5 kWh, such as a Tesla Powerwall 3 (13.5 kWh), a FranklinWH aPower 2 (15 kWh), or two stacked Enphase IQ 5P modules (10 kWh), with 5–11.5 kW of continuous output. It feeds a critical-loads panel: fridge, lights, internet, furnace fan, a few outlets.
• Whole-home (~$18,000 and up): two or three stacked units for 27–40 kWh and 15–30 kW, backing up the main panel so central air conditioning, an electric range, or a heat pump keeps running. $18,000 is a realistic floor for two units; three units or a main-panel upgrade can push the total past $25,000.

Sizing backup capacity from your essential loads

List what must keep running during an outage and how many kilowatt-hours each load uses per day. A typical essentials list:

• Refrigerator and freezer: about 1.5 kWh per day
• LED lighting in a few rooms: about 0.5 kWh per day
• Modem, router, and phone charging: about 0.3 kWh per day
• Gas furnace blower fan in winter: about 3 kWh per day
• TV, laptop, and a few small appliances: about 1 kWh per day

That adds up to roughly 6–7 kWh per day. A 13.5 kWh battery covers two days of it with no sun, and on a clear day the solar system recharges it by early afternoon, so essentials can run through a multi-day outage. A well pump or sump pump can draw 1–2 kW while running, so check the battery’s power rating as well as its capacity.

Quick reference: battery capacity vs. hours of backup

Assumes 90% usable capacity and no solar recharge. Real run time depends on weather and appliance duty cycles.

When a battery improves the financial return

The calculator treats the battery as pure cost, but in several rate environments it earns money this model does not count:

• Time-of-use arbitrage: charge from solar at midday and discharge during the 4–9 p.m. peak. With a 20–30 cent gap between off-peak and peak rates and a 10 kWh daily cycle, that is worth $700–$1,000 per year.
• Net billing states: under California’s NEM 3.0, exported solar earns an avoided-cost rate of often 5–8 cents per kWh while evening imports cost 40 cents or more. Storing the surplus is why most new California systems include a battery, and solar-plus-storage there often pays back faster than solar alone.
• Demand charges: some residential tariffs bill on your highest 15-minute draw of the month. A battery that caps that peak can cut a $50–$100 monthly charge.
• Utility battery programs and virtual power plants: ConnectedSolutions in Massachusetts, Connecticut, and Rhode Island pays roughly $200–$275 per kW of average summer discharge each year; Green Mountain Power in Vermont leases Powerwalls cheaply in exchange for grid access; similar programs exist in California, Texas, Utah, and Hawaii. Enrollment can return $500–$1,500 per year for one battery.

Where a battery does not help: full retail net metering with no time-of-use component. If every exported kWh is credited at the price you pay to import one, the grid already stores your surplus for free and the battery only adds cost and about 10% round-trip losses. The decision then comes down to how much you value not losing power.

Battery degradation and warranty basics

Nearly all current home batteries use lithium iron phosphate (LFP) cells, which tolerate daily cycling well, but capacity still fades with use and time. Most warranties run 10 years and guarantee at least 70% of original capacity at the end of the term, with one of two usage limits attached:

• Throughput warranties cap total energy delivered, for example 37 MWh for a 13.5 kWh unit, about 2,700 full cycles.
• Cycle warranties cap full charge-discharge cycles, commonly 4,000–6,000. One cycle a day reaches 4,000 in about 11 years.

For planning, assume 70–80% of original capacity at year 10 and a replacement somewhere between years 12 and 15, well inside the 25-year life of the panels. Budgeting that second battery is the honest way to compare a solar-plus-storage quote against solar alone.