Battery Storage: Do You Actually Need One?

The battery is the most emotionally appealing and financially ambiguous part of a modern solar quote. It promises independence: your own power, stored from your own roof, keeping your home alive when the grid fails. It also typically adds $8,000–$15,000 to the project — roughly the price of the panels themselves — for a box that stores less energy than most people guess. This guide separates the three genuinely different reasons to buy one, because a battery can be a bargain for one of them and a poor buy for the other two.

First, the physics nobody puts in the brochure

A typical home battery (the Tesla Powerwall class) stores about 13.5 kWh of usable energy. An average US home uses ≈30 kWh a day. So one battery holds well under half a day of normal consumption. In an outage that doesn’t mean 11 hours of normal life — it means a disciplined essentials mode: refrigerator, lights, Wi-Fi, phone charging, a fan, maybe a small medical device, which can stretch 13.5 kWh across a day or more. What it will not do, alone, is run central air conditioning (3–5 kW sustained), an electric oven, a clothes dryer or an EV charger for long. Whole-home backup through multi-day outages is a two-to-three-battery conversation, or a battery-plus-generator one.

Batteries also lose energy in translation: expect ≈10% round-trip losses, and capacity warranted typically to 70% after 10 years. None of this makes batteries bad — it makes them a specific tool. Buy the tool for the job it actually does.

Reason one: backup power (the resilience case)

If outages are rare and short where you live, a battery bought purely for backup is an expensive insurance policy — compare a portable generator at a tenth of the price. If outages are frequent, long, or dangerous for someone in the home (medical equipment, extreme heat, wildfire shutoffs in the West, hurricane country, ice storms), the calculus flips: the battery is silent, instant, fuel-free and recharges from your roof every sunny day of the outage. In this case, size for your critical loads panel, not the whole house, and be honest with yourself about air conditioning: in Phoenix or Houston summers, “essentials” arguably includes at least one cooled room, which usually means two batteries or a hybrid generator setup.

Reason two: beating net billing (the arbitrage case)

Where exports are paid far below retail — California’s Net Billing Tariff, Arizona’s export rates, Hawaii — a battery performs a daily trade: it stores midday surplus that would be sold for 3–8¢ and discharges it in the evening, displacing purchases at 25–45¢. Run the spread honestly: a 13.5 kWh battery cycling ≈12 kWh a day across a 25¢ spread earns ≈$3 a day, ≈$1,000–$1,100 a year, before degradation and losses. Against a $10,000 installed cost that’s roughly a 10-year standalone payback — plausible, especially paired with time-of-use optimisation, but not the 4-year miracle in some brochures, and it assumes the spread survives. In states still paying retail for exports, the same battery earns almost nothing financially: the grid already is your free battery, which is why our net metering guide is required reading before this purchase.

Reason three: time-of-use rates (the clock case)

On time-of-use plans, evening peak prices can run double or triple the midday rate — with or without solar. A battery charged from your panels (or even cheap overnight grid power, where rules allow) and discharged across the 4–9 p.m. peak shaves the most expensive hours of your bill. This case strengthens every year as more utilities push households onto time-of-use defaults, and it’s the main reason west-facing panels plus a modest battery often out-earn a bigger south-facing array in California and Arizona.

Sizing: the maths quotes skip

• For backup: list critical loads in watts, multiply by the hours you want them, divide by 1,000 = kWh needed. Fridge (150 W avg) + lights (200 W) + Wi-Fi/phones (50 W) + fan (75 W) for 12 hours ≈ 5.7 kWh — one 13.5 kWh battery covers it with margin, and your panels refill it daily.
• For arbitrage: match the battery to your typical exported surplus, not your total production. If you export 8 kWh on a typical day, a 20 kWh battery is mostly empty capital.
• For self-supply under net billing: work out your evening/night consumption (roughly 30–40% of daily use for most homes). One battery covers the typical 8–12 kWh evening block for a mid-size home.
• Mind the power rating, not just capacity. A battery’s continuous output (≈5–11.5 kW depending on model) decides how many appliances can run at once — kWh is the tank, kW is the tap.

The verdict, by situation

• Net billing / low export rates + high evening prices: usually yes — often the difference between a mediocre and a good solar investment. Model it in the Savings Projector by raising self-consumption toward 85–90%.
• Full retail net metering + reliable grid: financially no. Buy it only if backup itself is worth the price to you — a legitimate purchase, made with open eyes.
• Frequent or risky outages: yes, sized to critical loads, whatever the tariff says.
• Undecided: choose a hybrid (battery-ready) inverter now and add storage later. Battery prices keep drifting down and your tariff may change; the option is cheap, the commitment can wait.

A battery is neither the essential partner of solar nor a scam bolt-on. It is a priced tool with three distinct jobs. Decide which job you are hiring it for, size it from your own loads and exports, and it can be one of the best parts of the system. Buy it from a slogan, and it’s the most expensive ornament on your garage wall.