Guide · 8 min read · free tool
Solar, Battery, EV: Three Bills, One Roof, and a Sketch You Should Not Finance On
Aug 31, 2026 · Zelpura Editorial · All guides
A roof, a battery, and a car can be one project or three expensive hobbies. The interaction is real: midday solar can charge a car, a battery can move solar into the evening, and a rate plan can punish you for ignoring both. This page stacks a simplified version of those effects so you can see whether the bundle is in the right neighborhood. It is not a production model. Shading, a utility's export rules, and a battery warranty will move the result more than a careful average. Use the sketch to decide which quotes to request. Use the quotes to decide what to buy.
The inputs are the job, not a formality. The fields are solar kilowatts, battery kilowatt-hours, EV miles per year, and one grid rate in dollars per kilowatt-hour. There is no separate export-rate box. The sketch assumes about 4 full sun hours a day, values about 70 percent of solar production at the rate you typed, compares the car to gas at 13 cents a mile and 3.5 miles per kilowatt-hour, and gives the battery a simplified annual shift. System cost is baked in at about 2,500 dollars per kilowatt of solar and 900 dollars per kilowatt-hour of battery. Those are not your quote. If your utility pays a stingy export rate, this single-rate sketch will look kinder than your bill. Type this year's rate, then distrust the payback until a quote replaces the baked-in prices.
A house that already penciled solar on its own, plus an EV that charges at night on a cheap rate, may not need a battery to make the car cheap. A house on a time-of-use plan with a painful evening peak may want the battery even if the car is happy. Run the pieces. If the bundle's extra savings over solar-alone are smaller than the battery's price, the battery is a backup purchase, not a savings purchase. Backup is a valid reason. It is a different reason, and it should be said out loud so you do not expect the battery to pay for itself on a spreadsheet that was really about outages. Outages are an insurance question. Insurance questions disguised as payback questions produce angry owners.
The miss is adding solar savings, battery savings, and EV savings from three different articles that assumed three different rates. The second miss is ignoring the charger circuit, the panel upgrade, and the roof age. A beautiful kilowatt-hour model on a roof that needs replacement is a model of the wrong year. The third miss is a federal or local incentive typed as guaranteed. Incentives have deadlines, income tests, and equipment lists. If you are not sure, type zero and let the quote do the optimism. Zero is a conservative sketch. It is also a useful mood stabilizer in a showroom.
Gather this year's bill and the tariff PDF. Type the grid rate you can point to, then write down that the payback used 2,500 dollars per kilowatt and 900 dollars per kilowatt-hour, not your quote. Get two quotes that include the electrical work, not only the panels. Ask each installer what export rate they assumed. If it does not match your tariff PDF, you have learned something important about the quote. Compare warranties on the battery cycle count, because a savings model that assumes a battery lasts forever is a mood. Then decide whether the project is savings, backup, or a car you wanted anyway. Those can all be yes. They should not share one blurred number.
This is an educational sketch, not an engineering design, not a utility interconnection approval, and not tax advice. Incentives and net-metering rules change and are jurisdiction-specific. A calculator does not know your roof, your service panel, your shading, or whether your utility allows the setup. Do not buy equipment from a result. Do not size a breaker from a result. Talk to a licensed electrician and a solar installer who will put assumptions in writing. If the payback depends on an incentive, confirm the incentive with the agency that pays it, not with the person selling the system.
Save the tariff PDF next to the sketch. When the utility changes the export rate, rerun the page before you recommend the same bundle to a neighbor. Yesterday's payback is not a favor you pass along. It may be a stale assumption. Also save the quote's assumed production. At the one-year mark, compare it to actual production. That error is the best input you will ever have, and it arrived too late for the purchase. It is not too late for the next decision, or for honesty about this one.
Read the result back before you act. Say whether you are buying savings, backup, or a car. If the answer is 'all of it, the calculator said,' separate the reasons before you sign. A signature should have a primary reason you still believe on a cloudy Tuesday.
Charging the car at noon from solar is a behavior, not a hardware guarantee. If nobody is home at noon and the car is at work, the midday kilowatt-hours go to the grid at the export rate, not into the commute. Model the schedule you have. A bundle designed for a retired household will disappoint a commuting one. The hardware is the same. The load shape is the product. Know your load shape, which is a fancy way of saying know when you are actually home.
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