The 30-second answer: Solar questions get simple the moment you stop asking “how many panels?” and start asking what must run, how much power does it need, and for how long? Power (kW) is a rate. Energy (kWh) is an amount. Every calculator below — from a single appliance to a demand charge on a factory bill — is one of those two numbers, or the two divided by each other. Nothing here is a quote, a guarantee, or a savings promise. It is arithmetic you can check.
I spend my working days around power plant equipment and the electrical maintenance that keeps it running, and the thing that strikes me about most residential solar conversations is that they start in the wrong place. They start with a product — a panel count, a battery brand, a monthly payment — and then work backwards to justify it. Utility engineers do it the other way around. They start with the load. What has to run, how hard, and for how many hours? Everything else is a consequence of that answer.
This page is built to be used the same way. It is the interactive companion to a field guide that follows electricity from one outlet to the whole grid, and its whole job is to let you put your own numbers in. Every default below is an example, not a forecast. Every formula is printed next to its result so you can check the arithmetic against your own bill, your own equipment sheet, or a pencil. All of the math runs in your browser. Nothing is stored and nothing is sent anywhere — no address, no utility bill, no email.
The one distinction that fixes most solar confusion
A watt is a rate. A watt-hour is an amount. That is the whole thing, and almost every muddled energy argument you have ever read comes from mixing the two.
A kilowatt is not a bucket — it tells you how fast the bucket is filling or draining. A kilowatt-hour tells you how much passed through. Asking “how many kilowatts are in the battery?” is like asking how many miles per hour fit in a gas tank. Once that clicks, a lot of marketing language stops working on you. An 8 kW array does not make 8 kW all day; 8 kW is its rated power under defined test conditions, and its real output moves through the day and the year. Energy production is the area under that moving curve.
The same split explains why two limits, not one, decide whether a backup system works. A 5 kW inverter cannot support an 8 kW load just because the battery behind it holds enough energy for several hours. Energy says how long. Power says whether it runs at all.
The energy ladder: watts to gigawatts, same physics, bigger fence
The scale of this stuff spans about nine orders of magnitude, and people treat each rung as if it were a different subject. It isn’t. A portable power station and a utility battery plant look nothing alike, but both have to generate or receive energy, move it, store it, control it, measure it honestly, and eventually give the materials back. The questions repeat. Only the fence gets bigger.
| Rung | Typical power | What lives there |
|---|---|---|
| Pocket & portable | 5–100 W | Phones, lights, radios, laptops, medical devices, small backup kits |
| Home | 1–20 kW | Refrigeration, HVAC, pumps, appliances, rooftop solar, backup circuits |
| Road | 1–20 kW | Home EV charging, scheduled charging, solar miles, bidirectional use |
| Industry | 100 kW–MW | Motors, process heat, demand peaks, fleets, onsite generation, microgrids |
| Grid | MW–GW | Solar farms, substations, transmission, storage plants, markets, balancing |
Illustrative ranges only — real loads and project sizes vary widely. The point is the ladder, not the brackets.
Here is my favorite demonstration that the rungs are the same subject. A 400 MWh grid battery rated at 100 MW has four hours of nameplate duration: energy divided by power. A 10 kWh home battery delivering 2.5 kW also has four hours. The numbers differ by a factor of forty million. The division does not change.
Calculator 1: what a load actually uses
Start here, always. Before array size, before battery brand, before financing: how much energy does the thing you care about consume? Read the nameplate, or better, measure it — a plug-in meter on a refrigerator for a week will teach you more than any calculator, because a compressor cycles and the label only tells you the peak.
Formula: kWh = watts × hours × days ÷ 1000. Defaults shown are examples — a 150 W example load running 8 hours a day for 30 days — and every one of them is editable. Watts are power; kWh are energy. Excluded: duty cycling, startup surge, and standby draw, so use an average draw rather than the biggest number on the label. This is a planning estimate, not a bill. All math runs in your browser; nothing is collected.
Calculator 2: how long a battery runs it
Once you know the load, runtime is division — but only if you start from usable energy rather than the biggest number on the brochure. Batteries hold back a reserve, the conversion path from DC cells to AC outlets loses a slice as heat, and you probably want a floor left in the pack so a long outage does not end with a dead battery and a dead phone.
The warning built into this one matters more than the number. Energy tells you how long. Power tells you whether it starts. A well-drawn spreadsheet showing nine hours of runtime is worthless if the load is bigger than the inverter can deliver — the system will simply refuse, or trip, and no amount of stored kWh fixes it. Check both.
Formula: hours = usable kWh × 1000 × efficiency × (1 − reserve) ÷ watts. Defaults are examples — a 10 kWh usable pack, a 500 W load, a 90% delivery path, a 20% reserve, and a 5 kW inverter — and all are editable. Usable kWh is not the brochure number; check the spec sheet. Excluded: temperature effects, surge, load changes, and age, all of which shorten real runtime. Energy sets how long; the inverter’s power rating sets whether it runs at all. Planning estimate only. Runs entirely in your browser.
Calculator 3: what an array roughly produces
This is the calculator people want first and should reach third. It turns a rated array size into a planning energy figure using peak-sun-hours and a loss factor — the same shape as the first-pass math any installer does before the real modeling starts.
Treat the loss factor honestly. The 0.80 default is a rough allowance for temperature, wiring, inverter conversion, mismatch, and dirt. It is a convention, not a constant, and it does not know about your roof. Weather, season, azimuth, tilt, shading, and snow all move real production, and a sunny annual average does not guarantee enough energy during a dark winter storm. When you want a number worth putting in a contract discussion, run NREL’s PVWatts Calculator against your actual address, then compare its output to whatever an installer’s production guarantee assumes. If the two disagree, ask why before you sign.
A useful trick: run this one backwards. Take your annual kWh off your utility bill, divide by 12, and adjust the array size until the daily figure matches. That tells you the array that would offset your usage on an average day — a far better starting point than a panel count someone quoted you.
Formula: kWh = array kW × sun hours × loss factor × days. Defaults are examples — an 8 kW array, 5 peak-sun-hours, a 0.80 loss factor, 30 days — and all are editable. The 0.80 is a rough planning allowance for temperature, wiring, inverter conversion, mismatch, and dirt; it is not a universal constant. Excluded: weather, season, roof direction, tilt, shading, and snow — which is why this is a first pass, not a design. For an address-specific model, use NREL’s PVWatts Calculator. All math is client-side; nothing is collected.
Calculator 4: turning surplus solar into miles
This is the fun one, and the one where boundaries get sloppy fastest. Ten kilowatt-hours leaving your array is not ten kilowatt-hours in the car. Conversion and charging losses take a cut, so the calculator asks for charging efficiency separately rather than hiding it. It also asks for your vehicle’s consumption in kWh per mile, because the EPA figure and your actual figure are rarely the same number in February.
The efficiency gap between drivetrains is the reason this math works at all. The U.S. Department of Energy puts a typical EV at roughly 87–91% efficient once regenerative braking is counted, against about 30% for a conventional gasoline vehicle depending on the drive cycle. Most of the fuel energy in a gasoline car leaves as engine and exhaust heat instead of motion.
Boundary check, because this figure gets abused in both directions: those percentages compare energy stored onboard with motion at the wheels. They are not a lifecycle score. Well-to-wheels analysis — the framework Argonne National Laboratory formalized in GREET — adds upstream fuel and electricity production. Cradle-to-grave adds vehicle and battery manufacturing, maintenance, and end of life. The EPA notes in its Electric Vehicle Myths page that battery-electric vehicles have no tailpipe emissions while their upstream emissions depend on the electricity used for charging — and that the grid mix can change over the vehicle’s life. A fair comparison does not begin at the charger for one car and the tailpipe for the other.
Formula: miles = solar kWh × charging efficiency ÷ vehicle kWh per mile. Defaults are examples — 30 kWh of surplus, a 90% charging path, and 0.30 kWh/mile — and all are editable. Use your car’s own recent consumption; our home charging cost guide explains where to find it and why the wall-side number is the one that matters. Excluded: speed, temperature, terrain, and cabin heat. Planning estimate, not a range guarantee. Client-side only.
Calculator 5: what shifting a kilowatt-hour is worth
A solar kWh does not have one value. It has several, depending on when it shows up, where it goes, and what the tariff says. Saving energy and selling energy are not the same transaction, and the meter follows rules rather than enthusiasm.
| Mechanism | What it actually pays |
|---|---|
| Self-consumption | Solar replaces electricity you would have bought. Gross value is usually the retail rate avoided. |
| Net metering | Exports earn bill credits under local rules. The credit may approximate retail value; eligibility and accounting vary. |
| Net billing | Imported and exported kWh are valued separately. The export rate is often lower than the retail purchase rate. |
| Time-of-use shifting | A battery or scheduled load moves consumption off expensive hours. Losses and degradation still count. |
| Demand response | A utility or aggregator may pay for verified load reduction during events, subject to program rules and baselines. |
| Virtual power plant | Many batteries, thermostats, or chargers coordinate as one resource. Understand control rights, reserve settings, and compensation. |
The calculator below handles the simplest and most common of those: moving energy from a cheap hour to an expensive one. Note the division in the formula — it is the honest part. To deliver eight kilowatt-hours during the peak window you have to buy more than eight off-peak, because the round trip loses some as heat. Skipping that division is how a “savings” number gets inflated.
Formula: daily value = shifted kWh × peak rate − (shifted kWh ÷ efficiency) × opportunity cost. The division is the point: to deliver 8 kWh at peak you must buy more than 8 kWh off-peak, because the round trip loses some as heat. Defaults are examples and all are editable. Excluded: degradation, program fees, equipment cost, reserve requirements, and any rule limiting when stored energy can be used or exported. This is gross value, not profit — and not a savings promise. Nothing is collected.
What comes out is gross value, not profit. Degradation, program fees, the equipment itself, reserve requirements, and export restrictions all come out of it afterward. On the physics side, the U.S. Energy Information Administration reported that the U.S. utility-scale battery fleet averaged 82% monthly round-trip efficiency in 2019 (pumped storage averaged 79%), drawn from its Power Plant Operations Report — a historical fleet average, not a promise for your equipment. Home systems commonly assume something in the high 80s or low 90s. Use your own equipment’s spec.
If your rate structure is where the money is, the mechanics of scheduling loads overnight are the same ones covered in our home charging cost guide — and the same logic applies whether the flexible load is a car or a water heater.
Calculator 6: the demand charge nobody explains
Residential bills mostly charge for energy. Commercial and industrial bills also charge for power — a demand charge based on the highest short-interval peak you hit during the billing period. It is a genuinely different line item, and it is where storage often earns its keep at the industrial rung of the ladder.
Two numbers come out of this one on purpose. The kW figure is what the tariff bills. The kWh figure is the minimum energy your equipment must actually hold to sustain that reduction for the full event. Both have to pass. A battery sized to the kW and starved on the kWh runs out mid-peak and buys you nothing.
Formulas: gross = kW × $/kW × months and minimum kWh = kW × hours. Two separate numbers on purpose: the kW is what the tariff bills, the kWh is what the battery must actually hold to hit it. Defaults are examples and all are editable. Excluded: measurement intervals, ratchets, coincident-peak rules, seasonal rates, and dispatch accuracy — a battery that fires five minutes early can miss the peak that sets the bill. Gross reduction, not profit, and never a quote. Client-side only.
And the word “gross” is doing real work. The tariff may use a short measurement interval, a seasonal peak, a ratchet based on prior months, coincident system peaks, or separate charges by voltage level. A battery that fires five minutes too early can miss the peak that sets the bill entirely. This is a screening number to decide whether a real study is worth commissioning — nothing more.
Industrial and grid scale: megawatts handle the peak, megawatt-hours handle the clock
Large projects are not just bigger home systems. They live inside protection schemes, interconnection agreements, operating procedures, market rules, fire plans, warranties, and dispatch software. But the jobs are recognizable from everything above: peak shaving discharges storage during the facility peak that sets the demand charge. Solar shifting charges storage midday so energy can serve evening load. Energy arbitrage buys low and sells high — after losses, degradation, fees, and dispatch limits. Fast grid services use a battery’s quick response for frequency regulation and reserves, though finite energy still caps duration. Microgrid controls coordinate generation, storage, loads, and often a generator so critical operations can island safely. And Megapack-class containerized systems bundle cells, inverters, controls, cooling, and protection into a plant.
Aggregation is what connects the rungs. FERC’s Order No. 2222 opened the door for distributed resources — home batteries, thermostats, chargers — to participate in wholesale markets through aggregators. That is why the demand-charge calculator and the TOU calculator are describing the same physics from opposite ends of a nine-order-of-magnitude ladder.
The battery loop: mine it once, keep it working, recover it responsibly
The circular goal is simple to say and hard to operate: keep already-mined minerals productive through repair, first life, second life where appropriate, and safe recycling into material clean enough to build with again. The chain runs extract, manufacture, first life, evaluate, recycle, build again — and each arrow is a business that has to actually pencil out.
Second life is not delayed disposal. It is only valuable when a battery has a verified, safe job. The EPA describes pyrometallurgical, hydrometallurgical, and emerging direct-recycling pathways; chemistry and pack design change which route fits. The timing problem is the interesting part: recycling supply arrives late by definition. A battery built today may work in a vehicle for years and serve somewhere else afterward before any recycler sees it. The IEA reported that global energy-sector battery demand reached 1 TWh in 2024 and projected EV battery demand above 3 TWh in 2030 under its Stated Policies Scenario. Those are scenarios, not guarantees.
Numbers worth keeping in your head
| Number | What it means |
|---|---|
| 8 min 20 s | Roughly how long sunlight takes to reach Earth, per NASA. |
| ~95% | Silicon’s share of solar modules sold today, per DOE. |
| 25+ years | DOE says silicon cells can last 25 years or more and still produce over 80% of original power. |
| 4 hours | Duration of a 400 MWh battery rated at 100 MW — energy divided by power. |
| 1 TWh | Global energy-sector battery demand in 2024, per the IEA. |
| 1 kW ≠ 1 kWh | A kilowatt is a rate. A kilowatt-hour is what one kilowatt delivers in one hour. |
Gear that makes these numbers real
None of the calculators above need a purchase to be useful. But the fastest way to stop guessing at inputs is to measure them, and the tools that do it are cheap. A plug-in energy meter tells you what a load actually draws over a week rather than what its label claims at peak — that single measurement fixes the input to calculator one. A whole-home energy monitor does the same job at the panel, which is where you find the peaks that drive demand-style pricing and the baseline your array is actually competing with; it is also the tool that answers the panel questions in our electrical panel capacity guide. And a portable power station is the cheapest possible version of the battery-runtime lesson: usable kWh, an inverter rating, and a reserve, all printed on one box.
Shop plug-in electricity usage meters → Shop whole-home energy monitors → Shop portable power stations →
These are category search links, not specific product endorsements — check the current specs, capacity, and inverter rating yourself before buying, and run the numbers above against the spec sheet.
A companion book is coming
This page is the fast, interactive layer of a longer field guide I have been writing: Solar Power in the Real World. It starts with watts and watt-hours and walks the same ladder in full — home backup, EV charging, homeowner economics, industrial demand, microgrids, solar farms, grid-scale storage, energy arbitrage, grid services, and the battery’s next life — with the calculators above worked out longhand in an appendix.
It is not published yet. There is nothing to buy, pre-order, or sign up for, and there is deliberately no link here. When it is actually available, this section will say so and point at it. Until then, the calculators are the useful part, and they are free.
FAQ: solar, storage, and the numbers behind them
Will rooftop solar power my home during an outage?
Usually not on its own. Ordinary grid-tied solar shuts down during an outage so it cannot backfeed lines that utility crews are working on. Backup power requires equipment designed to form and protect a local island - typically a compatible inverter, transfer equipment, and a battery. If outage performance is the reason you want solar, say so before anyone quotes you a system, because it changes the equipment list and the price.
How many solar panels does a home need?
It depends on annual and seasonal energy use, location, shading, roof geometry, panel rating, and system losses. Start from your own kWh, not a panel count: the rough production calculator on this page turns an array size into a planning number, and you can run it backwards until the energy matches your bill. That is a first pass only. Use NREL PVWatts for an address-specific model and a qualified installer for the final design.
Can a homeowner sell solar power back to the grid?
Sometimes. Net metering, net billing, export tariffs, demand-response programs, and virtual power plants vary by state, utility, equipment, and interconnection agreement. Some pay bill credits near the retail rate; others value exports well below what you pay to buy the same kWh. Check interconnection permission, export rate, system-size limits, battery grid-charging and export restrictions, and program exit terms before assuming any number.
How long will a home battery run essential loads?
Approximately: usable battery energy, times the efficiency of the delivery path, times one minus your reserve setting, divided by the average load. The battery runtime calculator on this page does exactly that. Real runtime is usually shorter, because loads change, weather makes heating and cooling work harder, temperature affects performance, and batteries age. Energy is only one of two limits - the inverter’s power rating is the other.
What is the difference between a kilowatt and a kilowatt-hour?
A kilowatt is a rate; a kilowatt-hour is an amount. A kilowatt tells you how fast energy is moving, and a kilowatt-hour tells you how much moved. Asking how many kilowatts are in a battery is like asking how many miles per hour fit in a gas tank. Nearly every confused solar conversation traces back to this one distinction.
How efficient is an EV compared with a gasoline vehicle?
The U.S. Department of Energy reports roughly 87 to 91 percent for a typical EV once regenerative braking is counted, versus about 30 percent for a conventional gasoline vehicle, depending on the drive cycle. Those are onboard figures - energy stored in the vehicle versus motion at the wheels. A complete comparison also follows upstream fuel and electricity production and vehicle manufacturing.
What does 100 MW / 400 MWh mean?
The project can deliver up to 100 megawatts of power and holds 400 megawatt-hours of nameplate energy. Divide energy by power and you get four hours of nameplate duration. The same division works on your wall: a 10 kWh home battery delivering 2.5 kW also has four hours. The numbers change by a factor of forty million; the relationship does not.
What happens to lithium-ion batteries at end of life?
They can be evaluated for repair, reuse, repurposing, or recycling. The EPA describes recycling pathways that include collection, discharge, disassembly or shredding, production of black mass, and recovery through pyrometallurgical, hydrometallurgical, or emerging direct-recycling processes. Chemistry and pack design change which route makes sense.
If you want to keep pulling this thread: the home charging cost guide runs the money math on the flexible load most people already own, the panel capacity guide covers what your service can actually support before any of this gets installed, and the home charger guide ranks the adjustable-output hardware that lets you match a charger to whatever headroom you have.
Sources, checked July 15, 2026: U.S. DOE — Homeowner’s Guide to Solar; NREL PVWatts Calculator; DOE — Solar Photovoltaic Cell Basics; DOE — EV vs. gasoline efficiency; EPA — Electric Vehicle Myths; Argonne National Laboratory — GREET well-to-wheels; EPA — Lithium-Ion Battery Recycling; EIA — Energy Storage for Electricity Generation; FERC — Order No. 2222 Fact Sheet; IEA — Global EV Outlook 2025: EV Batteries; NASA — Quick Facts. Utility tariffs, incentive programs, product specifications, codes, and market rules all change — verify current local information before acting. Every calculator here produces a planning estimate for education, not an engineering design, a utility quote, a savings guarantee, or financial, tax, legal, or code advice. Consult a licensed electrician and your utility before installing generation or storage.