Most people get this wrong on the first try. They buy too few solar panels and run out of power, or way too many and waste thousands.
To calculate your solar energy needs, you only need four steps: add up how much electricity you use in a day, add a little extra for losses, divide by your sun hours to get the panels, and then size the storage. Not hard, and we'll do a full example.
What A kWh Is
A kilowatt-hour is what you'd use running a 100-watt bulb for 10 hours. (100W × 10h = 1,000Wh = 1 kWh.) Power × time = energy. The rest is bookkeeping.
Step 1: Add Up Your Daily Energy Use
Got a power bill? It already did most of this step for you, and it's the most accurate starting point. It shows your monthly consumption in kWh, so divide that by 30 for a daily figure.
For a sense of scale, the average US home bought about 899 kWh a month in 2022, according to the US Energy Information Administration. That's around 30 kWh a day, or 10,791 kWh in a whole year.
No bill, or building something new? Then list every device:
| Appliance | Power (W) | Hours/day | Daily energy (Wh) |
|---|---|---|---|
| LED light | 10W | 5h | 50Wh |
| Refrigerator | 150W | 24h | 3,600Wh |
| Laptop | 50W | 4h | 200Wh |
| Water pump | 500W | 1h | 500Wh |
| Total | 4,350Wh (4.35 kWh) |
Include chargers, the modem, and lights left on by accident.
We counted the fridge for all 24 hours, which is the safe worst case. Its motor switches on and off, so a real fridge often runs closer to 6 to 10 hours at the watts on its label. Our energy use calculator does this sum for you, per day, month and year.
Step 2: Add A Buffer
Real batteries and inverters aren't 100% efficient. Some energy turns into heat along the way, like a water hose with a few small leaks.
How much leaks out? A good inverter like the Victron MultiPlus-II reaches 93 to 96% efficiency at its best, and the European Commission's PVGIS solar tool assumes 14% overall losses on the panel side by default. So plan on about 20% additional energy to stay safe:
4.35 kWh × 1.2 = 5.22 kWh per day
That 5.22 is the daily production your solar panels are required to deliver.
Step 3: Turn kWh Into Solar Panels
To determine your panel size, you need one more figure. Your location gets a certain amount of "peak sun hours" per day. Not the hours the sun is up, but the day's sunlight squeezed into hours of full-strength noon sun (1,000 watts on every square meter).
It's very different depending on where you live. On average, Chicago gets about 4 peak sun hours a day, while Phoenix gets about 7.5.
In our example the house gets 5 sun hours:
5.22 kWh ÷ 5 hours = 1.04 kW of panels needed
Divide by your panel size. If you're using 300W panels:
1,040W ÷ 300W = 3.5 panels → round up to 4 panels
Always round up. Half a panel doesn't exist, and a little extra solar power covers the grey days. Want a more accurate estimate for your own roof? The solar panel calculator adds your region and which way your roof faces, two factors that change the result a lot.
Step 4: Size The Battery
Panels make electricity during the day. Your house uses it day and night, and the battery covers the gap.
For one day of backup at 5.22 kWh on a 12V system: 435 Ah. On a 24V system: 217 Ah. Higher voltage means thinner wires and cheaper installation, so many setups use 24V or 48V.
There's one catch, however. You shouldn't empty the bank all the way. If you only plan to use 80% of a lithium (LiFePO4) pack, divide by 0.8, and the 12V bank grows from 435 Ah to about 544 Ah of capacity.
For two days of backup, double it. For three days, triple. This is where the cost of a solar system balloons.
→ Size your bank with the calculator
Getting The Solar System Size Right
Too few panels: you run out of electricity on grey evenings and lean on the backup generator. Too many: you've spent thousands more than needed, and the extra solar energy has nowhere to go.
A well-sized solar system fills the storage on a sunny afternoon with a little headroom.
So what about a full week of clouds in winter? You have three choices: add an extra panel or two, add more storage, or keep a backup generator. Many off-gridders end up with a mix of all three.
Cut Your Energy Usage First
Every kWh you don't use is a kWh you don't have to make, keep, or pay for. LEDs everywhere. Unplug things that pull electricity when "off". Find the secret hog and decide if it's worth keeping.
A house using 5 kWh a day needs about half the solar system of one using 10. Half the panels, half the storage, and a much smaller investment.
What Is A Solar Calculator And How Does It Work?
A solar calculator does these same four steps for you. You enter your devices or your kWh, and it gives back how many solar panels you need or how big the bank should be.
Same math, just faster, and it's easy to try "what if" numbers before you purchase anything:
- Daily and yearly energy use calculator: your daily, monthly and annual kWh
- Panel count calculator: how many solar panels you need
- Battery size calculator: amp-hours for your backup days
- Fuse size calculator
- AWG wire size chart
Questions To Ask Before You Buy
- How many kWh do I use on a normal day, with 20% on top?
- How many peak sun hours does my location get, and in which month?
- How many days of backup do I want, and how deep can I empty my bank?
- What can I cut before I size anything?
→ How sunlight becomes electricity
→ How many solar panels do I actually need?
→ How much wattage do I use every day?
Sources
- US Energy Information Administration, How much electricity does an American home use?
- SolarReviews, What is a peak sun hour? Peak sun hour numbers for your state
- European Commission JRC, PVGIS 5 user manual
- Victron Energy, MultiPlus-II inverter/charger datasheet
- Victron Energy, Lithium Smart LiFePO4 datasheet
Hard Words
| Term | What it means |
|---|---|
| Kilowatt-hour (kWh) | How much energy you use over time. One kWh runs a 100-watt bulb for 10 hours. |
| Watt (W) | How much power something needs at one moment. A 50W laptop pulls 50 watts the whole time it is on. |
| Watt-hour (Wh) | Watts times hours. 1,000Wh equals 1 kWh. |
| Peak sun hour | Not the same as an hour of daylight. It is the equivalent of one hour of full-strength noon sun, 1,000 watts on every square meter. |
| Inverter | A box that changes DC power from your panels and storage into AC power your appliances can use. Loses a little energy as heat along the way. |
| Amp-hour (Ah) | How much current a battery can deliver over time. A 100Ah one puts out 1 amp for 100 hours. |
| Voltage (12V/24V/48V) | How hard the electricity pushes. Higher voltage means thinner wires for the same power. |
| Depth of discharge | How much of the stored energy you actually use. Lithium (LiFePO4) is often planned at 80 percent, lead-acid at about 50 percent. |
| Off-grid | Living without a connection to the public electricity grid. You make every kWh yourself. |
Common Questions
What is a kWh in plain language?
It is watts times hours. A 100-watt bulb running for 10 hours uses 1 kWh, and your electricity bill is measured in it.
Why do I add 20 percent on top of my number?
Inverters, wires and storage all lose a little energy as heat, and the buffer covers that.
How do I find my peak sun hours?
Look up your location on a solar map or sun-hour chart. Chicago gets about 4 a day and Phoenix about 7.5.
How many solar panels do I need?
Divide your daily kWh (with the buffer) by your sun hours, then by the size of one panel, and round up. The example needs 4 panels of 300W.
How big should my battery be?
Start with one day of backup at your daily kWh. Two days doubles the bank, three days triples it.
What if I get a cloudy week?
Add an extra panel or two, add more storage, or keep a backup generator. Many off-gridders use a mix of all three.
How can I make the whole system smaller?
Cut your energy usage first: LED bulbs, unplugging idle devices and finding the secret power hog.