Off-Grid Planner
Work out how much power your cabin, van, boat or off-grid home needs each day, then estimate the battery, solar and inverter capacity to supply it.
No sign-up required. Free to use, with every formula shown on this page. Last reviewed October 2026.
PLAN YOUR OFF-GRID SYSTEM
Off-Grid Power Planner
Estimate daily power needs and explore the battery, solar and inverter capacity you may need for a cabin, camper, or off-grid home.
Step 1: Appliances & Loads
No appliances added yet.
Add an appliance or pick a preset above to start calculating.
Step 2: System Choices
Days your battery can run without sun.
Suggested System
How it works
Size your system from your daily power use
Every off-grid system starts with the same question: how much energy do you use in a day? Build your appliance list, make three system choices, and the planner sizes the battery, solar array, charge controller and inverter to match. The sources and assumptions behind it are on our How we calculate page.
1. List your appliances
Pick from the presets or add your own. For each one, set the quantity, watts, hours a day and duty cycle (the share of time a fridge or freezer compressor actually runs). Mark whether it runs on mains AC or 12/24/48 V DC, and whether it is a motor with a start-up surge.
2. Make your system choices
Choose a 12, 24 or 48 V system, 1 to 4 days of autonomy (how long the battery runs with no sun) and lithium (LiFePO4) or lead-acid batteries. Advanced settings let you change the efficiency figures and motor surge.
3. Set your sunlight
Pick a peak sun hours preset, from 1.0 for a northern European winter to 6.5 for a southern European summer, or type your own figure. Use your darkest month if you need power all year.
Example daily energy use
How much energy do common off-grid appliances use?
Daily energy is quantity × watts × hours × duty cycle. These examples use the planner's own presets and show why heating and cooking quickly outgrow a small solar system.
*Daily energy = quantity × watts × hours × duty cycle. For mains (AC) appliances the planner also adds inverter losses (8% at the default 92% efficiency); DC appliances have no inverter loss.
The formulas
How the planner calculates your system
The planner uses standard off-grid sizing rules. These are the exact formulas it runs, so you can check every number it gives you.
Everything starts from your total daily energy:
Appliance Wh/day = quantity × watts × hours × duty cycle
AC appliances: Wh/day ÷ inverter efficiency (92%)
Daily Wh = DC total + AC total
Battery bank
Covers your daily use for the days of autonomy you choose, using only the share of the battery that is safe to drain. Inverter losses are already in the daily total, so they are not counted twice.
Battery (Wh) = daily Wh × days of autonomy ÷ usable depth of discharge
Battery (Ah) = battery Wh ÷ system voltage
Solar array and charge controller
Sized to replace one day's energy from the sun you get. The performance ratio covers heat, wiring, dirt and conversion losses. The planner counts panels at 400 W each.
Solar (W) = daily Wh ÷ (peak sun hours × performance ratio)
Panels = solar W ÷ 400, rounded up
MPPT (A) = solar W ÷ system voltage × 1.25
Inverter
Covers every AC appliance marked as running at the same time, plus headroom, and the start-up surge of motors such as pumps and fridges. The result is rounded up to a standard size.
Continuous (W) = simultaneous AC load × 1.25
Surge (W) = simultaneous AC load, with motors × 3
Inverter = next size up from the larger figure
Default values: 85% usable capacity for lithium (LiFePO4) and 50% for lead-acid or AGM, 92% inverter efficiency, a 75% solar performance ratio and a 3× motor surge. Standard inverter sizes are 300, 600, 1,000, 1,500, 2,000, 3,000, 5,000, 8,000 and 10,000 W. You can change the efficiency and surge figures in the planner's advanced settings.
Worked example
Sizing a weekend cabin, step by step
A small cabin runs five LED lights and a phone charger on DC, plus a fridge-freezer, laptop, water pump and TV on AC, about 1,470 Wh a day. The owner wants two days of autonomy from a 24 V lithium battery bank.
These are the figures the planner shows for this cabin using its presets, with every AC appliance marked as running at the same time. The 830 W surge is the fridge-freezer and pump at 3× plus the laptop and TV.
Understand your result
What to do with your system estimate
The worked example shows the biggest decision is usually winter solar. The same cabin needs five times the panels in a northern European winter as in summer. Use your estimate to decide where to compromise.
Trim your daily load
The cheapest watt-hour is the one you never use. Check the planner's top consumers, swap to LED lighting and an efficient fridge, and move heating and cooking to gas or wood.
Plan for your worst month
Size the solar for the season you will actually use the system. Fill a winter shortfall with a backup generator, which also lets you cut days of autonomy and battery cost.
Have it checked before you install
Use the estimate to compare kits and quotes. Have a qualified electrician or installer confirm cable sizes, fusing and earthing before you install, following your local wiring regulations.
Explore more tools
More practical tools, all in one place
Use simple checks, pickers and planners to make better decisions about your home, equipment and energy use.
Electricity Cost Calculator
Find out how much your appliances cost to run per day, month and year.
Replace or Keep
Compare the running cost of your current appliance with potential replacements.
Dehumidifier Picker
Find the right size and type of dehumidifier based on your room and usage.
Plug-in Solar Payback
See what a plug-in (balcony) solar kit would save and how many years it takes to pay for itself.
Damp & Mould Check
Check whether a window or wall will get condensation or mould, and what humidity keeps it dry.
Heating Cost Compare
Compare what it costs to heat a room with gas, oil, electric heaters, storage heaters or a heat pump.
Radiator & BTU Calculator
Work out what size radiator a room needs, in watts and BTU, for a boiler or a heat pump.
Paint Calculator
Work out how much paint a room or the outside of a house needs, and which tins to buy.
Concrete Calculator
Work out the concrete for a slab, footing, fence posts or steps, and the cement, ballast or bags to buy.
Tile Calculator
Work out how many tiles, boxes, adhesive and grout a floor or wall needs.
Wallpaper Calculator
Work out how many rolls of wallpaper a room or feature wall needs, allowing for the pattern repeat.
Square Metre Calculator
Work out the area of a room, floor or garden in m² and ft², including L-shapes and circles.
Gravel Calculator
Work out the tonnes and bags of gravel, chippings or sub-base for a path, border or driveway.
Decking Calculator
Work out the deck boards, joists and screws or clips for a timber or composite deck.
Flooring Calculator
Work out the packs of laminate, wood or click vinyl flooring a room needs, with underlay and beading.
Solar Panel Calculator
See what solar panels on your roof would make and save, and how many years they take to pay back.
Heat Loss Calculator
Estimate your whole home's heat loss, the heat pump size it needs and what heating costs a year.
Cable Size Calculator
Work out the cable size and fuse for a 12 V, 24 V or 48 V circuit in a van, boat or off-grid system.
EV Charging Cost
See what an electric car costs to charge per mile, per charge and per year, and the saving over petrol.
Laundry Symbols
Tap the symbols on a care label to see what they mean and the wash, dry and iron settings to use.
U-Value Calculator
Build up a wall, loft, roof or floor layer by layer to see its U-value and what its heat loss costs.
Curtain Size Calculator
Work out the pole length, curtain width and drop, the ready-made size to buy and the fabric to make them.
TV Size Calculator
Find the best TV size for where you sit, or the best seat for your TV, and how high to mount it.
Light Bulb Finder
Find the fitting, brightness and colour of the LED to replace any bulb, and keep a list of your bulbs.
Common questions
Frequently asked questions about off-grid power
Answers to the questions people ask most when sizing batteries, solar panels and inverters for an off-grid system.
How do I work out how much solar power I need?
Add up your daily energy use in watt-hours, then divide it by your peak sun hours multiplied by a performance ratio of about 75%. For example, 1,468 Wh a day with 5 peak sun hours needs about 392 W, or one 400 W panel. Plan with your darkest month's sun hours if you need the system all year.
How big a battery do I need for off-grid living?
Multiply your daily energy use by the number of days you want to run without sun, then divide by the share of the battery you can safely use: about 85% for lithium iron phosphate (LiFePO4) and 50% for lead-acid. For example, 1,468 Wh a day for two days on LiFePO4 needs about 3.5 kWh, or 144 Ah at 24 V.
What is the difference between watts and watt-hours?
Watts measure power, the rate at which an appliance uses energy at a given moment. Watt-hours measure energy, the amount used over time. A 100 W light running for 5 hours uses 500 Wh. Batteries are sized in watt-hours (or amp-hours), and inverters in watts.
What are peak sun hours?
Peak sun hours are the equivalent number of hours a day that sunlight reaches full strength of about 1,000 W per square metre. They depend on latitude, season, weather and panel angle. The planner's presets range from 1.0 for a northern European winter to 6.5 for a southern European summer.
What size inverter do I need?
Add up the power of every AC appliance that could run at the same time and add 25% headroom. Then check the start-up surge: the planner multiplies motor loads such as pumps and fridges by three. Choose the next standard inverter size above the larger of the two figures, and use a pure sine wave inverter for sensitive electronics.
Is the planner's estimate enough to buy and install a system?
No. It is a planning estimate to help you understand the scale and cost of a system and compare quotes. Actual performance depends on equipment, shading, temperature and installation. Have a qualified electrician or installer confirm the design and wiring before you install it.