Key Takeaways
- Off grid solar sizing starts with a load audit, not a panel count. Add up your daily watt-hours first, then work backwards to battery, array, inverter, and charge controller.
- Battery bank size depends on how many cloudy days you want to ride through (days of autonomy) and the usable depth of discharge of your chemistry.
- Lithium (LFP) lets you use 80 to 90 percent of rated capacity. Lead-acid caps out near 50 percent, so it needs a much bigger bank for the same job.
- Size the solar array to refill a full day of use in your worst realistic sun hours, then add roughly 30 percent margin for weather.
- The inverter is sized by peak simultaneous load plus motor surge, not by daily energy.
Get off grid solar sizing wrong and you feel it fast: lights dimming by 9 pm, a fridge that clicks off on the third cloudy morning, or a battery bank that cost twice what it needed to. An off-grid home carries all of its own supply, so every number has to hold up on the worst week of the monsoon, not the sunniest day in March. The good news is that the math is honest and repeatable. In this piece I will walk through the exact order I follow when I spec a standalone system for an Indian home, with real watt-hours and rupee figures.
Why off-grid sizing is a different job
On a grid-tied home the utility is your infinite backup. Undersize the array and you just import a little more; oversize it and you export. Off-grid has no such cushion. If the battery empties at 2 am, the house goes dark until the sun comes back.
That changes the priorities. You design for the shortest, cloudiest stretch you expect to see, and you buy storage to bridge it. Every appliance you add pushes four things up at once: panels, batteries, inverter rating, and charge controller current. So the first move is never shopping for panels. It is counting exactly what you plan to run.
Step 1: Run a real load audit
Write down every load, its power draw in watts, and the hours you actually run it per day. Multiply watts by hours to get watt-hours (Wh). Sum the column and you have your daily energy budget. This single number drives everything downstream, so be honest about it. People routinely forget the fridge runs 24 hours and that a water pump, though brief, is a heavy hitter.
| Appliance | Power (W) | Hours/day | Energy (Wh/day) |
|---|---|---|---|
| LED lights (8 x 9 W) | 72 | 5 | 360 |
| Ceiling fans (4 x 60 W) | 240 | 8 | 1,920 |
| Refrigerator (avg draw) | 150 | 8 | 1,200 |
| Television | 100 | 4 | 400 |
| Mixer and small kitchen loads | 500 | 0.5 | 250 |
| Phone and laptop charging | 100 | 3 | 300 |
| Water pump (0.75 kW) | 750 | 1 | 750 |
| Total | 5,180 |
That comes to about 5.2 kWh per day for a modest three-bedroom home. A fridge does not draw its rated wattage nonstop; it cycles, so I use an averaged figure of roughly 8 equivalent full-power hours. If you run an air conditioner, expect the daily total to jump past 10 kWh and rethink whether full off-grid is the right call.
Tip: If you own the appliances already, clamp a plug-in energy meter on the big loads for a week. Measured watt-hours beat nameplate ratings every time, and they usually reveal a phantom load or two you can simply switch off.
Step 2: Size the battery bank
The battery bank is usually the single most expensive part of an off-grid build, so getting it right matters. Two inputs decide its size: days of autonomy and usable depth of discharge (DoD).
Days of autonomy is how long the house can run on batteries alone with little or no sun. For most Indian homes I plan for 2 days, which covers a typical overcast monsoon spell. Push to 3 days if you are somewhere with long cloudy runs or if an outage genuinely cannot happen.
Depth of discharge is the fraction of the battery you can safely use. Draining a battery flat shortens its life, so chemistries have limits. Lead-acid tolerates about 50 percent regularly. Lithium iron phosphate (LFP) is happy at 80 to 90 percent. The formula is the same for both:
Battery capacity (Wh) = (Daily Wh x Days of autonomy) / (DoD x System efficiency)
Using our 5,180 Wh, 2 days of autonomy, and an 85 to 90 percent round-trip efficiency, here is how the two chemistries compare.
| Chemistry | Usable DoD | Required bank | At 48 V |
|---|---|---|---|
| Lead-acid (tubular) | 50% | ~24.4 kWh | ~510 Ah |
| Lithium (LFP) | 90% | ~12.8 kWh | ~267 Ah |
The lead-acid bank is nearly double the lithium one for the same job. That gap is why LFP has taken over new off-grid builds despite the higher upfront price. Lithium also handles deeper daily cycling and lasts far more cycles, which changes the long-run cost picture. I break the rupee-per-cycle comparison down in the guide on solar battery payback and cost in India, and the charge and discharge rate you pick matters too, which I cover in battery C-rating explained for solar storage.
Step 3: Size the solar array
The array has one daily job: refill everything you took out of the battery, plus cover the losses along the way. Those losses (wiring, controller, battery charge and discharge, heat) knock overall efficiency down to roughly 70 percent for a typical off-grid setup.
The other input is peak sun hours (PSH), the number of hours per day the sun delivers full 1,000 W per square metre equivalent. Most of India sees 4.5 to 5.5 PSH averaged over the year, but a monsoon week can drop well below that. I design against a conservative figure so the bank still fills on dull days.
Array size (Wp) = Daily Wh / (PSH x System efficiency)
Plugging in 5,180 Wh, 5 PSH, and 0.7 efficiency gives about 1,480 Wp. That is the bare minimum for an average day. Because off-grid has no grid to lean on, I round up by roughly 30 percent for cloudy-day margin and land on a 2 kW array, which is four 500 W panels or six 340 W panels. Extra panels are cheaper insurance than an oversized battery, and they shorten the recharge window after a bad stretch.
Step 4: Size the inverter and charge controller
Here is the part people mix up. The inverter is not sized by daily energy. It is sized by the largest bunch of loads that could run at the same instant, plus surge headroom for motors.
Add up your worst-case simultaneous draw: fridge compressor, pump, a few fans, TV, and lights might peak near 2,000 to 2,300 W. Add 20 to 25 percent headroom and you want a continuous rating around 2.4 kW, so a 3 kVA inverter fits. Motors like pumps and compressors pull two to three times their running current at startup for a split second, so confirm the inverter's surge rating covers that spike.
The MPPT charge controller is sized by array current into the battery. A 2 kW array charging a 48 V bank draws roughly 2,000 / 48 = 42 A, so a 60 A MPPT gives comfortable margin. Just as important, the controller's maximum input voltage must exceed your panel string's open-circuit voltage on the coldest morning, when voltage runs highest.
Safety warning: Off-grid battery banks store enormous energy at DC voltages that do not trip like AC. A shorted 48 V lithium bank can vaporise a spanner. Always fit a correctly rated DC breaker or fuse between battery and inverter, torque every lug to spec, and never work on live DC without insulated tools. Mains wiring and rooftop work should be done by a licensed electrician.
A worked example, start to finish
Last year I sized a standalone system for a farmhouse near Nashik with no reliable grid connection. The family wanted lights, fans, a fridge, a TV, and their borewell pump to run without a diesel genset. Their measured daily use came out close to the 5.2 kWh table above. Here is the order I worked in.
- Load audit: confirmed 5.2 kWh per day with a plug-in meter over ten days, including the pump running about an hour.
- Battery bank: chose LFP at 90 percent DoD, 2 days autonomy, landing on a 48 V, 280 Ah lithium bank (~13.4 kWh usable headroom).
- Solar array: installed 2 kW of panels (four 500 W modules) to refill the bank with margin on cloudy days.
- Inverter: fitted a 3 kVA, 48 V hybrid inverter to cover the pump surge alongside household loads.
- Charge controller: the inverter's built-in 60 A MPPT handled the array current with room to spare.
- Protection: added a DC breaker, AC distribution board with RCBO, and proper earthing before commissioning.
The finished system landed around 3.2 to 3.6 lakh rupees installed, most of it in the lithium bank and inverter. A year on, it has ridden through several three-day cloudy spells without the family touching the old genset. The one change I would flag for anyone with heavy pumping needs: a dedicated pump setup can be cheaper than oversizing the whole house, and I walk through that trade-off in the write-up on the solar water pump system.
Common sizing mistakes to avoid
- Sizing panels first. The array is an output of your load and battery numbers, not the starting point.
- Ignoring efficiency losses. A bank that stores 5 kWh does not deliver 5 kWh to your sockets. Bake the 70 to 85 percent losses into the math.
- Only one day of autonomy. One cloudy day is common. Design for at least two, or you will meet your low-voltage cutoff often.
- Underrating the inverter surge. A pump that starts fine on the bench can trip an undersized inverter every time in the field.
- Mixing old and new batteries. A tired cell drags the whole bank down. Commission the storage as one matched set.
For the broader picture of how these blocks fit together, from string layout to protection, see the pillar on how to design a solar system. Official load and irradiance references also help: the Ministry of New and Renewable Energy publishes India-specific off-grid guidance, and NREL hosts solar resource maps you can use to check peak sun hours for your district.
Frequently Asked Questions
How many solar panels do I need for an off-grid home?
It depends on your daily watt-hours, not the size of the house. A home using about 5 kWh per day typically needs a 2 kW array, roughly four 500 W panels, sized to refill the battery in average sun with a margin for cloudy days.
How big should the battery bank be?
Multiply your daily watt-hours by your days of autonomy, then divide by usable depth of discharge and system efficiency. For 5.2 kWh a day and two days autonomy, that is about 12.8 kWh of lithium or 24.4 kWh of lead-acid at 48 volts.
Is lithium or lead-acid better for off-grid?
Lithium (LFP) wins for most new builds. It uses 80 to 90 percent of its capacity versus 50 percent for lead-acid, lasts several times more cycles, and needs a smaller bank. Lead-acid still suits very tight upfront budgets and light usage.
What size inverter do I need?
Size it by your peak simultaneous load, not daily energy. Add up everything that could run at once, add 20 to 25 percent headroom, and confirm the surge rating covers motor startups. A typical 5 kWh-per-day home lands on a 3 kVA inverter.
Can I run an air conditioner off-grid?
You can, but it roughly doubles your daily energy and pushes battery and array costs up sharply. A 5-star inverter AC helps. Many homeowners keep the AC on grid or genset and run everything else off solar to control the budget.
Putting your numbers together
Off-grid sizing rewards patience with the arithmetic. Nail the load audit, decide how many cloudy days you must survive, and the battery, array, inverter, and controller fall out of that in order. Do it in that sequence and you avoid both the dark-at-9pm undersize and the wallet-draining oversize. If you want to see how this stage connects to panel layout, wiring, and protection for a full build, start with our guide on how to design a solar system and work outward from your own load table.
