Key Takeaways
- Sizing a solar battery bank starts with your real backup load and how many hours you want to run it, not with a panel or inverter rating.
- Work in watt-hours: add up the loads you must keep alive, multiply by backup hours, then divide by depth of discharge and inverter efficiency to get the true kWh you must buy.
- A lithium (LFP) bank lets you use roughly 90 percent of its rated capacity; a lead-acid bank realistically gives you 50 percent, so you buy far more nameplate kWh.
- Always add a 20 to 25 percent design margin for ageing, cold mornings, and future loads so the bank is not maxed out on day one.
- Match battery voltage and chemistry to your inverter, and confirm the inverter can deliver your peak surge, not just the average draw.
Sizing a solar battery bank is the step most people rush, and it is the one that decides whether your backup actually carries you through a 6 pm to 10 pm outage or dies with the TV mid-serial. Get it wrong and you either overspend on kWh you never touch, or you buy a bank that sags every evening. I have commissioned enough home systems around Pune and small towns nearby to know the failure is almost always in the maths, not the hardware. In this guide I will walk you through the exact watt-hour method I use on site, the depth-of-discharge and efficiency corrections most calculators skip, and a worked example you can copy for your own home.
Start with the load, not the battery
The single biggest mistake I see is people asking "how much battery storage should I buy?" before they know what they want to run. The battery bank exists to serve a load for a duration. Until you pin those two numbers down, any kWh figure is a guess. So we build from the bottom up: list the appliances, note their real wattage, and decide how long each must stay on during an outage.
Be honest about what is "essential" backup versus "nice to have". A backup bank sized to run your whole house including the AC and geyser will be enormous and expensive. Most Indian homes I size are built around lights, fans, router, TV, fridge, and phone charging, with the option to add one AC on a bigger system. Separating must-run loads from optional ones is where you save real money.
The watt-hour method: how much battery storage you really need
Battery capacity is energy, and energy is measured in watt-hours (Wh) or kilowatt-hours (kWh). One kWh is 1000 Wh. To find the kWh battery needed, you convert every load into watt-hours over your backup window and total them. This single habit, thinking in watt-hours instead of "how many amps" or "how big a battery", removes almost all the confusion around battery bank sizing.
The core formula is simple: energy for one appliance equals its wattage multiplied by the hours you run it. Do that for each load, add them up, and you have your daily backup energy demand. Then you apply two corrections, depth of discharge and system efficiency, to turn that demand into the nameplate capacity you must purchase.
Step 1: List every backup load and its wattage
Read the rating label or use realistic figures. A BLDC ceiling fan draws around 30 W, an LED bulb 9 W, a Wi-Fi router 15 W, a 43-inch LED TV about 80 W, and a five-star rated fridge averages roughly 100 to 150 W over a day once the compressor cycling is accounted for. Do not use the peak compressor draw as a continuous number, or you will oversize badly.
Step 2: Multiply each load by its backup hours
Decide how long you want each device to survive an outage. Lights and fans might need four hours; the fridge you may want on all night. Multiply watts by hours for each row to get watt-hours, then sum the column. That total is your energy demand at the appliance, before any battery losses.
Step 3: Correct for depth of discharge
You cannot safely drain a battery to zero. Depth of discharge (DoD) is the fraction you are allowed to use. Lithium LFP banks are typically rated for around 90 percent usable, while flooded lead-acid should be held to about 50 percent to protect its life. Divide your energy demand by the DoD (as a decimal) to find the rated capacity that supplies that usable energy. This one correction explains why a lead-acid bank always looks twice as big on paper.
Step 4: Correct for inverter and round-trip efficiency
Between the DC battery and your AC appliances you lose energy in the inverter and in the battery's own charge-discharge cycle. A practical combined figure is around 85 to 90 percent for a good hybrid inverter with lithium. Divide again by that efficiency (say 0.9) so the bank still delivers your target after losses. Skipping this step is why some undersized banks trip a few minutes early.
Step 5: Add a design margin
Batteries age and lose capacity, cold winter mornings reduce output, and your loads tend to grow over time. I add 20 to 25 percent headroom so the bank is comfortable, not maxed out on day one. This margin is the difference between a bank that lasts its warranty gracefully and one you are cursing in year three.
Tip: Do the whole calculation once for "essential loads only" and once for "essential plus one AC". Seeing both kWh numbers side by side makes the price of comfort obvious and stops scope creep from quietly doubling your budget.
A worked example: sizing a real home backup bank
Let me run the numbers the way I do on a site visit. Say a family wants evening and overnight backup for essentials: six LED lights, four fans for four hours, a router and TV, and a fridge kept on for ten hours overnight. Here is the load table I would build.
| Load | Watts | Hours | Watt-hours |
|---|---|---|---|
| 6 LED lights (9 W each) | 54 | 5 | 270 |
| 4 BLDC fans (30 W each) | 120 | 4 | 480 |
| Wi-Fi router | 15 | 10 | 150 |
| 43-inch LED TV | 80 | 3 | 240 |
| Fridge (avg draw) | 120 | 10 | 1200 |
| Total demand | 2340 Wh |
The appliance demand is about 2.34 kWh. Now apply the corrections for a lithium LFP bank. Divide by DoD: 2340 / 0.9 = 2600 Wh. Divide by efficiency: 2600 / 0.9 = 2889 Wh. Add a 20 percent margin: 2889 x 1.2 = about 3467 Wh, so roughly a 3.5 kWh usable LFP bank. In the real market that rounds neatly to a 48 V, 100 Ah lithium pack (which is about 4.8 kWh nameplate and comfortably covers it).
Run the same 2340 Wh demand on flooded lead-acid and the story changes. Divide by 0.5 DoD and 0.85 efficiency, then add margin, and you land near 6.6 kWh of nameplate capacity for the same job. That is why, once you factor usable energy and cycle life, lithium's higher sticker price often works out cheaper per usable kWh over the years. If you want the detail behind those numbers, read our guide on depth of discharge and usable battery capacity.
Battery bank sizing also depends on chemistry
Two banks with the same nameplate kWh can behave completely differently because chemistry changes usable depth, cycle life, and how they age. The table below is the quick comparison I keep in my head when advising a household. For the full trade-offs, see our breakdown of LFP vs NMC vs lead-acid solar batteries.
| Factor | LFP (lithium) | NMC (lithium) | Lead-acid |
|---|---|---|---|
| Usable DoD | ~90% | ~85% | ~50% |
| Typical cycles | Very high | High | Low to moderate |
| Nameplate needed for same job | Least | Low | Most |
| Upfront cost | High | High | Low |
| Cost per usable kWh over life | Often lowest | Low | Often highest |
Match the bank to your inverter and voltage
The kWh figure tells you how much energy to store; it does not tell you the whole story about power. Your inverter must handle the peak load, including the surge when a fridge compressor or pump kicks in, which can be two to three times its running watts. Size the inverter for that surge, and confirm its continuous rating covers all your simultaneous loads.
Voltage matters too. Small systems run at 12 V or 24 V; most modern home hybrid systems use 48 V because higher voltage means lower current, thinner cables, and less loss. Whatever you choose, the battery bank voltage, the inverter's battery-input voltage, and the charge controller must all agree. Mixing chemistries or old and new batteries in one bank is a reliability trap I always warn clients away from.
Safety warning: Battery banks store serious energy and DC wiring can arc and start fires if undersized or loose. Use correctly rated cables, fuses, and a battery isolator, and have the DC side, mains changeover, and earthing done or checked by a licensed electrician. This article is general guidance, not a substitute for professional installation. See our disclaimer.
Quick step-by-step recap
- List every load you want on backup and its realistic wattage.
- Multiply each load by the hours of backup you want, then total the watt-hours.
- Divide the total by your chemistry's depth of discharge (0.9 for LFP, 0.5 for lead-acid).
- Divide again by system efficiency (about 0.85 to 0.9) to cover inverter and round-trip losses.
- Add a 20 to 25 percent margin for ageing, temperature, and future loads.
- Round to a real product voltage and capacity (for example 48 V, 100 Ah), and confirm your inverter handles the peak surge.
For the wider picture on chemistries, charging, and lifespan, our complete guide to solar batteries ties all these pieces together. You can also cross-check subsidy and standards guidance on the official MNRE site and the NREL resources on battery storage.
Frequently Asked Questions
How many kWh battery do I need for a home backup?
For essential loads (lights, fans, router, TV, fridge) most Indian homes land around 3 to 5 kWh of usable capacity for an evening-and-overnight backup. Add one AC and you move toward 8 to 10 kWh. Always size from your own load table, not a generic number.
Why is depth of discharge so important in battery bank sizing?
Depth of discharge sets how much of the nameplate capacity you can actually use. At 50 percent DoD a lead-acid bank gives half its rating, so you must buy roughly double the kWh a 90 percent lithium bank needs for the same job. It directly drives cost.
Can I add more batteries to my bank later?
Sometimes, but only within limits. Adding modules of the same chemistry, age, and voltage from a compatible system usually works. Mixing old and new batteries, or different chemistries, drags the whole bank down to the weakest cells, so plan capacity ahead rather than patching later.
Does the inverter size need to match the battery size?
They are linked but different. The battery holds energy (kWh); the inverter delivers power (kW), including surge. You can pair a large bank with a modest inverter for long runtime at low power, but the inverter must still handle your peak simultaneous load and any motor surge.
Conclusion
Sizing a solar battery bank comes down to honest arithmetic: know your real loads, decide your backup hours, and correct properly for depth of discharge, efficiency, and ageing. Do that and you will buy the right kWh once instead of guessing twice. When you are ready to choose the actual battery, start with our complete guide to solar batteries and let your load table lead the decision.
