Abstract: Backup hours = usable battery capacity (kWh) ÷ critical load (kW), then adjust for inverter efficiency, depth of discharge, and battery age. The math is simple, but the three numbers most people get wrong are: they use nameplate instead of usable capacity, they include non-essential loads, and they forget the inverter and temperature derating.
The Basic Formula
Backup hours = (Usable capacity in kWh) ÷ (Critical load in kW)
That's it. But each of those three words needs definition.
Usable capacity is not the nameplate number. A Dawnice HZEB-LCT-10 is rated at 10.24 kWh nameplate. At 90% depth of discharge, usable energy is 9.2 kWh. Run the math on nameplate and you overstate runtime by 11%.
Critical load is not the whole house. It's the sum of the loads you actually want running during an outage — lights, fridge, router, well pump — not the air conditioner, oven, and EV charger. Add the wrong loads and the number is fantasy.
Inverter efficiency takes another haircut. A hybrid inverter runs at roughly 95–97% efficiency. Multiply usable kWh by 0.95 to account for conversion losses.
The Full Formula
Effective runtime (hours) =
Nameplate kWh × DoD × Inverter efficiency × Age derate
÷ Critical load (kW)
- DoD: 90% for Dawnice LFP batteries
- Inverter efficiency: 0.95
- Age derate: 0.9 for batteries past 5 years (capacity has faded to ~90% of new)
Worked Example: Residential
A UK home wants backup for:
- LED lights: 100W
- Refrigerator: 150W
- Router + charger: 65W
- Total critical load: 315W = 0.315 kW
Battery: one HZEB-LCT-10 (10.24 kWh nameplate)
Usable = 10.24 × 0.90 × 0.95 = 8.74 kWh
Runtime = 8.74 ÷ 0.315 = 27.7 hours
That's just over a full day of essential-load backup. Our UK 30 kWh case study — three HZEB-LCT-10 units paired with a Sunsynk hybrid — delivers roughly 80 hours of essential-load runtime. That covers a multi-day winter outage.
Worked Example: C&I
A small factory in Romania wants backup for:
- Emergency lighting: 2 kW
- Control systems: 3 kW
- Security and router: 1 kW
- Office outlets: 4 kW
- Total critical load: 10 kW
Battery: the Dawnice DBS-ESD-1000P-200 (1 MW / 2 MWh container)
Usable = 2000 × 0.90 × 0.95 = 1710 kWh
Runtime = 1710 ÷ 10 = 171 hours
That's over a week of backup for critical loads. The Romania 75 kWh wall-mounted install was sized the same way — the rural load list (well pump + workshop) drove the number, not a marketing kWh figure.
What This Calculation Misses
Startup surge. A refrigerator compressor draws 3–5× its running wattage for the first second. A well pump can draw 3000W startup on a 800W running rating. The battery and inverter handle the surge, but it doesn't reduce runtime significantly — it just has to be within the inverter's peak power rating.
Temperature. Below 0°C, LFP capacity drops temporarily. A garage in winter delivers 80–90% of rated capacity. The Romania rural install accounted for this by oversizing the battery by 20%.
Load growth. If the customer adds an EV charger or heat pump next year, the critical load grows and runtime shrinks. Build in 15–20% headroom.
Common Mistakes
- Using nameplate instead of usable kWh — overstates runtime by 10–15%
- Sizing to total home load instead of critical load — overstates runtime by 3–5×
- Forgetting inverter efficiency — loses 5%
- Ignoring battery age — a 5-year-old pack delivers ~90% of new
- Sizing to a single worst day instead of average — either overspends or under-delivers
Backup hour math is not the hard part. The hard part is getting the critical load list right. Get the load list wrong and the formula produces a precise but wrong number.
If you're sizing a system and not sure what counts as critical load, send us the appliance list. Ruibit Energy works through the load calculation with you before recommending a Dawnice battery size.
Frequently Asked Questions
Q1: What is the basic formula for backup hours?
Backup hours = usable capacity (kWh) ÷ critical load (kW). Then adjust for depth of discharge (90% for LFP), inverter efficiency (95%), and battery age (90% for units over 5 years old).
Q2: Should I use nameplate or usable kWh for the calculation?
Always use usable kWh. A 10.24 kWh battery at 90% DoD delivers 9.2 kWh, not 10.24. Using nameplate overstates runtime by 10–15%.
Q3: What counts as "critical load"?
The loads you need during an outage: lights, refrigerator, router, well pump, security. Not the air conditioner, oven, or EV charger. Adding non-essential loads overstates runtime by 3–5×.
Q4: How long will a 10 kWh battery last?
At 0.315 kW of essential loads (lights, fridge, router), a HZEB-LCT-10 delivers roughly 28 hours. Add a well pump or EV charger and runtime drops significantly. Always calculate from your actual critical load list.
Q5: Do I need to account for startup surge?
Yes, but it doesn't reduce runtime — it has to be within the inverter's peak power rating. A well pump drawing 3000W startup on an 800W running rating needs an inverter that can handle the surge. Runtime depends on running watts, not startup.
